SPECIES: Pinus ponderosa var. scopulorum
	

INTRODUCTORY

SPECIES: Pinus ponderosa var. scopulorum

 

  Big Bend National Park. Photo courtesy of James Manhart
AUTHORSHIP AND CITATION:
Howard, Janet L. 2003. Pinus ponderosa var. scopulorum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].

ABBREVIATION:
PINPONS
PINPON

SYNONYMS:
Pinus ponderosa ssp. scopulorum (Watson) Weber [309]
Pinus arizonica var. stormiae Mart. (pino real) [173,253]

NRCS PLANT CODE [295]:
PIPOS
PIPO

COMMON NAMES:
interior ponderosa pine
Rocky Mountain ponderosa pine
pino real

TAXONOMY:
The scientific name of interior ponderosa pine is Pinus ponderosa var. scopulorum Engel. (Pinaceae) [80,116,177,179,212,274]. It is 1 of 3 widely recognized varieties of ponderosa pine (P. ponderosa Laws.); the other 2 are Pacific ponderosa pine (P. p. var. ponderosa) and Arizona pine (P. p. var. arizonica) [116,177]. The taxonomy of the ponderosa pine complex is not completely resolved. There are morphological and distributional overlaps among the varieties, and disagreement among authorities regarding the geographical boundaries of ponderosa pine infrataxa including interior ponderosa pine [82,116,191,253]. Although distinct, varieties of ponderosa pine show differences on broad latitudinal clines. Dodge [97] described the species as "a continuation of a large group of populations from the Central American Highlands to British Columbia." Ponderosa pine in the northern interior are either classified entirely as P. p. var. scopulorum [116], or as P. p. var. scopulorum east of the Continental Divide and as P. p. var. ponderosa west of the Continental Divide [191].  Ponderosa pine populations that extend northward into western Texas from Coahuila are either classified as P. p. var. scopulorum [116] or as P. arizonica var. stormiae Mart. [173,177,253].

In this report, "ponderosa pine" refers to the species as a whole. "Southwestern ponderosa pine" refers to both Arizona pine and interior ponderosa pine in the Southwest. Information on ponderosa pine in Texas and east of the Continental Divide in Montana and Wyoming is included herein; Pacific ponderosa pine and Arizona pine are covered in separate Fire Effects Information System reviews.

Intraspecies hybridization and introgression occur between all 3 varieties of ponderosa pine. Interior ponderosa x Arizona pine hybrids may bear 3- and 4-needle fascicles; 4- and 5-needle fascicles; or 3-, 4-, and 5-needle fascicles, sometimes on the same spur branch [38,97,116,236]. Interior ponderosa and Arizona pines also hybridize and introgress with Apache pine (P. engelmannii); 3-taxa hybrids (interior ponderosa x Arizona x Apache pines) occur occasionally [236].

LIFE FORM:
Tree

FEDERAL LEGAL STATUS:
No special status

OTHER STATUS:
No entry


DISTRIBUTION AND OCCURRENCE

SPECIES: Pinus ponderosa var. scopulorum
GENERAL DISTRIBUTION:
Ponderosa pine is the most widely distributed pine species in North America, ranging north-south from southern British Columbia to central Mexico and east-west from central Nebraska to the west coast [202]. Ponderosa pine ecosystems occupy about 38 million acres across 14 states [122]. The U.S. Geological Survey provides a distributional map of ponderosa pine.

Interior ponderosa pine's general distribution is from the eastern slope of the northern Rocky Mountains in Montana east to central Nebraska and Kansas and south to eastern Nevada, western Texas, and Mexico. The distribution continues south from Texas into Coahuila, Tamaulipas, and San Luis Potos, Mexico [81,82,274]. However, interior ponderosa pine is not a geographically distinct taxon [163]. There is a broad zone of intergradation between interior and Pacific ponderosa pines in southeastern Idaho and on both sides of the Continental Divide in Montana and Wyoming [38,73,97,235,236]. Interior ponderosa x Arizona pine intergrades occur in southeastern Arizona, southeastern New Mexico, and Mexico [38,97,116,236].  On sky islands of southeastern Arizona, interior ponderosa pine mostly occurs above 7,000 feet (2100 m) elevation. A transition zone of intergraded interior ponderosa pine × Arizona pine occurs between 5000 and 7000 feet (1500-2100 m) [97].

ECOSYSTEMS [122]:
FRES20 Douglas-fir
FRES21 Ponderosa pine
FRES23 Fir-spruce
FRES26 Lodgepole pine
FRES29 Sagebrush
FRES33 Southwestern shrubsteppe
FRES34 Chaparral-mountain shrub
FRES35 Pinyon-juniper
FRES36 Mountain grasslands
FRES38 Plains grasslands
FRES39 Prairie

STATES/PROVINCES: (key to state/province abbreviations)
AZ CO KS MT NE NV NM ND SD TX UT WY
MEXICO

BLM PHYSIOGRAPHIC REGIONS [40]:
6 Upper Basin and Range
7 Lower Basin and Range
8 Northern Rocky Mountains
9 Middle Rocky Mountains
10 Wyoming Basin
11 Southern Rocky Mountains
12 Colorado Plateau
13 Rocky Mountain Piedmont
14 Great Plains
15 Black Hills Uplift
16 Upper Missouri Basin and Broken Lands

KUCHLER [189] PLANT ASSOCIATIONS:
K012 Douglas-fir forest
K015 Western spruce-fir forest
K016 Eastern ponderosa forest
K017 Black Hills pine forest
K018 Pine-Douglas-fir forest
K019 Arizona pine forest
K020 Spruce-fir-Douglas-fir forest
K021 Southwestern spruce-fir forest
K023 Juniper-pinyon woodland
K024 Juniper steppe woodland
K031 Oak-juniper woodland
K032 Transition between K031 and K037
K037 Mountain-mahogany-oak scrub
K038 Great Basin sagebrush
K053 Grama-galleta steppe
K055 Sagebrush steppe
K063 Foothills prairie
K064 Grama-needlegrass-wheatgrass
K065 Grama-buffalo grass
K066 Wheatgrass-needlegrass
K067 Wheatgrass-bluestem-needlegrass
K075 Nebraska Sandhills prairie

SAF COVER TYPES [106]:
201 White spruce
206 Engelmann spruce-subalpine fir
210 Interior Douglas-fir
211 White fir
216 Blue spruce
218 Lodgepole pine
219 Limber pine
220 Rocky Mountain juniper
237 Interior ponderosa pine
239 Pinyon-juniper
240 Arizona cypress

SRM (RANGELAND) COVER TYPES [266]:
301 Bluebunch wheatgrass-blue grama
303 Bluebunch wheatgrass-western wheatgrass
309 Idaho fescue-western wheatgrass
310 Needle-and-thread-blue grama
320 Black sagebrush-bluebunch wheatgrass
321 Black sagebrush-Idaho fescue
402 Mountain big sagebrush
403 Wyoming big sagebrush
409 Tall forb
412 Juniper-pinyon woodland
413 Gambel oak
415 Curlleaf mountain-mahogany
502 Grama-galleta
503 Arizona chaparral
504 Juniper-pinyon pine woodland
509 Transition between K031 and K037
602 Bluestem-prairie sandreed
607 Wheatgrass-needlegrass
608 Wheatgrass-grama-needlegrass
609 Wheatgrass-grama
610 Wheatgrass
611 Blue grama-buffalo grass
612 Sagebrush-grass
715 Grama-buffalo grass

HABITAT TYPES AND PLANT COMMUNITIES:
The interior ponderosa pine/bunchgrass type is the most common association throughout interior ponderosa pine's range. It is characterized by open grassland interspersed with widely spaced trees. Under pristine conditions, the tree canopy usually covers no more than 25% of the ground. North of the Colorado border, Idaho fescue (Festuca idahoensis) is usually the dominant bunchgrass; Arizona fescue (Festuca arizonica) is generally dominant to the south [84,208]. In mountainous areas, stand structure becomes increasingly savanna-like at lower elevations and forested at higher elevations. Interior ponderosa pine merges with pinyon-juniper (Pinus-Juniperus spp.), chaparral-mountain shrubland, or grassland at lower elevations and with Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca), subalpine fir-Engelmann spruce (Abies lasiocarpa-Picea engelmannii), Rocky Mountain lodgepole pine (Pinus contorta var. latifolia), and/or limber pine (P. flexilis) at higher elevations [88].  Species composition of interior ponderosa pine communities is described below by state or region.

MT: In central Montana the interior ponderosa pine type merges into plains grassland at lower elevations and limber pine at higher elevations. Limber pine, Rocky Mountain Douglas-fir, and Rocky Mountain juniper (Juniperus scopulorum) are common overstory associates. Plains grassland species include blue grama (Bouteloua gracilis), sideoats grama (B. curtipendula), and prairie Junegrass (Koeleria macrantha). Western snowberry (Symphoricarpos occidentalis) and skunkbush sumac (Rhus trilobata) are common shrub associates. Soapweed yucca (Yucca glauca), pygmy prickly-pear (Opuntia fragilis), and plains prickly-pear (O. polyacantha) occur on the driest sites [16,230]. Rocky Mountain juniper, bluebunch wheatgrass  (Pseudoroegneria spicata), and longleaf wormwood (Artemisia longifolia) are dominant species in interior ponderosa pine of the Missouri River Breaks [208].

Interior ponderosa pine is the only forest tree in southeastern Montana, where it forms several diverse habitats. On dry sites it supports an understory of plains grassland species such as big bluestem (Andropogon gerardii var. gerardii), little bluestem (Schizachyrium scoparium), and blue grama, and of mountain grassland species such as bluebunch wheatgrass and elk sedge (Carex geyeri). Understories are typically dense on wetter sites, and include species characteristic of Pacific ponderosa pine forests to the west. Russet buffaloberry (Shepherdia canadensis), bearberry (Arctostaphylos uva-ursi), twinflower (Linnaea borealis), and heartleaf arnica (Arnica cordifolia) are common wet-site associates [16,230].

WY: Interior ponderosa pine is usually the only tree at lower timberline in the Bighorn Mountains. Shrubs or grasses may dominate the understory. The most common understory dominants are bluebunch wheatgrass, Idaho fescue, white spirea (Spiraea betulifolia), and mountain ninebark (Physocarpus monogynus) [164]. Interior ponderosa pine communities are uncommon in southeastern Wyoming. Where they occur, elk sedge most commonly dominates the understory. Ross' sedge (C. rossii) and bearberry are occasional understory dominants. Common graminoids and forbs include prairie Junegrass, spike fescue (Leucopoa kingii), western yarrow (Achillea millefolium), and heartleaf arnica [8].

White spruce (Picea glauca) may co-occur with interior ponderosa pine in the Black Hills of Wyoming and South Dakota. Bur oak (Quercus macrocarpa), quaking aspen (Populus tremuloides), and paper birch (Betula papyrifera) are overstory associates [52].

Dakotas: Interior ponderosa pine understories in the Black Hills of South Dakota are usually dominated by shrubs including common juniper (Juniperus communis), russet buffaloberry, common snowberry (Symphoricarpos albus), chokecherry (Prunus virginiana), and bur oak. A few stands have grassy understories of timber oatgrass (Danthonia intermedia), Kentucky bluegrass (Poa pratensis), and/or little bluestem [289]. Interior ponderosa pine forests and woodlands are not abundant in North Dakota. Understory composition is similar to that of interior ponderosa pine in the Black Hills [125,138].

CO: On the Colorado Front Range, interior ponderosa pine generally occurs with Rocky Mountain Douglas-fir on north slopes and Rocky Mountain juniper on south slopes [215]. Quaking aspen commonly co-occurs on sites that have experienced past fires or other reoccurring disturbance [84]. Understory species include common juniper, wax currant (Ribes cereum), bearberry, spike fescue, blue grama, and buffalo grass (Buchloe dactyloides). At the forest-plains grassland ecotone, understory species may also include black grama (Bouteloua eriopoda), little bluestem, needlegrasses (Hesperostipa and Achnatherum spp.), western wheatgrass (Pascopyrum smithii), and cheatgrass (Bromus tectorum) [215]. Interior ponderosa pine is a minor species in western Colorado, where it occurs in Rocky Mountain Douglas-fir forests [165].

In southern Colorado, interior ponderosa pine lies between lower-elevation grassland or pinyon-juniper (Pinus-Juniperus spp.) and higher-elevation Rocky Mountain Douglas-fir. Grassy understories with blue grama, mountain muhly (Muhlenbergia montana), mutton grass (Poa fendleriana), Arizona fescue, and little bluestem are common on dry sites. On moister sites, shrubs such as Gambel oak (Quercus gambelii), wavyleaf oak (Q. pauciloba), and bearberry may also be important [91].

UT: Interior ponderosa pine/shrub communities in central and southern Utah are usually the lowest coniferous forest type, and border shrublands or Colorado pinyon-Utah juniper (Pinus edulis-Juniperus osteosperma) woodland. Dominant understory species include curlleaf mountain-mahogany (Cercocarpus ledifolius), greenleaf manzanita (Arctostaphylos patula), black sagebrush (Artemisia nova), Gambel oak, and mountain snowberry (S. oreophilus). Interior ponderosa pine/mountain muhly occurs in central and southern Utah [322]. Interior ponderosa pine/grassland series also occur in northeastern Utah: dominant grasses are Idaho fescue, sheep fescue (F. ovina), and/or mutton grass. Rocky Mountain lodgepole pine, Utah juniper, and quaking aspen are seral in these habitat types [216].

Interior ponderosa pine in southeastern Nevada occurs mostly on northerly aspects of canyons. Associated species include Utah juniper, Gambel oak, rubber rabbitbrush (Chrysothamnus nauseosus), big sagebrush (Artemisia tridentata), bottlebrush squirreltail (Elymus elymoides), western wheatgrass, and cheatgrass [45].

Southwest: Interior ponderosa pine is the most common forest tree in the Southwest [323]. Vegetation patterns in southern New Mexico and Arizona are mostly determined by aspect and elevation. In southeastern Arizona, interior ponderosa pine forest lies on elevational and moisture gradients between dry, low-elevation Mexican pinyon-Chihuahua pine (Pinus cembroides-P. leiophylla var. chihuahuana)-Apache pine forest and/or Madrean oak woodland and moister, higher-elevation southwestern white pine (P. strobiformis)-Rocky Mountain Douglas-fir forest [30,35,135]. At 7,500 to 8,500 feet (2300 to 2600 m) elevation, interior ponderosa pine, Arizona pine, and encinal oaks tend to dominate dry, south- and west-facing slopes, while Rocky Mountain Douglas-fir and corkbark fir (Abies lasiocarpa var. arizonica) occupy north- and east-facing slopes. Madrean oak associates include Arizona white oak (Quercus arizonica), Gambel oak, Emory oak (Q. emoryi), and silverleaf oak (Q. hypoleucoides). Understories are dominated by grasses including pine dropseed (Blepharoneuron tricholepis), Arizona wheatgrass (Elymus arizonicus), prairie Junegrass, and screwleaf muhly (M. virescens) [30]. In high-elevation (8,400 to 9,300 feet (2500-2800 m)) mixed-conifer forest, Engelmann spruce, blue spruce (P. pungens), corkbark fir, Rocky Mountain Douglas-fir, southwestern white pine, and quaking aspen co-occur with interior ponderosa pine [127].

AZ: Associates in interior ponderosa pine/bunchgrass in northern Arizona include mountain muhly, mutton grass, Arizona fescue, bottlebrush squirreltail, blue grama, pine dropseed, deergrass (M. rigens), and prairie Junegrass [25]. Shrubs may be nearly absent to fairly common. If present, they are usually widely and irregularly spaced compared to more northern ponderosa pine communities. Fendler ceanothus (Ceanothus fendleri), wax currant, skunkbush sumac, greenleaf manzanita, Stansbury cliffrose (Purshia mexicana var. stansburiana), Apache plume (Fallugia paradoxa), and mountain-mahogany (Cercocarpus spp.) are among the most common shrubs in interior ponderosa pine/bunchgrass. The ponderosa pine type usually merges into Arizona chaparral at lower elevations of northern and central Arizona. It may also finger into riparian zones [60]. At higher elevations, the type merges into corkbark fir-spruce (Picea spp.) forest. Associates in the upper ponderosa pine zone include those shrubs also found in fir-spruce forest: common juniper, Oregon boxwood (Paxistima myrsinites), red raspberry (Rubus idaeus), mountain ninebark, and snowberry (Symphoricarpos spp.). Rocky Mountain Douglas-fir and quaking aspen may co-occur with interior ponderosa pine above 7,000 feet (2000 m) [68,77,229].

TX: Netleaf oak (Q. rugosa), silverleaf oak, and quaking aspen are common associates throughout western Texas. Colorado pinyon and alligator juniper (Juniperus deppeana) often form a woodland association with interior ponderosa pine on south slopes; Douglas-fir and southwestern white pine join interior ponderosa pine in a forest/bunchgrass association on moister slopes [2]. Understory associates include finestem tussockgrass (Nassella tenuissima), California brome (Bromus carinatus), Big Bend bluegrass (Poa strictiramea), bulb panicgrass (Panicum bulbosum), and pinyon ricegrass (Piptochaetium fimbriatum) [90]. In the Guadalupe and Chisos mountains, interior ponderosa pine dominates ponderosa pine-Douglas-fir-southwestern white pine parklands and is an associate in Chisos red oak (Q. gravesii)-Colorado pinyon woodlands. In the Davis Mountains, it occurs in ponderosa pine-southwestern white pine-Gambel oak associations [2,90,92,267].

Vegetation typings describing plant communities dominated by interior ponderosa pine are:

AZ [36,114,199,223,226,277,313]
CO [91,159,160,184,186,187,245]
MT [137,139,208,230,237,241,242,243]
NE [176]
NV [45]
NM [4,5,91,114,136,199,277]
ND [125,138]
SD [138,166,289]
TX [156,288]
UT [322]
WY [8,166,290]

Regional: [87,170,276,303]

MANAGEMENT CONSIDERATIONS

SPECIES: Pinus ponderosa var. scopulorum
WOOD PRODUCTS VALUE:
Interior ponderosa pine is the most commercially valuable and productive timber tree in the inland west and Southwest [48,127]. High-quality logs are used for high-grade boards and a wide variety of other products including cabinets, molding, and cut stock. Lower-quality logs are used for dimension lumber and other construction products [48].

IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Interior ponderosa pine communities are important wildlife habitat. The forest understory provides valuable browsing and grazing for wildlife and livestock [101,102,291,294,296]. Wildlife also use interior ponderosa pine woodland-grassland mosaics heavily. Wooded draws of interior ponderosa pine in the high plains grasslands of the Dakotas and Nebraska provide valuable habitat for a variety of wildlife [44].

Birds: Interior ponderosa pine communities are critical habitat for a wide variety of birds [32,39,120] including owls [113,120,168], other cavity nesters, and wild turkey [247]. Cavity-nesting birds use interior ponderosa pine snags for foraging and roosting as well as nesting [32,99]. Balda [32] estimated that secondary cavity nesters comprised 68% of the total density of wintering birds in southwestern ponderosa pine forests. Cunningham and others [83] provide a list of secondary cavity nesters on the Coconino National Forest of Arizona. They recommend a density of 5.2 interior ponderosa pine snags per hectare to sustain habitat for cavity-nesting birds.

Bennetts and Hawksworth [39] found that bird diversity and number of nests observed in interior ponderosa pine stands in northern Colorado were positively correlated with interior ponderosa pine's level of infestation with southwestern pine dwarf-mistletoe (Arceuthobium vaginatum ssp. cryptopodum). Western tanagers, American robins, chipping sparrows, hermit thrush, and Cassin's finches nested in witch's brooms. Among guilds, number of insectivores, canopy, foliage, and ground omnivores, and ground granivores species was significantly higher in infected stands, while flycatchers and nectivores showed no significant effect due to level of stand infection. In northern Arizona, Dwyer [99] found that in postfire years 1 and 2, secondary cavity-nesters were more abundant in unburned, light-, and moderate-severity wildfire sites compared to sites that experienced severe wildfire. She attributed the difference to lack of suitable interior ponderosa pine snags on the severe burn. Secondary cavity-nesters used the severe burn when artificial nest boxes were attached to burned snags. In the short term, light and moderate wildfire promoted mountain bluebird and had little effect on pygmy nuthatches, white-breasted nuthatches, and violet-green swallows. Moderate wildfire had a short-term negative effect on mountain chickadees. Other studies [169,287] have shown that mountain chickadees increase or return to prefire numbers 3 to 5 years after fire.

Interior ponderosa pine provides roosting, nesting, and foraging habitat for the Mexican spotted owl. The mixed-conifer, interior ponderosa pine, and interior ponderosa pine/Gambel oak types provide optimal habitat for the owl, which is federally listed as threatened [121]. Ganey and Balda [119] found that for roosting habitat, Mexican spotted owls in northern Arizona preferred closed-canopy, virgin stands of mixed conifers with a high density of snags and large logs. The owls preferred both interior ponderosa pine and mixed-conifer old growth forests for foraging. A study on the Coconino National Forest showed the owls preferentially selected nest sites in areas with 41 to 70% canopy closure [129]. Fiedler and Cully [113] provide tree basal area and age class silvicultural prescriptions that optimize Mexican spotted owl habitat and promote old-growth interior ponderosa pine.

The flammulated owl, a cavity nester, is also dependent on old-growth interior ponderosa pine forest, but favors a more open canopy for nesting and foraging than the Mexican spotted owl [168].

Reintroduced and rare in southeastern Arizona, thick-billed parrots are ecologically dependent on Chihuahua and southwestern ponderosa pines for food and shelter. Pine seeds are their primary diet item, and the parrots nest in pine snag cavities [270].

Small mammals: Interior ponderosa pine provides habitat for many rodent species, and the seeds are an important food source for some rodents and shrews [126,182]. Tree squirrels (Sciurus and Tamiasciurus spp.) use interior ponderosa pine for nesting, and the seeds are among their most important foods. Abert's squirrel is ecologically dependent upon southwestern ponderosa pine [107,109,181,269]. Red squirrels beyond the range of Abert's squirrel use interior ponderosa pine heavily for food and nesting. Where ranges of the 2 squirrels overlap, Abert's squirrels tend to displace red squirrels to higher-elevation forests [109].

Deer: Interior ponderosa pine communities provide valuable deer habitat. Their understories provide forage; production is highest in open stands. A study on the Apache-Sitgreaves National Forest of Arizona showed mean production of mule deer forage in interior ponderosa pine-Douglas-fir-southwestern white pine was 82 pounds/acre (92 kg/ha) at 50 feet2/acre (11 m2/ha) basal area and 4 pounds/acre (4.5 kg/ha) at 400 feet2/acre (90 m2/ha) basal area [291]. The tree itself provides minor browse [297]. White-tailed deer on Missouri River bottomlands of north-central Montana consumed interior ponderosa pine in trace amounts during fall. In winter, browsing frequency increased to 33%, comprising 2% of total volume intake. White-tailed deer were not detected browsing interior ponderosa pine in spring or summer [10].

PALATABILITY:
Interior ponderosa pine is unpalatable to domestic livestock. Cattle generally browse interior ponderosa pine seedlings only when herbaceous forage is scarce [172,297]. When herbaceous vegetation is sparse, however, livestock may browse interior ponderosa pine enough to slow or stop interior ponderosa pine seedling recruitment [215]. Pregnant cows that consume large amounts of interior ponderosa pine needles show an increased incidence of abortion and other reproductive anomalies [314]. Interior ponderosa pine is not highly palatable to wild ungulates either, but they generally prefer interior ponderosa pine to associated conifer species [172]. Wild ungulate use of interior ponderosa pine is mostly seasonal, with heaviest use occurring in winter and spring [84,162,172]. White-tailed deer in the Black Hills of South Dakota did not use interior ponderosa pine during July to September but browsed trees from October to December.  Use was lowest from January to March, and highest from April to June [161].

Rodents and lagomorphs browse seedlings year-round [84]. Common porcupines consume the phloem of mature interior ponderosa pine; Abert's squirrels consume the seed, bud, and twigs as well as the phloem.  Palatability of the phloem varies among trees within a stand, with both rodent species preferring trees with relatively low monoterpene concentrations. Common porcupines also prefer trees infected with southwestern pine dwarf-mistletoe to uninfected trees [201].

NUTRITIONAL VALUE:
Interior ponderosa pine near Flagstaff, Arizona, comprised 6% of the mule deer diet. Chemical content (%) of interior ponderosa pine leaves and buds was follows [297]:

Protein Acid-detergent fiber Ca P Digestible dry matter*
8.0 41 0.13 0.15 41
*in-vitro digestibility

COVER VALUE:
Interior ponderosa pine snags provide sites for cavity-nesting birds and mammals. A study on the Coconino National Forest of Arizona determined that an average of 2.6 ponderosa pine snags per acre (6.5/ha) was needed to maintain woodpecker (Picidae) populations. The most frequently used snags were trees that had been dead at least 6 years and were 16 inches (41 cm) or more dbh [15]. Cunningham and others [83] recommend a density of 2.1 interior ponderosa pine snags per acre (5.2 /ha) to sustain habitat for cavity-nesting songbirds on the Coconino.  

Mule deer in Colorado use open-canopy interior ponderosa pine/grassland habitat for feeding. Their foraging activity decreases as canopy cover increases [190]. On the Missouri River Breaks of Montana, interior ponderosa pine-Rocky Mountain juniper/bluebunch wheatgrass communities comprised the single most important mule deer summer feeding grounds compared to grassland, shrub, and Rocky Mountain Douglas-fir types. Mule deer also used interior ponderosa pine/longleaf wormwood communities for summer foraging, and for year-round escape and bedding cover. Elk used interior ponderosa pine-Rocky Mountain juniper/bluebunch wheatgrass communities moderately for summer feeding and extensively for summer bedding. Over 3 years, overall elk use of the type was moderate, with elk use peaking during a year of high forb production  [208].

Interior ponderosa pine provides shade and resting cover. Amstrup [12] found that pronghorn in south-central Montana and north-central Wyoming used upland ponderosa pine woodlands significantly more than expected (p<0.05) during summer, while use did not differ from expected in other seasons. He often saw 12 or more pronghorn crowded within the shade of pine clumps during hot summer afternoons. Elk in the Black Hills of South Dakota sought interior ponderosa pine stands with 54% or more canopy closure during the summer months [219].

VALUE FOR REHABILITATION OF DISTURBED SITES:
As a valuable wildlife and commercial timber tree, interior ponderosa pine is widely planted on erodable and other disturbed sites including burns. Artificial regeneration is often difficult due to droughty soils and competition from other vegetation. Heidmann [155] provides regeneration strategies for ponderosa pine.

Some exotic herbs that are commonly planted for rehabilitation may interfere with growth of interior ponderosa pine seedlings. Artificial interior ponderosa pine regeneration planted north of Flagstaff showed no significant difference (p < 0.05) in height and stem diameter on plots seeded to native blue grama or bottlebrush squirreltail and then weeded of other species compared to control plots kept free of all species but interior ponderosa pine; however, interior ponderosa pine seedlings on plots seeded with the exotics small burnet (Sanguisorba minor), yellow sweetclover (Melilotus officinalis), orchard grass (Dactylis glomerata), or desert wheatgrass (Agropyron desertorum) were significantly shorter than seedlings on control plots or plots planted to the native grasses. Seedlings planted with yellow sweetclover or desert wheatgrass were significantly smaller in girth compared to those on weeded plots or plots seeded to other herbaceous species [105].

OTHER USES AND VALUES:
Interior ponderosa pine is valued as a drought-resistant ornamental [67]. It is widely planted for windbreaks, especially on the plains grasslands [58,240].

Native Americans ate interior ponderosa pine seeds and the sweet, edible phloem in the inner bark [151,152]. The Cheyenne of Montana applied interior ponderosa pine pitch inside whistles and flutes to improve the instruments' tone. They made blue dye from a root extract [152]. The Nez Perce used the pitch as a torch fuel; the Nez Perce and Crow also used pitch as glue [151].

OTHER MANAGEMENT CONSIDERATIONS:
Interior ponderosa pine ecosystems are under considerable stress, both from natural and anthropogenic factors including past management practices.

Pests and diseases: Southwestern pine dwarf-mistletoe is a serious disease agent of interior ponderosa pine south of Wyoming [153,200]. Infection rates in the Southwest were at 33% in the 1990s [200], with about 20% infection of interior ponderosa pine on the Colorado Front Range. Stands that have had partial cuts or mountain pine beetle attacks are most susceptible to dwarf-mistletoe infections. New stands that have regenerated after stand-replacing fire are least likely to become infected [6]. Dwarf-mistletoes (Arceuthobium spp.) and interior ponderosa pine do not co-occur north of Colorado because the northern interior continental climate is inhospitable to dwarf-mistletoe growth [7,52].

At least 59 species of insects attack interior ponderosa pine. The mountain pine beetle (Dendroctonus ponderosae) is the most serious pest in the Black Hills and the central and southern Rocky Mountains; D. adjunctus is a closely related pine beetle that damages trees in the Southwest. Pine beetle epidemics have occurred throughout recorded history. Epidemic outbreaks are usually associated with large (> 6-inch (15 cm) diameter), stressed tress in overcrowded stands [7,52]. Pine engraver beetles (Ips spp.) are usually secondary infesters in the central and southern Rocky Mountains but are often more damaging than pine beetles in the Southwest [7,227].

Bark beetle (Ips spp. and D. adjunctus) kill has escalated greatly in the Southwest. Prolonged drought and high tree density have probably increased the susceptibility of southwestern ponderosa pine to attacks. A 2002 survey of National Forests in Arizona and New Mexico showed a 4-fold increase in ponderosa pine mortality in 2002, with over 2 million pines killed on approximately a half-million acres. A combination of control actions including removing infested trees, application of insecticide, and thinning is recommended. Because the beetle outbreak is so large, control efforts are best targeted to accessible, high-value areas [14].

The most serious wood-decaying fungi are red rot and western gall rust. Shoestring root rot occasionally infects pole-sized and younger trees [7]. Armillaria (Armillaria spp.) is a serious fungal pathogen in the Black Hills and New Mexico [154]. Management and harvesting guidelines to minimize infection are available [6,7,154].

Age classes and stand structure: Interior ponderosa pine is rapidly losing its oldest age class. Mortality of old-growth interior ponderosa pine is high due to increased interference from dense postsettlement trees and stagnated nutrient cycling in the absence of fire. For example, mortality rate of old-growth interior ponderosa pine on the Fort Valley Experimental Forest of Arizona increased 7-fold from the 1930s level (0.75 tree/ha/decade) to the 1970s (5.75 trees/ha/decade) [213]. Changes to interior ponderosa pine and mixed-conifer forests due to fire exclusion are discussed in the Fire Ecology section of this report.

Range: Open stands of interior ponderosa pine produce more forage than dense stands.  Periodic surface fire promotes grass production by reducing shrubs and pine litter, which retard graminoid production. Postfire increases in forage production last about 5 to 10 years. Other benefits of range fires include better livestock distribution and range utilization, increased water yield, and improved wildlife habitat.

Forage productivity varies with understory, with blue grama understories generally the least productive and Arizona fescue and screwleaf muhly the most productive [228]. Stands of large-diameter trees in a patchy distribution help maintain graminoid diversity  [70,225]. Overstory thinning increased production in the grass and forb understory on the Coconino National Forest. Herbage production was significantly* greater (p=0.10) on plots thinned to basal areas of less than 70 ft2/acre (16.3 m2/ha) compared to unthinned plots with similar basal areas. Herbage production 6 years after treatment was as follows [70]:

Basal area (ft2/acre) Herb production (lbs/acre)
thinned unthinned
Difference
20 566.6 376.8 189.8*
40 384.2 277.1 107.1*
60 277.5 218.8 58.7*
80 201.8 177.4 24.4
100 143.0 145.2 -2.2
120 95.1 119.0 -23.9

Cattle and elk use is inversely proportional to tree basal area [71,72]. On thinned interior ponderosa pine sites in Arizona, elk preferred sites where some alligator juniper was left to sites where it had been completely removed. Mule deer showed no preference for thinned vs. unthinned areas, but tended to congregate on sites with alligator juniper, southwestern pine dwarf-mistletoe, or acorn-producing Gambel oak [72].

After monitoring cattle use on interior ponderosa pine/Arizona fescue-mountain muhly on the Manitou Experimental Forest near Colorado Springs, Colorado, Smith [268] recommended 30 to 40% utilization of the bunchgrasses by the end of the grazing season to maximize cattle weight gain while maintaining palatable bunchgrasses. Currie [84] provides suggestions for managing interior ponderosa pine/bunchgrass lands in the Central Rocky Mountains for cattle grazing.

Response of to interior ponderosa pine grazing: Light- to moderate-intensity grazing does not greatly impact advanced interior ponderosa pine regeneration on rangelands in good condition. On the Manitou Experimental Forest, neither rest-rotation nor season-long grazing resulted in much damage to natural and artificial regeneration under light or moderate use, but interior ponderosa pine seedlings were heavily browsed and incurred severe damage to terminal buds under heavy rest-rotation or season-long grazing use [84]. Thirteen years after exclosures were built, ponderosa pine in northern and central Arizona had increased slightly on both ungrazed and grazed plots, at 6% and 5% frequency, respectively. Initial frequency was 2% on both plots [26].

Mast and others [215] found that moderate cattle grazing combined with fire exclusion has favored interior ponderosa pine seedling establishment on ponderosa pine/grassland and ecotonal communities of the Colorado Front Range. Prior to fire exclusion, frequent fire probably excluded ponderosa pine seedlings at the ecotonal boundary.

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Pinus ponderosa var. scopulorum
GENERAL BOTANICAL CHARACTERISTICS:
Morphology: Interior ponderosa pine is a native conifer. Young, fast-growing trees have pyramidal crowns that become broader and more rounded as they mature. Height at maturity varies but this variety tends to be shorter than Pacific ponderosa pine. Trees in the Great Basin seldom exceed 100 feet (30 m) [192], while specimens in the southern Rocky Mountains may reach 160 feet (50 m) in height and 50 inches (130 cm) in diameter [262]. Interior ponderosa pine branches are open and self-pruning. Needles are from 3 to 7 inches (7-17 cm) long. The seed cones are 2 to 4 inches (5-10 cm) in length, and bear winged seeds with a body length of 3 to 4 millimeters and wing length of up to 15 millimeters [116]. Bark of mature trees is about 3 inches (8 cm) thick [192]. The root system is deep and spreading; interior ponderosa pine is one of the most wind-firm trees in the inland Northwest [6,7,73]. An 85-year-old tree in Colorado that was 19 feet (5.7 m) in height and 4.5 inches (114.3 cm) dbh had a maximum root depth of 5 feet (1.5 m) and a maximum lateral spread of 10 feet (3 m), with the majority of roots growing down slope. Fine roots were absent [41]. In Nebraska plains grassland clay, a 32-year-old, 25-foot (8 m) tree had lateral roots extending 20 feet (6 m) from the stem. Greatest depth of vertical roots was 12 feet (4 m), with most roots concentrated 3 to 4 feet (0.9-1.2 m) below ground [320].

There are 2 races of interior ponderosa pine: the northern and southwestern types. Northern interior ponderosa pine has a majority of 2-needle fascicles [16,73,310]. The variety's scientific name derives from the stiff, tufted (scopulate) foliage and compact crown characteristic of this race [73]. The southwestern race has a majority of 3-needle fascicles and a relatively open crown [179,310]. 

Structure: Stands were historically open with varied age class distributions [213,215]. Cooper [75] found that on the Fort Apache Reservation of east-central Arizona, where frequent prescribed burning is practiced, interior ponderosa pine grows in a mosaic of small (about 0.2-acre (0.08 ha)), even-aged groups that are maintained by fire. Trees are widely and randomly spaced. In contrast, structure in an unburned virgin stand on the Fort Valley Experimental Forest was uneven-aged with trees in small, uneven-aged groups. Pole-sized trees predominated, with scattered presettlement trees 14.5 to 41 inches (37-104 cm) dbh, and dense thickets of saplings [77]. (The Fire Ecology section of this report compares of pre- and postsettlement structure of this stand.) Laven and others [198] found presettlement stands on the central Colorado Front Range tended to have widely spaced, uneven-aged trees. Grouping patterns were not evident.

Age: Interior ponderosa pine is long lived. Larger trees may attain ages of 700 or more years [192,262], although ponderosa pine between 350 and 425 years of age has a high mortality rate [221]. In a reconstruction of presettlement stand age classes near Flagstaff, Mast and others [213] found that prior to 1876, trees greater than 100 years old made up 68% of the overstory.  The oldest interior ponderosa pine on record was a 1,047-year-old tree from Colorado [192]. In the Southwest, Swetnam and Brown [285] recorded a 742-year-old tree from northwestern Arizona. 

Physiology: The tough, compact foliage of the northern race of interior ponderosa pine probably confers cold tolerance [73]. A deep, extensive root system, high sapwood:heartwood ratio, and sunken stomata make interior ponderosa pine highly drought tolerant throughout its range [36,73]. In the northern and central portions of interior ponderosa pine's distribution, only limber pine and Rocky Mountain bristlecone pine (Pinus aristida) better withstand extended drought [36]. In the Southwest, Barton and Teeri [35] found that interior ponderosa pine seedlings from the Chiricahua Mountains of southeastern Arizona were more drought resistant than southwestern white pine but less so than Mexican pinyon, Chihuahua, and Apache pines.

RAUNKIAER [239] LIFE FORM:
Phanerophyte

REGENERATION PROCESSES:
Pollination and seeds: Mating system is outcrossing of monoecious trees by wind pollination. Interior ponderosa pine 1st produces cones at 10 to 20 years of age [188,193]. Seed production is cyclic: trees in the Black Hills produce good seed crops every 2 to 5 years [52]. The cycle varies with climate and is not reliably periodic [182]. High temperatures during strobili formation have been correlated with good cone crops [86,210]. Jones [172] reported that in the Southwest, cone crops tend to be lighter in mixed-conifer compared to lower-elevation interior ponderosa pine forest. Seed- and cone-feeding insects may greatly reduce the seed crop. On the Kaibab and Coconino national forests of northern Arizona, degree of cone and seed infestation differed significantly (p<0.05) by site. The study did not address how the sites differed [46]. Shrews, rodents, and birds may also greatly reduce the cone crop [182,182,196]. During a 10-year study near Flagstaff, predation by Abert's squirrels varied from 0.3 to 75% of the total cone crop. The number of cones clipped was not related to the size of the total crop [196].

Seeds are mostly wind dispersed and do not usually carry more than 120 feet (37 m) from the parent tree [227]. Seeds cached by rodents may result in some seedling establishment, but rodents are not important dispersal agents of interior ponderosa pine seed. Clark's nutcrackers play a minor but important role in seed dispersal because seeds they cache are more likely to establish than rodent-cached seeds [194]. Seeds require mineral soil and do not germinate until the soil is continuously warm and moist [195]. There is some evidence that unlike Pacific ponderosa pine, interior ponderosa pine does not require cold/damp stratification prior to germination (M. G. Harrington 2003, pers. comm.). Schmid and Mitchell [259] found the number of sound seeds collected from trees on the Coconino and Kaibab national forests did not vary significantly (p=0.05) between sites, but did vary significantly among trees. Mean number of sound seeds/cone/tree ranged from 35 to 58. In field trials, germination rate of seed collected and outplanted on the Fort Valley Experimental Forest showed 61 to 90% germination, with germination time varying from 12 to 37 days. First-year survivorship was greatest (13-19%) in seedlings that germinated in early to mid-July, and least (2-7%) in seedlings that germinated in August. Predation of germinants by dark-eyed juncos was responsible for over half of germinant mortality [195]. Seed germination is usually lowest, and seed predation highest, in years when seed crops are low [182]. Ponderosa pine is not a seed banking species, although a minor amount of seed may germinate in its 2nd spring or summer [172]. 

Seedling establishment: Seedlings are relatively shade intolerant and require canopy-opening disturbances such as fire, logging, or tree death to establish [34,192,214,271]. Establishment pulses may occur on open sites when wet years follow a fire year, but wet postfire years are not always required for good establishment in the central Rocky Mountains and north. Two major pulses of seedling establishment in Rocky Mountain National Park, Colorado (1870-1890 and 1970-1980), occurred when fire was followed by normal precipitation [215]. Seedlings quickly develop a long taproot that enables them to reach moisture even on hot, burned-over soils [73,192]. Nevertheless, fall drought is a major factor in seedling mortality [171,197] and drought limits seedling establishment at interior ponderosa pine's lower elevational limit [34]. Since interior ponderosa pine in the Southwest has only a 2-month growing season before the onset of fall drought, winter dormancy, and spring drought, seedling mortality is usually highest in regions with bimodal precipitation [195]. Regeneration is further hindered in southwestern ponderosa pine on volcanic soils, which produce smaller seedlings (1 to 2 inches (3-5 cm)) that are more susceptible to frost-heaving compared to seedlings on other soils [155]. Rodent and ungulate browsing may result in considerable seedling loss. Artificial interior ponderosa pine regeneration on the Apache National Forest of east-central Arizona showed 95% mortality, and surviving seedlings showed little height gain, with mule deer and elk browsing. Seedlings in exclosures showed 2 to 3 times the height growth of unprotected seedlings [171]. However, certain gazing regimes favor seedling establishment. Pearson [232] found that in the Southwest, heavy cattle grazing that reduced grass interference with interior ponderosa pine growth, followed by light cattle grazing that allowed tree seedlings to survive, favored ponderosa pine over herbaceous and shrub species.  

On open sites with favorable moisture conditions, interior ponderosa pine seedlings often establish in large numbers. Dense seedlings often develop into "dog-hair" sapling thickets if stands are not thinned by fire or other means [77]. Boldt and Van Deusen [52] report that high tree densities (> 1 tree/ft2) are common in naturally regenerated interior ponderosa pine stands in the Black Hills. An even-aged, 63-year-old stand that had never experienced fire contained 6,600 trees per acre (16 500/ha), with average d.b.h. of 2.4 inches (6.1 cm). A number of researchers [61,257,284,285] have noted that ponderosa pine establishment in the Southwest tends to occur in pulses, during periods of relatively cooler, wetter climate conditions, but the current rate of establishment appears unprecedented. Mast and others [213] found that rate of interior ponderosa pine seedling establishment varied 8-fold over the 3 centuries prior to fire exclusion; however, presettlement variability in seedling establishment was dwarfed by a 2-orders-of-magnitude increase in seedling establishment in the early 20th century compared to presettlement rates of interior ponderosa pine establishment. Brown and others [61] suggest that these pulses of dense tree establishment and rapid growth result in ladder fuels that increase the severity of the next fire. 

Growth and mortality: Biomass allocation shifts as trees mature. As seedlings, ponderosa pine allocates relatively more biomass to roots compared to stems and leaves. Saplings tend to allocate relatively more of their biomass to foliage, and pole-sized trees allocate more biomass to woody tissue [132]. Open-grown interior ponderosa pine generally gain height rapidly in the sapling and pole stages [172], and mortality rates tend to level off at maturity. Knowles and Grant [184] found that interior ponderosa pine on the Colorado Front Range showed a sharp drop in mortality at age 115. The oldest trees sampled were 300+ years old. Minor [220] developed site index curves for interior ponderosa pine in the Southwest.

Dendrochronological and basal area analyses of trees in central Colorado showed that interior ponderosa pine growth was positively correlated with presence of Gambel oak and negatively correlated with presence of other interior ponderosa pine (r > 0.93). The authors [42] attributed increased growth of interior ponderosa pine associated with Gambel oak to better soil nutrient status conferred by oak litter and/or increased density of interior ponderosa pine facilitated by Gambel oak nurse trees. Klemmedson [183] also noted the benefit of Gambel oak litter to soil nutrient status and interior ponderosa pine growth. Gambel oak also grows in shrub form. Data on relative growth rate of young interior ponderosa pine in Gambel oak shrub stands are not available; however, Gambel oak shrubs may hinder growth of interior ponderosa pine rather than increase it (M. G. Harrington 2003, pers. comm).

SITE CHARACTERISTICS:
Interior ponderosa pine occupies relatively dry, nutrient-poor sites compared to other montane conifers but shows wide ecological amplitude throughout its distribution. Interior ponderosa pine in the Sangre de Cristo Mountains of Colorado is associated with dry sites. Detrended correspondence analysis determined that of 5 co-occurring tree species, only limber pine occurred on sites that were subject to more solar radiation, and had higher soil pH, than interior ponderosa pine [11]. In contrast, interior ponderosa pine in southeastern Arizona occurs on relatively moist sites that are higher in elevation than the habitats of most southwestern pine species [35].

Climate: Temperature regimens do not vary greatly across ponderosa pine's range: Pearson [233] found only a 2 oFahrenheit difference in mean annual temperature among ponderosa pine sites in Arizona, the Black Hills, Washington, and California. Length of frost-free period in interior ponderosa pine sites depends upon latitude and elevation. In much of its northern range, the frost-free period averages about 120 days. In the extreme Southwest, the frost-free period may lengthen to 240 days [69].

A common climatic factor occurs across interior ponderosa pine's geographical range: Interior ponderosa pine grows on sites too dry to support other timber species. Although the drought season occurs at different times in different regions, periodic prolonged drought is characteristic across interior ponderosa pine's range and limits its expansion into drier types [33,69]. Overall, annual precipitation ranges from 15 to 20 inches (380-510 mm) per year. In low-elevation interior ponderosa pine at desert edges, mean precipitation is as low as 9 inches (230 mm) per year. In mixed-conifer forests at moderate elevations, mean annual precipitation may exceed 25 inches (640 mm) [33]. Climate grades from dry continental interior in the north, to subhumid in south-central Wyoming, to semi-arid in the Southwest and the plains grassland-interior ponderosa pine woodland interface [33,84]. Climate is most favorable for regeneration in the Black Hills, where the wettest months are May and June and summer rains are frequent [33,68]. The Medicine Bow Range of Wyoming, which averages 25 inches (640 mm) annual precipitation, is the wettest portion of interior ponderosa pine's range. Precipitation on the east slope of the Medicine Bow ranges between 12 and 20 inches (300-510 mm), with 2/3rds received from April through September [84]. Transplant studies and abundant natural regeneration under fire exclusion suggest that climate is not limiting for interior ponderosa pine establishment on many plains grassland sites [134]. Climate in the Southwest has a distinct bimodal pattern of winter and summer precipitation. May and June, the wettest months in the northern portion of interior ponderosa pine's range, are the driest months in Arizona and New Mexico [68,84].  Mean annual precipitation in interior ponderosa pine communities in western Texas is 16 to 20 inches (400-510 mm) [267].

Soils: Interior ponderosa pine occurs widely on igneous and sedimentary parent materials including basalt, volcanic cinder, limestone, and sandstone soils. Textures include clayey and silty loams, sandy loams, gravels, and cobbles [28,68,138,267]. Steep slopes with restricted soil profile development may support at least some interior ponderosa pine [68]. Degree of slope varies and can be steep: sites in the Santa Catalina Mountains of southeastern Arizona ranged from 10 to 36o [97].

Elevation: Interior ponderosa pine occupies a narrower, often higher elevational range than Pacific ponderosa pine. It is most common from 6,000 to 8,500 feet (1800-2600 m) [33,84]. Elevation by state is:

Arizona 7,000-10, 000 feet 2300-3050 m [29,35,77,84,97]
Colorado 6,360-9,500 feet   2120-2900 m [11,245]
Nebraska around 4,000 feet  1200 m [307]
New Mexico 7,600-8,900 feet   2300-2700 m [135]
Nevada 5,600-8,500 feet   1700-2600 m [45,192]
Texas  6,000-7,000 feet   1800-2000 m [90,124]
Utah 700-8,900 feet   200-2700 m [216]

SUCCESSIONAL STATUS:
Interior ponderosa pine is mostly a seral species. It is highly light and drought tolerant, and is usually the 1st conifer to establish on disturbed sites [172]. Lundquist [204] identified fire as the primary force setting back succession in interior ponderosa pine/grassland communities in the Black Hills of South Dakota. Shallow soils that promote tree fall, tree harvest, Armillaria infection, cattle grazing, and storm damage were other major factors setting back succession  [215,245]. Catastrophic winds and mountain pine beetle outbreaks have also been identified as stand-initiating events in interior ponderosa pine forests [205]. One or a combination of these events creates growing space for seedlings. Lundquist and Negron [205]  predict that interior ponderosa pine seedling establishment stops about 70 years after a stand-replacing event due to canopy closure. Even spacing of mature trees in this "stem exclusion stage" is maintained by frequent, low-severity surface fires. Fine-scale succession occurs in canopy gaps created by death of a group of mature interior ponderosa pine. On most sites, more shade-tolerant trees eventually establish beneath the interior ponderosa pine canopy without fire or other stand-initiating disturbance, although interior ponderosa pine is the climax forest on some harsh, dry sites [215,237,245]. Interior ponderosa pine may be completely replaced by later-successional conifers such as Rocky Mountain Douglas-fir, fir (Abies spp.), and spruce (Picea spp.) after several decades without disturbance. Multistoried stands of interior ponderosa pine have increased as a result of fire exclusion. They are typically structured with seedlings, saplings, and small poles of interior ponderosa pine and later-successional, shade-tolerant conifers, clustered around widely scattered mature interior ponderosa pine. In later stages of succession, the clumps merge to form continuous stands of irregular structure [7].

Interior ponderosa pine is moderately shade tolerant. It tolerates shade better than quaking aspen, whitebark pine, and bur oak but less than western white pine, white spruce, and Rocky Mountain juniper. Unlike the first 4 species, interior ponderosa pine can establish beneath its own canopy [7,31].

On some sites, interior ponderosa pine can be a secondary colonizer in the absence of disturbance. GIS modelling of historic photographs of interior ponderosa pine-plains grassland ecotones on the Colorado Front Range shows a pattern of tree invasion into historic grassland, particularly on north slopes. Interior ponderosa pine invasion began around 1930, when effective fire exclusion began. Two strong pulses of interior ponderosa pine establishment onto grassland occurred in the 1970s and 1980s during years of above-average precipitation. Decreased livestock grazing may have also aided tree invasion into plains grassland in the 1970s and 1980s [214].

Interior ponderosa pine is typically seral to white spruce in the Black Hills of Wyoming and South Dakota. It is climax on sites that are too dry to support white spruce. Quaking aspen may dominate a site following stand-replacing fire, but interior ponderosa pine typically establishes dominance after a few postfire decades. Bur oak may dominate some low-elevation stands in the northern Black Hills and Bearlodge Mountains of eastern Wyoming. Succession to interior ponderosa pine is "very slow" on sites where oak scrub stands develop after fire or other stand-replacing events [7].

East of the Continental Divide in Montana, interior ponderosa pine is a climax species. It forms heavily stocked, uneven-aged stands in the absence of fire [19,237].  In the Missouri Breaks region of Montana, bluebunch wheatgrass-western wheatgrass grasslands succeed to interior ponderosa pine/bluebunch wheatgrass habitats in the absence of fire. Shrubland areas of the region may succeed to interior ponderosa pine/Rocky Mountain juniper without fire [104].

Succession to conifers may be slow in southern New Mexico and Arizona. Aspect strongly influences rate of succession. In the White Mountains of south-central New Mexico, interior ponderosa pine establishment began 15 to 20 years after fire on northern and eastern exposures, while 75 or more years were sometimes required for forest development after fire on southern and western slopes. Once established, however, interior ponderosa pine cover was generally higher on drier slopes than on north-facing slopes [135]. Moir and Dietrich [221] present successional models for interior ponderosa pine in Arizona and New Mexico, both with fire and with fire exclusion.

SEASONAL DEVELOPMENT:
Timing of cone development, pollination, and seed dispersal events are similar across interior ponderosa pine's range. Germination and seedling establishment differ north to south, with establishment occurring earlier in the year in more northern latitudes. Strobili of interior ponderosa pine in the Great Plains elongate in May and June [274]; late May elongation of cones is reported from northern Arizona [261]. Female cones mature in their 2nd summer [274]. Mean dates of pollination of interior ponderosa pine on the Wyoming-Colorado border were May 24th and 25th [98], while pollination occurred in mid-June in northern Arizona [261]. Seeds are dispersed in fall. They germinate in spring in the Great Basin and the North [192] and after summer rains in July and  August  in the Southwest  [133,171,195].

Phenological development of interior ponderosa pine on the Fort Valley Experimental Forest of northern Arizona was as follows [261]:

Phenological event          Date                  
diameter growth begins 15-30 May
vegetative buds swell 1-15 May
vegetative bugs elongate 15-25 May
root growth starts 15-30 May
shoot elongation 10-30 June
needle elongation 15 June-30 July
shoot growth stops 1 July-30 Aug.
seed germinates 1 July-Aug. 30
staminate buds appear 20-31 May
pollination 10-20 June
diameter growth stops 1-20 Sept.
female cones developed 15-20 Sept.
seeds ripe 1-20 Oct.
needle shed 1-30 Oct.
root growth stops 15-30 Nov.

FIRE ECOLOGY

SPECIES: Pinus ponderosa var. scopulorum
FIRE ECOLOGY OR ADAPTATIONS:
Fire adaptations: Interior ponderosa pine is rated "very resistant" to fire. No other conifer within its range is better adapted to survive surface fires [55,115], which often char but usually do not kill mature trees [52,281,307]. Adaptations to survive surface fires include open crowns; self-pruning branches; thick, insulative, relatively unflammable bark; thick bud scales; tight needle bunches that enclose and protect meristems, then open into a loose arrangement that does not favor combustion or propagation of flames; high foliar moisture; and a deep rooting habit [55,180,323].  Trees in widely spaced stands are typically better equipped to survive surface fire than trees in denser stands because they develop thicker bark  [52,55]. Ponderosa pine cannot survive crown fire [6,7,89,148,316,323], but mature trees can survive a considerable amount of scorching [94,144].

Surface fire often kills interior ponderosa pine seedlings and saplings [50,57,87]; however, the effect is dependent upon fire severity and stand structure. Young trees in open canopies acquire fire-resistant traits rapidly, and 6-year-old saplings often survive low-severity surface fire. Fire is especially damaging in overcrowded young stands: the relatively denser foliage and thinner bark of trees in thick stands reduce resistance to surface fire. Such stands are also prone to crown fire [55].

Fire prepares a favorable seedbed for interior ponderosa pine regeneration. Periodic surface fire removes the heavy litter and duff that accumulate in ponderosa pine forests. Wind-borne seeds falling from the crowns of surviving or fire-killed trees land on a nutrient-enriched mineral seedbed under an open canopy that favors germination and seedling establishment [251,275,323]. Seedling-water relations may be enhanced when fire removes competing vegetation [133].

Fire regimes: Interior ponderosa pine evolved under a regime of frequent surface fires and infrequent mixed-severity and stand-replacement fires [52,281,307]. Presettlement fires in lower-elevation (<7920 feet (2400 m)) ponderosa pine communities were mostly low- to moderate-severity surface fires that  maintained open-grown, parklike stands [33,61,63,74,77,117,149,278,280,298,305,311]. Prior to the 1900s interior ponderosa pine was perpetuated by surface fires that recurred every 5 to 30 years. Fire return intervals tended to be shorter in the warm, dry forests of the Southwest than in the cool, dry forests of the central Rocky Mountains or the cool, relatively moist forests east of the northern Rocky Mountains [61,77,95,141,198]. For example, Dieterich and Swetnam [96] report a 2-year mean fire return interval for presettlement interior ponderosa pine on the Fort Valley Experimental Forest near Flagstaff; Laven and others [198] report a 45.8-year mean fire return interval (range=20.9-66.0 years) for the Front Range of Colorado; and Brown and Sieg [63] report an average fire return interval of 22 years for presettlement interior ponderosa pine forests of South Dakota. Gruell [131] provides an annotated record of wildfires that occurred throughout interior ponderosa pine's range during the settlement period (1776-1900). Historic fire regimes are summarized by state and region in the ending paragraphs of this section.

Fire history studies show mixed-severity fire regimes for some interior ponderosa pine forests. Many forests experienced infrequent, large stand-replacement fires prior to the European-American settlement period [61]. For example, Laven and others [198] reported a range of 3 to 161 years (mean (m)=45.8) for the central Colorado Front Range. Small fires occurred on average every 20.9 years; large fire occurrence averaged 41.7 years. Higher-elevation  (>7920 feet (2400 m)), relatively mesic mixed-conifer forests with interior ponderosa pine, Rocky Mountain Douglas-fir, and Rocky Mountain lodgepole pine tend to have more mixed-severity fires than lower-elevation interior ponderosa pine forests  [280,315]. This is probably because herbaceous species recover from fire more quickly, and dry out earlier in the season, at low elevations. In the northern Colorado Front Range, mean fire interval for widespread (> 10 trees scarred), mixed-severity fire in higher-elevation forests during 1650 to 1920 ranged from 34 to 43 years; mean fire interval for widespread, mixed-severity fire in lower-elevation interior ponderosa pine forest was 14 to 24 years. During the same period, mean fire interval for localized (2-9 trees scarred) fires in higher- and lower-elevation ponderosa pine forest was 17 to 22 years and 8 to 18 years, respectively [298]. Interior ponderosa pine at 5,633 to 5,919 feet (1707-1804 m) in the Chiricahua Mountains experienced an historical fire return interval ranging from 1 to 15 years (m=6.17 years) compared to a range of 1 to 31 years (m=7.96 years) in higher-elevation (6,801-7,002 feet (2073-2134 m)) mixed-conifer forest in the Chiricahua Mountains. Some higher-elevation mixed-conifer forests show an historical fire regime similar to lower-elevation ponderosa pine, however. Swetnam and Basian [280] suggest that mixed-conifer forests on dry, steep slopes, where fire can easily ignite and spread upslope from many directions, are most likely to experience frequent surface fire.

Native American burning influenced fire regimes in ponderosa pine ecosystems prior to and during European-American settlement, mostly by increasing fire frequency. For example, in studying fire history in the Chiricahua Mountains of southeastern Arizona, Seklecki and others [264] found that southwestern ponderosa pine showed a shorter fire return interval (mean=3.0 years, range=1.0-16.0 years) between 1700 and 1900, when Chiricahua Apaches inhabited the area, than in earlier periods when Apaches did not reside there. Dormant-season (spring) fires were also more frequent during that time period in southwestern ponderosa pine sites of the Chiricahua Mountains compared to southwestern ponderosa pine sites near the Mexican border, which were used as travel corridors but mostly unoccupied at that time [264]. Near the border, most fires occurred in the late May to early June growing season [280]. Fire frequency increased to 1 per year (an uncommonly low number for southwestern ponderosa pine) during the Spanish-Apache wars of 1760 to 1780, when Apaches probably used fire as a method of warfare. Fire occurrence terminated abruptly in the late 1880s, coincident with Apache resettlement to reservations (Geronimo surrendered in 1886) and increased livestock grazing by European-Americans [264,280].

Fire regimes in interior ponderosa pine also affect regimes of adjacent communities. In many cases, fire frequency has been reduced in adjacent communities because ignitions in interior ponderosa pine are suppressed and fire does not spread into adjacent communities. For example, stand structure of a shrub live oak-hairy mountain-mahogany (Quercus turbinella-Cercocarpus montanus var. paucidentatus) community on the Prescott National Forest of Arizona was historically a fire-maintained mosaic of different-aged chaparral. Mean fire frequency in the adjacent interior ponderosa pine/Arizona white oak (Q. arizonica) stand was 2 years. After over 100 years of fire exclusion, the chaparral stand is even-aged and senescent, with heavy accumulations of dead material. Interior ponderosa pine is encroaching into the chaparral [93].

Climate and fire frequency: Long-term fire history studies on the northern Colorado Front Range show that interannual variability in soil moisture is more conducive to widespread fire than drought alone. Fire occurrence, especially widespread fire, tends to increase 1 to 4 years after above-average moisture availability in spring-summer [298]. Similarly, fire occurrence tends to increase 2 to 3 years after above-average precipitation in winter-spring  [30,281]. Climatic variation that produces widespread, stand-replacing fire has been associated with southern oscillation events. El Nino is associated with greater soil moisture and herbaceous fuel production in spring, with fire occurrence peaking several years after El Nino events. La Nina events are associated with dry springs, with fire occurrence peaking in the same year [298]. A decline in fire frequency in interior ponderosa pine forests of the Southwest coincided with reduced El Nino-La Nina events between 1780 and 1830 [284,298]. Alternating wet and dry years resulting from El Nino-La Nina events in the mid- to late 1800s increased fire frequency [298].

Fire exclusion: The ecological changes that have occurred in ponderosa pine forests over the last century have been well documented by a number of researchers [23,74,95,117,224,306,311,312]. Frequent, mostly light-severity surface fires thinned small trees, especially the less fire-resistant Rocky Mountain Douglas-fir and firs. The combined effects of 60 to 80 years of fire exclusion, logging that removed many overstory pines, heavy livestock grazing, and climate change have created closed-canopy stands with dense understories and ladder fuels [57,62,63,76,77,117,256,281,292].  These changes have been documented throughout interior ponderosa pine's range [76,209,271], and have also occurred in interior ponderosa pine/Douglas-fir and mixed-conifer types [178,298]. A fire history study of interior ponderosa pine stands near Flagstaff, Arizona, documented changes in stand structure over a 116-year period [77].

Stand structure in 1876 (reconstructed) and in 1992:
  Trees/acre               
Dbh class (inches) 1876 1992
0-3.9  0.3 945
4-7.9 0.7 243
8-11.9 1.0 46
12-15.9 1.4 6.7
16-19.9 1.7 1.6
20-23.9 2.1 2.5
24-27.9 2.4 2.4
28-31.9 2.8 4.1
32-35.9 3.1 1.7
36-39.9 3.5 0.3
40-43.9 3.8   0.2       
Total 22.8 1,253.5

When wildfire burns these dense interior ponderosa pine stands under dry conditions, the abundant fuel quickly allows it to develop a high intensity and to spread into tree crowns. Severe, stand-replacing fires were infrequent in interior ponderosa pine forests in the past; now they are common [22,55,62]. Abundant litter and living and dead woody fuels feed explosive wildfires of intensities and sizes that have not occurred for many centuries, if ever [280]. The increasingly frequent occurrence of large, crowning wildfires in interior ponderosa pine may indicate a shift to a fire regime characterized by very large (> 100 000-acre (4000 ha)) crown fires [76,255]. Data in Sackett and others [255] show a great increase in the number of acres burned by wildfire in Arizona and New Mexico since 1970. Over 100,000 acres (40 000 ha) burned from 1915 to 1990, with 70% of the fires occurring after 1970. Before 1970, total acreage burned per year never exceeded 130,000 acres (52 000 ha). After 1970 there were 8 years in which total acreage burned exceeded 119,000 acres (47 600 ha), with nearly 500,000 acres (200 000 ha) burning in 1989. On the Mexican side of the international border, where there is a lack of effective fire suppression, frequent, widespread surface fires have persisted in southwestern ponderosa pine and mixed-conifer forests [280].

Besides unprecedented, large-acreage severe fires, other ecological consequences of fire suppression in interior ponderosa pine ecosystems include [77,79,312]:

Organisms within interior ponderosa pine ecosystems have evolved with fire, and frequent fires are probably required to maintain ecosystem health [54,76,121,221,222]. Some researchers have questioned whether ponderosa pine ecosystems are sustainable under current conditions [23,77,265,280].

Fuels: Even within the same provenance, fuel loadings in ponderosa pine stands may vary greatly depending upon age class, stand structure, and understory composition [252]. Prediction equations for fuel loads in ponderosa pine are available  [3,110,111,112,143,248]. Mean fuel loadings (tons/acre, 0-1 inch and > 1-inch fuels) have been calculated for ponderosa pine stands on 3 Reservations, 2 National Parks, and 8 National Forests of Arizona and New Mexico. The study involved 62 sites: mean forest floor loading for the entire 62 stands was 12.5 tons/acre (4.1 t/ha) [249].

Absence of fire in interior ponderosa pine and mixed-conifer forests has led to uncharacteristically large accumulations of surface and ground fuels [29,175,252]. Structurally, fire exclusion has led to vertical continuity, with Douglas-fir, firs, and other shade-tolerant, less fire-resistant species in the understory. These late-successional species become ladder fuels that encourage crown fires in interior ponderosa pine and mixed-conifer forests [29,55]. In the dry southwestern climate, the natural accumulation of pine needles and woody fuels is exacerbated by slow decomposition [149].

State and regional fire regime studies
Black Hills and interior Northwest: Fire season on the interior ponderosa pine-grassland savannas of eastern Montana and the Dakotas peaks in July and August, when the majority (73%) of lightning-strike ignitions occur. Wildfire season generally extends from April to September [161]. Frequent surface fires historically burned litter and killed young interior ponderosa pine and other non-sprouting woody species encroaching into grasslands [50,53]. Interior ponderosa pine in the Black Hills was historically characterized by 2 communities: interior forest and savanna. These treed landscapes were described as "islands" surrounded by plains grasslands. Brown and Sieg [63] found that in interior forest sites at Jewel Cave National Monument, South Dakota, fire return intervals from the 1500s to the late 1800s averaged 20 to 24 years, with a range of 1 to 93 years. Fire return intervals at savanna sites in Wind Cave National Park, South Dakota, averaged 10 to 12 years, with a range of 2 to 23 years. The fire return interval of interior ponderosa pine savannas is the shortest documented for northern interior ponderosa pine ecosystems, and is similar to the short-return intervals of interior ponderosa pine forests in the Southwest.

Invasion of interior ponderosa pine onto grasslands, and increased tree density in formerly open savanna, is thought to be largely attributable to reduced fire frequency, although grazing has probably contributed to increased interior ponderosa pine density on forest and grassland margins [50,53]. A little less than one-half of interior ponderosa pine in the Black Hills are single-storied, even-aged stands that developed after crown fires or mountain pine beetle epidemics [7].  "Dog-hair" interior ponderosa pine thickets are common on many sites [50].

Brown and others [62] report the following measures of fire frequency on interior ponderosa pine sites less than 50 acres (20 ha) in size:

Site Period of Analysis No. of Intervals Median Fire Interval (years) Range of Intervals (years) Years Since Last Fire
Black Hills, SD 1580-1887 9 23 11-74 110 (1887-1997)
Black Hills, SD 1668-1890 7 22 13-72 107 (1890-1997)
Medicine Bow NF, WY 1436-1911 15 26 8-74 86 (1911-1997)
Medicine Bow NF, WY 1460-1909 12 33.5 8-82 88 (1909-1997)
Arapahoe-Roosevelt NF, WY 1568-1861 4 80.5 10-122 136 (1861-1997)
Arapahoe-Roosevelt NF, WY 1568-1887 3 117 80-122 110 (1887-1997)
Rio Grande NF, CO 1528-1896 26 9.5 2-41 101 (1896-1997)

Colorado: A fire history study of a 10,000-acre (4000 ha) interior ponderosa pine-Rocky Mountain Douglas-fir site in central Colorado showed a pattern of frequent surface fires from 1197 to 1851. Large stand-replacement fires were rare, but several landscape-level fires are documented. Intervals between fire years ranged from 1 to 128 years at  the landscape scale and from 1 to 58 years for individual stands. Fires occurred throughout the growing season. Fire size varied across time; for example, numerous small fires occurred in the 1500s, while landscape-level fires occurred in 1631, 1696, and 1723. After 1723, there were few fires until 1851; that fire was a stand-replacement, mixed crown and severe surface fire that covered most of the landscape. There have been no extensive fires in the study area since 1851, and most stands have not experienced fire for nearly 100 years [61].

Utah: Fire history studies show a range of 3 to 47 years for interior ponderosa pine in southern Utah [64,209,272]. Studies in Zion National Park show a presettlement fire frequency range of 3 to 12 years for interior ponderosa pine and mixed-conifer forests. All study sites had experienced fire at least once a decade [64,209]. Large fires that burned more than 1,000 acres (400 ha) occurred nearly every 3 years on the Horse Pasture Plateau. Fire return intervals declined greatly in the Park beginning in the  late 1800s, when the study sites were subject to intense livestock grazing and grassy fuels were sparse. Although grazing has not been practiced in the Park for over half a century, fire events remained infrequent under fire exclusion [209].

Nebraska: Bragg [56] evaluated fire frequency for upland interior ponderosa pine-bur oak forest adjacent to sandhills prairie. He found a mean fire return interval of 3.5 years from 1851 to 1900, and a mean of 8.5 years for 1901 to 1950. Without the frequent fires that killed the seedlings, upland interior ponderosa pine has invaded sandhills prairie communities [176].

Texas: A fire history study in mixed interior ponderosa pine-Rocky Mountain Douglas-fir-southwestern white pine in Guadalupe Mountain National Park showed a mean return interval between major fires of 17 years prior to the 1920s; mean return interval for all fires detected between 1696 and 1922 was 4.7 years. Evidence of fire scarring was not found after 1922. Portions of the montane forest have a 2-tiered structure of overstory interior ponderosa pine and Rocky Mountain Douglas-fir, and dog-hair thickets of ponderosa pine and Douglas-fir saplings in the understory [1,2].

New Mexico: Swetnam and Dieterich [286] found that extensive surface fires were common in the Gila Wilderness of southwestern New Mexico before 1900. Mean fire return interval from 1633 to 1900 was 4 to 8 years, with a range of 1 to 26 years. Most fires burned throughout the 40.5-hectare study sites, although a few fires appeared to be smaller. Fire history was similar in northern New Mexico, with extensive high-frequency, low-severity surface fires in low-elevation (< 5,545 feet (1690 m)) interior ponderosa pine forests and less frequent surface fires, along with some patchy crown fires, in higher-elevation mixed-conifer Rocky Mountain Douglas-fir-interior ponderosa pine-white fir forest. Across sites, the minimum fire interval for major fires (> 10% of trees scarred) in interior ponderosa pine before 1900 ranged from 1 to 9 years; maximum fire interval for major fires ranged from 16 to 51 years (m=9.4 years). In mixed-conifer forests where interior ponderosa pine was codominant, minimum and maximum fire interval ranges were 1 to 4 and 21 to 46 years, respectively (m=10.4 years). Fires were rare in interior ponderosa and mixed-conifer forest after the 1850s. The authors attribute the initial drop in fire frequency to domestic sheep grazing by the Navajos, and by the 1900s, to fire exclusion by European-Americans [292].

Arizona: The sky islands of southeastern Arizona have among the highest incidences of lightning-caused fires in the United States [185,260]. The lightning fire season begins in late April, peaks in June, and runs into October. Maximum lightning fire incidence is above 6,000 feet (1800 m): directly within the interior ponderosa pine zone [29]. Prior to the 1880s, surface fires burned through ponderosa pine sky islands once or twice a decade, but occurred at highly variable intervals. Fires were somewhat less frequent in higher-elevation, mixed-conifer forests [281]. Swetnam and others [282] reported a 9- to 22-year range (m=14.6) for presettlement fire return intervals in interior ponderosa pine-Douglas fir in Chiricahua National Monument. Fire frequency in interior ponderosa pine-Arizona pine forests in southeastern Arizona was greatly reduced after Euro-American settlement in the 1870s. Intense livestock overgrazing coupled with fire exclusion caused both the number of fires and the area burned to decline dramatically [238].

Historical fire frequency in the interior ponderosa pine-encinal oak woodland interface has been documented at least 1 fire per decade [85,174]. In interior ponderosa pine-oak woodlands of the Rincon Mountain Wilderness, fire regime was dominated by large-scale (> 500 acre (200 ha)), early-season (May-July) surface fires. Mean fire return interval from 1657 to 1893 was 6.1 years, with a range of 1 to 13 years. From 1748 to 1996, mean fire return interval in the high-elevation mixed-conifer type was 9.9 years, with a range of 3 to 19 years [30].

There are many western plant communities and ecosystems in which interior ponderosa pine is either dominant, an important component of the vegetation, or an invader. Historic fire return intervals for these communities and ecosystems are summarized below. Please refer to the Fire Effects Information System report on the dominant species listed here for further information on fire regimes in these communities and ecosystems.

Community or Ecosystem Dominant Species Fire Return Interval Range (years)
Nebraska sandhills prairie Andropogon gerardii var. paucipilus-Schizachyrium scoparium < 10 
sagebrush steppe Artemisia tridentata/Pseudoroegneria spicata 20-70 [231]
mountain big sagebrush A. t. var. vaseyana 20-60 [21,65]
Wyoming big sagebrush A. t. var. wyomingensis 10-70 (40**) [299,321]
plains grasslands Bouteloua spp. < 35
blue grama-needle-and-thread grass-western wheatgrass B. gracilis-Hesperostipa comata-Pascopyrum smithii < 35 
blue grama-buffalo grass B. g.-Buchloe dactyloides < 35
grama-galleta steppe B. g.-Pleuraphis jamesii < 35 to < 100 [231]
curlleaf mountain-mahogany* Cercocarpus ledifolius 13-1000 [24,263]
mountain-mahogany-Gambel oak scrub C. l.-Quercus gambelii < 35 to < 100 
Rocky Mountain juniper Juniperus scopulorum < 35
wheatgrass plains grasslands Pascopyrum smithii < 35 [231]
Engelmann spruce-subalpine fir Picea engelmannii-Abies lasiocarpa 35 to > 200 [20]
pinyon-juniper Pinus-Juniperus spp. < 35 [231]
Rocky Mountain lodgepole pine* Pinus contorta var. latifolia 25-300+ [17,20,244]
Colorado pinyon P. edulis 10-49 [231]
interior ponderosa pine* P. ponderosa var. scopulorum 2-46  [20,30,198]
Arizona pine* P. p. var. arizonica 2-10 [20]
quaking aspen (west of the Great Plains)* Populus tremuloides 7-120 [20,130,218]
mountain grasslands Pseudoroegneria spicata 3-40 (10**) [17,20]
Rocky Mountain Douglas-fir* Pseudotsuga menziesii var. glauca 25-100 [20]
bur oak Quercus macrocarpa < 10 [300]
oak savanna Q. m./Andropogon gerardii-Schizachyrium scoparium 2-14 [231,300]
*fire return interval varies widely; trends in variation are noted in the species report
**mean

POSTFIRE REGENERATION STRATEGY [275]:
Tree without adventitious bud/root crown
Crown residual colonizer (on-site, initial community)
Initial off-site colonizer (off-site, initial community)
Secondary colonizer - off-site seed

FIRE EFFECTS

SPECIES: Pinus ponderosa var. scopulorum
IMMEDIATE FIRE EFFECT ON PLANT:
The effect of fire is related to tree size, fire severity, and stand density [7]. Low-severity surface fires usually kill trees less than 3 to 5 years of age or less than 6 inches (15 cm) dbh, and mortality in the 6- to 30-inch (15-76 cm) dbh class is not unusual [6,7,89,148,316,323]. Researchers in Zion National Park, Utah, found surviving 10-year-old interior ponderosa pine seedlings that were scarred from low-severity surface fire [64]. Trees in dense stands and trees infected with southwestern dwarf-mistletoe are most susceptible to mortality, particularly in the smaller size classes. Pole-sized and larger trees are resistant to low-severity surface fires. Thick bark affords protection against cambial damage, and foliage and buds are usually elevated away from the flame zone.  However, intense radiant heat produced by moderate-severity fire, or flames that reach buds, can damage or kill mature trees. Severe surface or crown fires generally kill interior ponderosa pine of all size classes [6,7,89,148,316,323], although some sawtimber-sized trees may survive severe surface fire [66].  Heavy accumulations of litter at the base of trees increase the duration and intensity of fire, making trees more susceptible to scarring. Resin deposits around an old "cat-face" may increase bark flammability and promote further injury [55].

On the Coconino National Forest, a mixed-severity wildfire on 7-9 May 1972 killed about 1/4th of the standing interior ponderosa pine where surface fire severity was moderate. Mortality was greatest in the smaller diameter classes: 90% of  "pulpwood" and 7% of  "sawtimber" were killed by moderate-severity fire. Approximately 2/3rds of the trees were killed where severe surface or crown fire occurred. Ninety percent of pulpwood and 50% of sawtimber were killed where fire was severe. The summer after the fire, basal area was 211 ft2/acre (47.5 m2/ha) on an adjacent unburned area; 103 ft2/acre (23 m2/ha) on the moderate burn; and 40 ft2/acre (9 m2/ha) on the severe burn [66].

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
With its high foliar moisture content, interior ponderosa pine can withstand extensive scorching as long as bud and twigs, which tolerate higher temperatures than needles, are not badly scorched [94,258,323]. Ponderosa pine may recover from as much as 90% scorching as long as 50% of buds and twigs survive to maintain shoot growth on defoliated branches [323]. Extensive scorching of ponderosa pine crowns may cause mortality within 3 postfire years [142,157,207]. Generally, ponderosa pine recovery is best after dormant-season scorching; trees scorched in the growing season show poorer survivorship [142,144]. After a growing-season (July) wildfire in northern Arizona, Herman [158] noted at least 65% survival for trees greater than 8 inches (20 cm) dbh that had less than 60% crown scorch, while Dieterich [94] observed 89% recovery of 6 to 14-inch (15-36 cm) dbh trees that had been up to 90% scorched by dormant-season (November) wildfire on the Coconino National Forest. Studies of postfire survivorship after scorching show mixed results, however. Davis and others [89] reported that more than 75% of ponderosa pines (5- to 11-inch (13-28 cm) dbh class) scorched more than 67% died within 2 years following a dormant-season (October) prescribed fire on the Coconino National Forest.

Dormant-season studies indicate that bud kill, which is related to fire season, is more important than foliage kill in determining chances of ponderosa pine survival after burning [142,301,302]. Wagener [302] and Harrington [144] found the minimum requirement for ponderosa pine survival was 90% or less scorch with 50% or more of bud and twigs remaining. Five years after prescribed burning on the San Juan National Forest of Colorado, Harrington [144] found significant (p=0.05) differences in mortality of scorched interior ponderosa pine, depending upon season of burning. Mortality was lowest for fall-scorched trees (5%), and spring-scorched trees showed less mortality than summer-scorched trees (17 vs. 21%, respectively). Ninety percent of fire-damaged interior ponderosa pine that died had done so by postfire year 4. Most trees greater than 7.2 inches (18 cm) diameter survived fall burning even with 90% scorching. With spring and summer burning, trees less than 4 inches (10 cm) diameter died with greater than 50% scorching, while at least 90% scorching was required before trees larger than 4 inches (10 cm) in diameter were killed by spring or summer fire [142].

Mortality models: McHugh and Kolb [217] found a model using total crown damage by fire (scorch + consumption) and bole char severity as independent variables gave the best 2-way variable model for predicting individual tree mortality for prescribed and wildfires in northern Arizona. A study on the Colorado Front Range found that crown scorch, expressed as a percentage of the prefire live crown length, was the best determinant of postfire mortality of interior ponderosa pine [318]. Wyant and Zimmerman [319] found that degree of crown scorch and tree size was the most effective predictors of postfire survival potential after September prescribed burning in Colorado. They provide a model for estimating mortality based upon percent crown scorch and dbh.

Mortality studies: Harrington and Hawksworth [148] conducted prescribed burning treatments to interior ponderosa pine on the South Rim of Grand Canyon National Park, Arizona. They found that mortality generally increased with decreasing tree size and increasing crown scorch, bole char, and southwestern dwarf-mistletoe infection. Trees with greater than 87% crown scorch experienced 100% mortality, even in the 30- to 36-inch (76-92 cm) size class. Severe bole char resulted in 67% mortality. Mortality by size class is indicated in the leftmost section of the graph below.

 
   Char rating: 0=unburned around the tree, 1=duff burned but no char, 2=light bole char, 3=moderate bold char,  4=severe bole char with much bark consumed
DMR (dwarf-mistletoe rating): 0,=no mistletoe, 1=trace, 2=light, 3=moderate but not throughout tree, 4=moderate throughout tree, 5=most of tree heavily infected, 6=entire tree heavily infected

On the Coconino National Forest, the general effect of a 3 October prescribed fire was a thinning from below and a 70% reduction in duff. No interior ponderosa pine less than 4.5 inches (10 cm) tall survived the fall fire. Mortality was less in pole-sized and larger trees. Severely damaged trees died within 2 years. Percent damage and postfire mortality of interior ponderosa pine by size class* and fire severity are given below [89].

  Saplings Poles Sawtimber All trees
 
Site A Site B
Site A Site B
Site A Site B
Site A Site B
Crown damage:        
   none
0 0
8 10
8 8
7 12
   light
20 16
20 37
22 72
22 50
   moderate
10 16
20 16
35 11
18 18
   severe
70 68
52 37
35 9
53 30
Dead after 2 years:        
   none
0 0
0 0
0 0
0 0
   light
0 0
0 0
0 0
0 0
   moderate
0 5
0 6
0 0
0 10
   severe
60 68
42 30
25 6
43 20
*saplings are < 5 in. dbh, poles are 5-11 in. dbh, sawtimber is > 11 in. dbh

PLANT RESPONSE TO FIRE:
Fire prepares a mineral soil seedbed favorable to interior ponderosa pine regeneration [133,275,323]. Germinants require mineral soil so the emerging root radicle immediately contacts soil moisture [323]. On the Fort Valley Experimental Forest near Flagstaff, moisture in the upper 2 inches (5 cm) of soil was consistently greater on burned sites than on unburned sites. Plots were seeded to interior ponderosa pine 4 weeks after a November prescribed fire. The 1st postfire growing season was droughty, and although overall seedling establishment was poor, it was significantly greater (p=0.05) on burned than on unburned plots. Of 62 surviving seedlings, 95% (59 seedlings) were on burned plots, and 5% (3 seedlings) were on unburned plots [133]. A study on the Coconino National Forest showed a frequency of 90% for natural regeneration the year following moderate-severity prescribed surface fire. At postfire year 12, frequency of interior ponderosa pine regeneration had dropped to 25% on burned plots. Adjacent unburned plots showed no seedling establishment during the 12 years of the study [111].

Mature interior ponderosa pine may show increased growth following low-severity surface fires. Following prescribed fall burning on the Front Range of Colorado, mean fascicle lengths and bud sizes (length and diameter) were significantly greater (p < 0.01) on trees that received burning treatment than on trees on unburned control sites. Sutherland and others [279] present a linear regression model to predict postfire radial growth of southwestern ponderosa pine after prescribed fire.

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
The open canopy created by frequent fire produces physiological and morphological changes that significantly increased interior ponderosa pine's resistance to bark beetles and foliar insects, increased nutrient uptake, and promoted growth on a northern Arizona site. These changes occur even in old growth. A year following thinning and prescribed burning near Flagstaff, resin flow was higher in presettlement trees (established before 1876) on burned plots compared to presettlement trees on untreated control plots, with flows of 12.2 mL/24 h and 3.6 mL/24 h, respectively. Foliar toughness, which  increases resistance to pine sawflies, was greater on burned compared to control plots (76 vs. 69.8 g tension). Trees on burned plots also had greater leaf nitrogen content (1.59 vs. 1.44 g/m2) and showed more basal area growth  (20.7 vs. 14.3 cm2) on burned than on control plots. Thinning without burning produced similar changes in tree physiology and morphology, except that with a mean resin flow of 4.3 milliliters per day, presettlement trees on plots that were thinned but not burned developed less resistance to bark beetles than trees on thinned and burned plots [77,108,109].

FIRE MANAGEMENT CONSIDERATIONS:
Current fire return intervals are greater than the historical range of variability for ponderosa pine forests. Magnitude of fire decline is greater at lower than higher elevations, which may aid managers in determining where management actions to reduce fuels and restore more natural fire regimes might be of highest priority [298].  Millions of acres of forest burns in the western United States during fire years. Roughly half of those acres are in ponderosa pine. The expense of excluding fire from ponderosa pine forests in an active fire year can easily exceed a billion dollars, and these costly attempts at fire suppression are not always successful. In comparison, treatments to restore ponderosa pine structure and ecological processes are modest in cost [19].

Thinning to remove small-diameter trees, accompanied by prescribed fire, has been suggested as a means of restoring structure and function to degraded ponderosa pine ecosystems [18,43,76,103,103,140,147,255,306]. Frequent low-severity surface fires restore ecosystem function by thinning dense stands and reducing woody debris and other organic matter on the forest floor. This can result in increased soil moisture, increased soil temperature (with accompanying rates of increased litter decomposition, soil nutrient cycling, and fine root growth), increased productivity of understory herbs and shrubs, increased basal diameter growth of overstory ponderosa pine, and favorable seedbeds [6,7,77,78]. Fire-pruning of lower pine branches opens the canopy, aiding production of wildlife shrubs such as snowberry and chokecherry [128]. Frequent prescribed fires reduce fire hazard without damaging overstory ponderosa pine [6,7]. Biswell [43] listed several ways in which prescribed burning reduces wildfire hazard in interior ponderosa pine:


Grand Canyon National Park prescribed fire hand crews "stringing fire"
in a North Rim burn unit, in cautious incremental strips, to keep burn
severity to prescribed levels.

If several fire cycles have been missed, thinning presettlement trees and manually removing heavy fuels from the base of large trees may be necessary to in order to protect old growth from severe scorching or death [76]. Harrington [144] recommends growing-season (spring or summer) burning in interior ponderosa pine forests if the management objective is thinning from below, and fall prescribed burning if stand losses must be minimized. Weather parameters for prescribed burning in southwestern ponderosa pine [149], and a logistic regression model predicting probability of interior ponderosa pine mortality by tree size, scorch class, and season of injury are available [142].

Allen and others [9] provide ecologically based recommendations for restoring southwestern ponderosa pine. They stress that restoration programs should include natural variability in southwestern ponderosa pine stands and the reestablishment of natural processes. Managers are encouraged to fully review their recommendations. A synopsis of their principles for restoration follows [9]:

  • Reduce the threat of crown fire
  • Prioritize and strategically target treatment areas
  • Develop site-specific reference (historical) conditions
  • Implement multiple, conservative treatments
  • Utilize and incorporate existing forest structure
  • Restore ecosystem composition
  • Retain trees of significant size or age
  • Consider demographic processes (regeneration pulses)
  • Integrate ecological processes and stand structure
  • Control and avoid using exotic species
  • Foster regional heterogeneity
  • Protect sensitive species
  • Assess cumulative effects
  • Protect from overgrazing
  • Establish monitoring and research programs
  • Implement long-term, adaptive management

Swetnam and Dieterich [286] recommend allowing large (> 3000 acres (1200 ha)) prescribed natural surface fires in southwestern ponderosa pine in wilderness areas such as the Gila Wilderness. Based upon their fire history research, which showed evidence of mostly extensive but also small fires, they also recommend allowing small and patchy mixed-severity fires in approved areas, subject to the limitations of wilderness boundaries, visitor safety, and management and suppression capabilities.

Mixed and stand-replacing fire: High-elevation interior ponderosa pine forests were historically denser, with lower fire frequencies and greater severities, than lower-elevation forests. Even at low elevations, north-facing slopes and mesic ravines probably supported dense forests that experienced occasional mixed and stand-replacing fires. Evidence of a mixed regime of frequent surface fires and occasional mixed or stand-replacement fires suggests that a single prescription cannot capture historical variability of fire regimes in interior ponderosa pine types [298].

Historical occurrence of large, widespread prehistoric fires in interior ponderosa pine demonstrates the potential for large portions of montane zones to burn during a single year. Interannual climatic variability, particularly El Nino-La Nina events, can greatly increase fine fuels. Fuels management through thinning and prescribed burning reduces the probability of widespread wildfire in most years, but it is uncertain that low to moderate levels of fuels management will reduce fire hazard enough to prevent stand-replacing fire during years when the weather is exceptionally conducive to rapid fire spread.  It may be helpful for public education programs on fire hazard reduction to emphasize that although frequent surface fires were most common in interior ponderosa pine stands, mixed and stand-replacement fires did occur in prehistoric times, and that during years of high fire hazard, stand-replacement fires may occur again despite fuels reduction programs [298].

Fuels: Fuel loads in pole-sized thickets of interior ponderosa pine can be high. Ponderosa pine's long needles form a loose litter layer that burns readily [246]. Near Flagstaff, fuels averaged 28.3 tons per acre (63.4 t/ha), 21.3 tons (19.2 t) of which was duff [77].  Eakle and Wagle [100] provide a model, developed on the Fort Apache Reservation, for estimating fine fuels in southwestern ponderosa pine stands. Harrington [145] presents a model for estimating forest floor consumption in southwestern ponderosa pine forest based upon moisture content of the H surface soil layer.

Beaufait [37] found that backfires in ponderosa pine needles spread more slowly and had less flame depth, longer residence time, and a higher rate of energy release than headfires.

Boldt [51] provides prescriptions for thinning pole-sized "dog-hair" thickets of interior ponderosa pine using sequential treatments. Additional information on thinning "dog-hair" stands is found in Sackett and others [254].

Range: Prescribed fire can enhance understory forage production by reducing forest floor depth, tree density, and allelopathic toxins, and by increasing nutrient availability [13,150]. On the Fort Apache Reservation in Arizona, Gambel oak had reached mid-story in an interior ponderosa pine forest. Prescribed surface fires at 5- to 7-year intervals reduced Gambel oak to an understory shrub that was  readily available to browsing animals. Coverage of the dominant grass, mountain muhly, increased under the prescribed burning regime [43].

Pearson and others [234] reported that on the Wild Bill Range of Arizona, an area of interior ponderosa pine that had been thinned to 20 square feet basal area per acre (4.5 m2/ha) did not show a significant reduction in density following a wildfire. The fire crowned and killed the trees in an adjacent unthinned stand, however. Forage production increased as much as 578 pounds per acre (650 kg/ha) from prefire levels on sites were stand-replacement fire occurred, while pre- and postfire forage production were similar on thinned and burned sites.

Wild ungulate foraging, especially that of bighorn sheep, increased after late April prescribed fire in a interior ponderosa pine/sedge-bluegrass (Carex-Poa spp.) community in Custer State Park, South Dakota. Grass production did not increase after the fire, but forb production was significantly increased (p < 0.01) on burned compared to unburned sites [102].

October prescribed burning brought a 2-fold increase in nitrogen content of Arizona fescue and associated grasses on an interior ponderosa pine/Arizona fescue community on the Fort Valley Experimental Forest near Flagstaff. Concentrations of potassium, phosphorus, calcium, and magnesium were also generally greater in forb and grass species on burned plots than in herbs on unburned control plots. Understory biomass was significantly (p < 0.05) greater on burned plots. The 1st fall after burning, understory yield was twice as great on burned plots compared to unburned plots [150].

Overgrazing can greatly reduce fire frequency by removing understory fuels. Fire history studies of southwestern ponderosa pine document the near-cessation of fire in the mid-1800s due to livestock grazing in Arizona and New Mexico [292,293].

Exclusion of the recurrent fires that once swept the interior ponderosa pine-plains grasslands interface has resulted in interior ponderosa pine invasion into the grassland. Expansion is likely to continue without application of prescribed fire [50,273].

Since fire kills interior ponderosa pine seedlings and some saplings, prescribed fire can be used to reduce the density of encroaching pines. Late-April fire in little bluestem-hairy grama-sideoats grama in the Black Hills of South Dakota caused 79% mortality of pine seedlings. Mean density of interior ponderosa pine seedlings was 8,132 per acre (3251/ha) before the fire and 1,669 per acre (4173/ha) in late summer, after burning. The fire reduced ground fuels by 32% [123].

Wildlife: Fire benefits the majority of wildlife species inhabiting interior ponderosa pine ecosystems. Among bird species, fires in interior ponderosa pine tend to increase guilds that use open stands and snags. On the Prescott National Forest in Arizona, a stand-replacement burn in interior ponderosa pine attracted relatively more granivores, aerial insectivores such as flycatchers, and bark-feeding insectivores such as woodpeckers, while unburned areas attracted more ground and foliage insectivores [47]. A 20-year wildlife study on 4 stand-replacement burns near Flagstaff had similar findings for birds. Most rodent species, except chipmunks, also increased on burned interior ponderosa pine sites relative to adjacent unburned control sites. Wild ungulate use increased greatly on burned sites compared to unburned sites. Mule deer use of burned sites declined at postfire year 1, but  increased to 2.5 times that of unburned sites for the next 19 years. Elk use also dropped at postfire year 1 relative to unburned sites, but increased to 3 times that of controls until postfire year 20, when use of burned and unburned sites was nearly equal. Elk use of the burns peaked at postfire year 7 [203].

Availability of interior ponderosa pines as nesting sites for cavity-nesters, particularly secondary cavity-nesters, is limited due to the scarcity of large, old-growth trees. Brawn and Balda [59] reported that violet-green swallow, pygmy nuthatch, and western bluebird populations increased after artificial nest boxes were placed in open and thinned plots, but not on untreated (dense) plots. Prescribed underburning that leaves large trees may encourage nesting. Prescribed fire in interior ponderosa pine of Wind Cave National Park, South Dakota, significantly  (p< 0.05) increased the number of breeding bird pairs and deer mice on burned sites compared to unburned control sites [49]. Creating small openings with prescribed fire may also promote nesting. Aulenbach and O'Shea-Stone [27] noted differential songbird use of burned sites created by a small, crowning wildfire compared to adjacent unburned sites in interior ponderosa pine on the Front Range of Colorado. Several species including red-breasted nuthatch, chipping sparrow, yellow-rumped warbler, and northern flicker occurred only on the burn. American robin, Steller's jay, and dark-eyed junco preferred the burn but used the control, while pygmy nuthatch, downy woodpecker, white-breasted nuthatch, and mountain chickadee occurred only on the unburned site.

Standing time of interior ponderosa pine snags is somewhat predictable. Harrington [146] found that 75% of fire-killed interior ponderosa pine snags on the San Juan National Forest of southwestern Colorado fell within 10 postfire years. Fall rates were not significantly different (p=0.1) among trees 2 to 16 inches (5-41 cm) dbh, but large trees that died quickly after 80% or greater crown scorch were likely to fall more quickly, while large trees that survived 2 or 3 postfire years before succumbing were likely to remain standing for longer periods of time.

FIRE CASE STUDIES

SPECIES: Pinus ponderosa var. scopulorum


  • 1st CASE STUDY:
  • Effects of fall and spring fires on interior ponderosa pine on the pine-grassland ecotone of the southern Black Hills
  • 2nd CASE STUDY:
  • Interior ponderosa pine mortality from seasonal prescribed fires in southwestern Colorado
  • 3rd CASE STUDY:
  • Natural interior ponderosa pine regeneration following prescribed fire in north-central Arizona
  • OTHERS

1st CASE STUDY:

FIRE CASE STUDY CITATION:
Howard, Janet L., compiler. 2003. Effects of fall and spring fires on interior ponderosa pine on the pine-grassland ecotone of the southern Black Hills. In: Pinus ponderosa var. scopulorum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [ ].

REFERENCES:
Bock, Carl E.; Bock, Jane H. 1983. Responses of birds and deer mice to prescribed burning in ponderosa pine. Journal of Wildlife Management. 47(3): 836-840. [49].

Bock, Jane H.; Bock, Carl E. 1984. Effects of fires on woody vegetation in the pine-grassland ecotone of the southern Black Hills. The American Midland Naturalist. 112(1): 35-42. [50].

SEASON/SEVERITY CLASSIFICATION:
Fall and spring/Severity not stated

STUDY LOCATION:
The study was conducted on Wind Cave National Park and adjacent Black Hills National Forest lands in Custer County, South Dakota.

PREFIRE VEGETATIVE COMMUNITY:
Slopes above the study sites were forested with interior ponderosa pine (Pinus ponderosa var. scopulorum); lower slopes were mixed plains grasslands composed of sedges (Carex spp.), big bluestem (Andropogon gerardii var. gerardii), little bluestem, (Schizachyrium scoparium), Sandberg bluegrass (Poa secunda), blue, hairy, and sideoats grama (Bouteloua gracilis, B. hirsuta, B. curtipendula), and western wheatgrass (Pascopyrum smithii). The study site was a mosaic of interior ponderosa pine savanna and interior ponderosa pine forest with an understory of graminoids and shrubs. Important shrubs on the study site were western snowberry (Symphoricarpos occidentalis), currant (Ribes spp.), chokecherry (Prunus virginiana), leadplant (Amorpha canescens), and Saskatoon serviceberry (Amelanchier alnifolia). Based upon dendrochronological analysis, presettlement fire return interval at the Park was 13 to 21 years. Before prescribed fire treatments, mean interior ponderosa pine density on study plots was 3.2 stems/17 m2 (~762 trees/acre); most of those trees on study plots were saplings less than 4 inches (10 cm) dbh [49,50].

TARGET SPECIES PHENOLOGICAL STATE:
Interior ponderosa pine was probably dormant during both the fall and early spring fires.

SITE DESCRIPTION:
elevation: 4,900 feet (1500 m)

FIRE DESCRIPTION:
The purpose of the study was to determine the effects of cool-season fires on woody vegetation at the interior ponderosa pine-grassland ecotone. Objectives were: (1) determine if prescribed fire increases understory shrub and tree density, (2) quantify mortality of immature interior ponderosa pine on the ecotone, and (3) determine if prescribed fires had consistent and predictable results throughout the study area.

Two cool-season fires were conducted. The fall fire was a 157-acre (63 ha) burn on 17 October 1979. The fire was set at mid-day and passed over the area in 4 hours. A few snags smoldered until the next morning, when it started raining. Fire weather conditions were [49]:

windspeed 10 miles/h (16 km/h)
temperature 57.9 oFahrenheit (14.4 oC)
relative humidity: 45%
soil/fuel moisture: 4-29%

The spring fire was an 815-acre (326 ha) burn on 14 April 1980. The fire was set at 10:30 a.m. and burned most of area within control lines within 36 hours. It was declared out on 15 May 1980. Fire weather conditions were [49]:

windspeed: 5 miles/h (8 km/h)
temperature: 57 oFahrenheit (14 oC)
relative humidity: 32%
soil/fuel moisture: 21-36%

The effects of the 2 fires were similar, so pre- and postfire litter and vegetation data were pooled for the 2 sites. Litter depth averaged 1.61 inches (4.02 cm) before fire [49].

FIRE EFFECTS ON TARGET SPECIES:
Interior ponderosa pine canopies, trunks, and saplings were reduced through the 2 postfire years of the study. Nearly all pines less than 4.6 feet (1.4 m) tall were killed. Among taller trees, most mortality was in trees less than 2 inches (5 cm) dbh. Pine canopy was reduced 12% by the fires. Changes in percent interior ponderosa pine cover over the study period are given below. Chi-square values were calculated using actual numbers.

  Prefire Postfire year 1 Postfire year 2
  Burn Control Burn Control Burn Control Chi-square
Canopy  56.2 57.4 48.3 57.2 47.3 56.5 10.44
Trunks and immatures  3.3 2.5 1.2 2.3 1.2 2.4 22.26

Litter depth was significantly reduced (p < 0.01) to a 0.76-inch (1.89 cm) depth by burning [49].

FIRE MANAGEMENT IMPLICATIONS:
The study demonstrates that prescribed spring or fall fire can control interior ponderosa pine encroaching plains grasslands. Outcome of the study objectives, listed by number above, follow. (1) A single fire treatment had little effect on the density of shrubs and deciduous understory trees. Overall shrub cover was reduced in the 1st but not the 2nd postfire year [49]. Density of currants was reduced and leadplant increased, but density of other shrubs was unaffected by fire. Overall graminoid cover was also unaffected by burning, although species showed different responses. (2) The prescribed fires reduced fuels and dramatically slowed interior ponderosa pine invasion and recruitment. (3) The prescribed fires gave consistent and predictable results across the study areas. Grassland fires are more predictable than fires in woody vegetation [50]. The Bocks' hypothesis of consistent and predictable results was also borne out in an earlier Wind Cave National Park study by Gartner and Thompson [123].


2nd CASE STUDY:

FIRE CASE STUDY CITATION:
McMurray, Nancy, compiler. 1988. Interior ponderosa pine mortality from seasonal prescribed fires in southwestern Colorado. In: Pinus ponderosa var. scopulorum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [ ].

REFERENCES:
Harrington, M. G. 1985. The effects of spring, summer, and fall burning on Gambel oak in a southwestern ponderosa pine stand. Forest Science. 31(1): 156-163. [141].

Harrington, Michael G. 1987. Ponderosa pine mortality from spring, summer, and fall crown scorching. Western Journal of Applied Forestry. 2: 14-16. [144].

SEASON/SEVERITY CLASSIFICATION:
fall (20 Oct. 1977)/light-severity
spring (12 June 1978)/light- to moderate-severity
summer (18 Aug. 1978)/light- to moderate-severity
STUDY LOCATION:
The study site is on the San Juan National Forest.

PREFIRE VEGETATIVE COMMUNITY:
The study sites were in an interior ponderosa pine/Gambel oak (Pinus ponderosa var. scopulorum/Quercus gambelii) stand. A fire history study showed that the last fire occurred in the stand in 1877. Prior to 1877, fires burned at 1- to 16-year intervals [141], and stand structure was presumably open. Pretreatment stand structure in 1977 was a dense overstory of pole-sized pines, and a dense understory of shrubby oaks. Herbaceous species and shrubs other than Gambel oak were sparse. Tree density was 300 per acre (750 trees/ha); basal area averaged 124 square feet per acre (28 m2/ha). Size class distribution of interior ponderosa pine was [141,144]:

Size class Cover (%)
0.5 to 4 inches (1-10 cm) 8
4 to 7 inches (10-18 cm) 31
7 to 10 inches (18-28 cm) 42
> 11 inches (> 28 cm) 19

TARGET SPECIES PHENOLOGICAL STATE:
fall: dormant - buds set; seeds ripe
spring: active growth beginning - buds broken; stems expanding; needles 1 to 2 inches (2.5-5 cm) long and expanding
summer: active growth - stem diameter growth; cones and seeds developing [144]

SITE DESCRIPTION:
Elevation: 7,600 feet (2317 m)
Aspect: south
slope: < 5% [141,144]

Precipitation was above the 16.8-inch (427 mm) annual mean 3 months preceding each burn, ranging from 2.8 inches (710 m) above normal before the spring burn to 0.4 inch (102 mm) above normal for the summer burn (fall ppt. not stated). The study site received about 2.2 inches (560 mm) of rain the July prior to summer burning, which probably encouraged tree growth. Precipitation following the fire treatments was 0.5 inch (130 mm), 3.8 inches (965 mm), and 3.5 inches (890 mm) above average for fall, spring, and summer treatments, respectively [144].

FIRE DESCRIPTION:
Backing fires were used until 15- to 20-foot (4.5-6 m) strips had burned inside firelines. Strip headfires were then ignited at 15- to 30-foot-wide (4.5-9 m) strips, depending upon weather and fuel conditions. Weather, fuel* moisture, and fire behavior during prescribed burning were [141]:

  Fall Spring Summer
Temperature (oF) 56-63 65-79 72-77
Humidity (%) 32-41 14-41 21-37
Wind speed (mph) 2-5 1-6 1-5
L-layer (% moisture) 7.2 5.4 6.0
F-layer (% moisture) 11.8 6.9 7.5
H-layer (% moisture) 18.9 11.5 9.2
Spread rate (ft/min) 6 8 5
Flame length (ft) 3.0 3.5 3.5
*L=litter layer, F=fermentation layer, H=humus layer                                  

FIRE EFFECTS ON TARGET SPECIES:
The study objective was to compare interior ponderosa pine mortality resulting from crown scorch incurred during fall, spring, and summer prescribed surface fires. Tree scorch classes, defined as the percentage of total crown height on all sides of the tree in which foliage was killed, were 10-30%, 34-66%, 67-89, 90-98%, 99%, and 100%. Study trees were monitored for 5 postfire years. In order to avoid the interactive effects of cambium and crown scorch damage, tree with boles that were charred enough to sustain damage to the cambium were not included in the study [141]. The management objective was to reduce the density of understory Gambel oak with 2nd-year reburns. Treatment effects to Gambel oak are summarized in the Fire Case Studies section of the FEIS report on that species.

At the end of 5 years, overall interior ponderosa pine mortality from fall crown scorch was significantly less (p=0.05) than that from spring or summer. Tree mortality equaled 12, 26, and 29% for fall, spring, and summer fires, respectively. Differences in mortality between seasons were most substantial during the 1st postfire season: only 5% of fall-scorched trees died, compared with 17% of spring-scorched trees and 21% of summer-scorched trees. Subsequent mortality for all seasons was relatively similar over the length of the study, suggesting good survival potential for those trees not killed outright by scorching. Generally mortality decreased as d.b.h. increased regardless of season. Trees less than 7 inches (17.5 cm) d.b.h. accounted for approximately 85% of the observed mortality. Mortality from fall burns was comparable to that recorded on the control plots. Mortality in trees in the < 4-inch and 7 to 10.9-inch d.b.h. classes was significantly greater on spring and summer burn plots than on fall burn and unburned plots. Percent mortality of scorched trees by diameter class and season of fire is given below. Means in rows followed by different letters differ at the 5% significance level. Means in columns followed by different numbers differ at the 5% level [144].

  Tree diameter class (inches)
Treatment < 4.0 4.0-6.9 7.0-10.9  10.9
fall 28a,1 18ab,1 0c,1 7bc,1
spring 55a,2 25b,1 11b,2 7b,1
summer 60a,2 37a,1 15b,2 5b,1
control 17a,1 6b,2 5b,1 1b,1

Scorch damage of up to 90% produced minimal mortality regardless of season of burn. When more than 90% of the crown was scorched, fall burning caused only 1/3rd the mortality produced by spring or summer burning. Percent mortality of scorched trees by scorch class and season of burn is presented below. Means in rows followed by different letters differ at the 5% significance level. Means in columns followed by different numbers differ at the 5% level [144].

  Scorch class
Treatment 0 10-33 34-66 67-89 90-99 100
fall 5a 8a,1 5a,1 0a,1 13a,1 39b,1
spring 5a 5a,1 13a,1 12a,2 54ab,2 100b,1
summer 5a 11a,1 12a,1 11a,2 42a,2 89a,1

Trees subjected to dormant-season burning were resistant to severe crown scorch damage. Of those trees sustaining 100% damage, only 10 of 28 (39%) ultimately died. All 8 trees sampled in the 7 to 10.9-inch (17.5-27.3 cm) category survived. Harrington [141] cautioned that this resistance to fire mortality is demonstrated for trees where crown scorch was the only type of fire damage recorded; such extensive damage would probably be fatal when combined with bole damage.

FIRE MANAGEMENT IMPLICATIONS:
Fall burning is the best option if minimizing mortality in the smaller size classes is a management objective; this is particularly true in burn units where fuel concentrations are heavy. If management objectives include large reductions in fuels or the elimination of undesirable understory brush species, growing-season fire may be the most effective. Although trees are generally more susceptible to fire damage during the growing season, most mortality due exclusively to crown scorch occurs in the smaller size classes. Such mortality can be beneficial if thinning from below is desired.


3rd CASE STUDY:

FIRE CASE STUDY CITATION:
Howard, Janet L., compiler. 2001. Natural interior ponderosa pine regeneration following prescribed fire in north-central Arizona. In: Pinus ponderosa var. scopulorum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [ ].

REFERENCES:
Sackett, Stephen S. 1980. Reducing natural ponderosa pine fuels using prescribed fire: two case studies. Res. Note RM-392. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 6 p. [250].

Sackett, Stephen S. 1984. Observations on natural regeneration in ponderosa pine following a prescribed fire in Arizona. Res. Note RM-435. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 8 p. [251].

Sackett, Stephen; Haase, Sally; Harrington, M. G. 1993. Restoration of southwestern ponderosa pine ecosystems with fire. In: Covington, M. Wallace; Debano, Leonard F.; Covington, W. W., tech. coords. Sustainable ecological systems: implementing an ecological approach to land management: Proceedings; 1993 July 12-15; Flagstaff, AZ. Gen. Tech. Rep. RM-247. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station: 115-121. [255].

Sackett, Stephen S.; Haase, Sally M.; Harrington, Michael G. 1996. Lessons learned from fire use for restoring southwestern ponderosa pine ecosystems. In: Covington, Wallace; Wagner, Pamela K., technical coordinators. Conference on adaptive ecosystem restoration and management: restoration of Cordilleran conifer landscapes of North America: Proceedings; 1996 June 6-8; Flagstaff, AZ. Gen. Tech. Rep. RM-GTR-278. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station: 54-61. [254].

SEASON/SEVERITY CLASSIFICATION:
fall/moderate

STUDY LOCATION:
The study took place on the Fort Valley Experimental Forest near Flagstaff, Arizona.

PREFIRE VEGETATIVE COMMUNITY:
The study site was in an interior ponderosa pine/Arizona fescue (Pinus ponderosa var. scopulorum/Festuca arizonica) forest. Interior ponderosa pine was the only overstory tree species. Stand structure was small, even-aged groups in an uneven-aged stand. Site index was 82, based on 100 years. Mean density of interior ponderosa pine on study sites was 39 per acre (96/ha). Size class distribution is presented below [251]:

Seedlings: < 4.5 feet tall, 395 stems/acre (< 1.4 m, 975 stems/ha)

Saplings: 0.6 to 3.9 inches d.b.h., 1,114 stems/acre (1.5-9.9 cm d.b.h., 2,750 stems/ha), age = app. 57 years

Poles: 4.0 to 10.9 inches d.b.h., 195 stems/acre (10.0-27.5 cm d.b.h., 700 stems/ha), age = 57 to 150 years

Sawtimber: > 11 inches d.b.h., 54 stems/acre ( >27.5 cm d.b.h., 130 stems/ha), age = 200 to 500 years

Relative tree cover on the experimental plots consisted of the following: sapling 17%; pole 62%, sawtimber 17.3%; small openings comprised the remaining 3.7%. Irregular distribution of the pine understory resulted in numerous grass-dominated openings [251]. Fendler's ceanothus (Ceanothus fendleri), mountain muhly (Muhlenbergia montana), bottlebrush squirreltail (Elymus elymoides), pine dropseed (Blepharoneuron tricholepis), Cainville thistle (Cirsium calcareum), groundsel (Senecio spp.), and goldenrod (Solidago spp.) were understory associates [255].

TARGET SPECIES PHENOLOGICAL STATE:
dormant

SITE DESCRIPTION:
Elevation: 7,200 to 7,400 feet (2195-2255 m)
Slope: 0 to 5%
Mean annual temperature: 43 degrees Fahrenheit (6 oC)
Average annual precipitation: 22 inches (570 mm); bimodal distribution; summer drought is common
Soils: Brolliar stony clay loam, tentatively classified as a fine, montmorillonitic, frigid, typic Argiboroll; depth to basalt flow rock usually 8 to 40 inches (20-100 cm) [251]

Results of fire scar analysis indicate that the study area had not burned since 1876; prior to that time, mean fire return interval was 2 years [255].

FIRE DESCRIPTION:
Rainfall was below normal during the late summer and fall. The following precipitation amounts were recorded at the fire site just prior to prescribed burning on November 5, 1976:

October 21 - 0.27 inch (69 mm)
October 25 - 0.03 inch (8 mm).

Ignition began at 5:30 p.m. and continued until 11:00 p.m. Backing fires were used initially, but later in the evening when these ceased to carry effectively, short strip headfires with 30 to 40 feet (9-12 m) between strips were utilized. Although rate of spread approximated 4 to 6 feet per minute (1.2-1.8 m/min), it sometimes increased to as much as 12 feet per minute (3.7 m/min). Flame lengths in surface needles rarely exceeded 16 inches (40 cm). Air temperature dropped from 59 degrees Fahrenheit (15 oC) at 5:30 p.m. to 37 degrees Fahrenheit (3 oC) at 12:00 a.m., when most of the flaming combustion was complete. Measured surface needle moisture content increased from 8 to 12% during the night as temperatures fell; similarly, ground fuel (F & H layers) moisture content of the forest floor increased from 10 to 19%. Glowing combustion in deep duff fuels continued for 2 to 3 days; some large woody debris and stumps smoldered for up to 4 weeks. Forest floor material less than 1 inch (2.5 cm) in diameter was reduced 63%; material greater than 1 inch (2.5 cm) was reduced 69%. Mineral soil was exposed on about 19% of the area, mostly around large mature trees and rotten logs. Pre- and postfire fuel loads were as follows [251]:

Dead fuels Prefire fuel load (tons/acre) Postfire fuel load (tons/acre) fuel reduction (%)
Surface & ground fuels      
  needles/humus 12.19 4.06 67
  0 to 1/4th in. woody 0.79 0.27 66
  1-4th to 1-in. woody 1.31 1.99 24
  other material 0.88   1.35 60
  Total 15.17 5.67 63
Large woody fuel      
  1 to 3-in. 0.81 0.37 54
  > 3 in., sound 2.09 1.79 14
  > 3 in., rotten 4.26 0.04 99
  Total 7.16 2.20 69
Total, all dead fuels 22.33 7.87 65

FIRE EFFECTS ON TARGET SPECIES:
Seedlings appeared soon after summer rains started in mid-July, 1977. High seedling concentrations occurred under large, mature pines and near heavy fuel accumulations where the forest floor layers had been completely consumed. Although partial shade was provided by the overstory, charcoal chunks, and unburned debris, seedlings appeared to be most frequent on the northeast side of seed trees. Needles cast over the period from November to July covered seeds with a fresh litter layer of 300 to 475 pounds/acre (268-424 kg/ha). In comparison to unburned controls, burned areas produced better germination and stocking; burns had approximately 2,600 seedlings/acre, with 38% of sample quadrats stocked versus control plots where seedlings equaled 833 seedlings/acre with a 17% stocking. Germination was most successful in areas where fire disturbance was most pronounced, and correlation between quadrat bare area (feet2) for each burning treatment and quadrats stocked with interior ponderosa pine seedlings was high (r=0.92). Number of seedlings in relation to seedbed is presented below:

  Seedbed classification*
  LFH L F FH H M A CF CH Total
unburned 5  0 0 0 0 0 0 0 0 5
burned 2 1 25 8 5 25 3 13 14 96
*LFH=all 3 forest floor layers intact; no fire; L=newly cast needles on naturally bare soil, no fire; F=F layer intact but charred on top surface from burnt L layer, no H layer below; FH=F and H layers intact but charred on top surface from burnt L layer; H=H layer intact but charred on top surface from burnt L and F layers; M=mineral soil, all materials consumed; A=mineral soil, chunks of charcoal; CF=F layer heavily charred, L layer burnt off, no H layer below; CH=H layer heavily charred, L and F layers burnt off

Germination and initial establishment were excellent on burned areas despite below normal precipitation during the spring and summer of 1977. The enhanced survival under these conditions may have resulted from the improved moisture relations, increased nutrient availability, and more favorable soil temperatures associated with mineral soil seedbeds. However, subsequent mortality was quite high due to the combined effects of drought, frost heaving, animal damage, and interval burning (see below). By the fall of 1981, only 12% of tagged seedlings (25 on each of 18 plots treatment plots and 8 on controls) remained. Survival of 4-year-old tagged seedlings on plots subjected to interval burning was 6%; 19% of tagged seedlings survived on plots receiving the initial treatment only. Probable cause of death (by %) of seedlings  selected 1 year after 1977 postfire germination is presented below [254].

  Drought/ winter kill Frost  heaving Fire Browsing Unknown/      other
Control 37 13 NA 0 50
1-yr burn rotation1 70 9 17 0 4
2-yr burn rotation2 25 10 56 3 6
4-yr burn rotation3 50 20 20 2 8
6-yr burn rotation4 52 13 NA 3 32
8-yr burn rotation4 48 28 NA 6 18
10-yr burn rotation4 67 9 NA 2 22
13 fires after seedling establishment
22 fires after seedling establishment
31 fire after seedling establishment
4No fires after seedling establishment   
   

Although very few sample seedlings were still alive by the fall of 1980, many 1977 seedlings remained on treatment plots. As a result, prior to interval burning in the fall of 1980, all seedlings on the 1-, 2-, and 4-year rotation plots were inventoried; a 100% inventory of 1977 seedlings on projected 6-, 8-, and 10-year rotations was done in the spring of 1981. Dry burning conditions in the fall of 1980 created more intense fires than had previously occurred during the study. Fireline intensities ranging between 260 to 650 btu/sec/fireline foot were produced on 4-year rotation plots, killing 94% of 3-year-old seedlings. Reduced fuel accumulations on annual and biennial plots produced less intense fires that resulted in 50 and 87% seedling mortality, respectively. Regardless of interval treatment, most surviving seedlings were located in areas of sparse or discontinuous fuels. Number of seedlings remaining from 1977 germination is presented below by treatment [254].

  Fall 1980 Spring 1981 Spring 1982
  number   number number height, in (mean) height (range) Stocking (#/acre)
Control 0   0 0     0
1-yr burn rotation 1431 burned 72 43 5.0 1.6-9.3 94
2-yr burn rotation 3862 burned 49 43 5.9 2.1-15.3 24
4-yr burn rotation 7253 burned 43 40 8.7 3.9-29.8 30
6-yr burn rotation   projected fire 1952 222 5.6 1.9-17.1 110
8-yr burn rotation   projected fire 2462 283 4.6 1.7-27.1 120
10-yr burn rotation   projected fire 1832 165 5.1 1.5-14.4 130
1three fires after seedling selection
2one fire after seedling selection
3No fires after seedling selection

FIRE MANAGEMENT IMPLICATIONS:
Prescribed fire represents an effective means of site preparation for interior ponderosa pine establishment in the Southwest, especially when regeneration is needed in dispersed, small openings where hand planting is not cost effective. Despite abundant initial establishment on mineral soil seedbeds created by burning, drought, frost heaving, and animal damage still constituted obstacles to long-term establishment. Seedbeds remained favorable for natural regeneration for at least 4 years after fire, allowing managers some leeway in attempting to coordinate site preparation with good seed crop years. Burning can be deferred for several years after germination so interior ponderosa pine seedlings are large enough to resistant to surface fire damage.

Sackett and others [254] concluded that 4-year interval burning was most effective for fire hazard reduction while protecting the overstory. Annual burning or 2-year interval burning was highly successful because conditions were too damp for light fuels, even though scarcity of fuels was not limiting. Six-year rotations allowed fuels to build up to the point that overstory could be damaged, and wildfires may be difficult to control under extreme fire weather conditions. They noted that mullein (Verbascum thapsus), Dalamation toadflax (Linaria dalmatica), and Cainville thistle (Cirsium calcareum) were most likely to invade severely burned sites, favoring bases of large, old-growth snags as microsites [255].


OTHERS:

There are several other Fire Cases Studies on interior ponderosa pine. Studies that may be of particular interest are listed below. Contact your local agency, university, or public library for copies of these articles.
  • Restoring ecosystem health in ponderosa pine forests of the Southwest (Fort Valley Experimental Forest, AZ, study [77]
  • A fire prescription for consuming ponderosa pine duff (Coconino National Forest, AZ) [89]
  • Forest restoration in southwestern ponderosa pine (San Juan National Forest, CO) [206]
  • Effects of fire on fuels (Apache-Sitgreaves and Coconino national forests, AZ) [211]

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