SPECIES: Opuntia fragilis


SPECIES: Opuntia fragilis

 2005 Jerry Murray

Taylor, Jane E. 2005. Opuntia fragilis. 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/ [].




brittle pricklypear
brittle prickly-pear
fragile pricklypear
little pricklypear
pygmy pricklypear

The scientific name of brittle pricklypear is Opuntia fragilis (Nutt.) Haw. (Cactaceae) [9,21,38,39,45,80].

Infrataxa: Based upon differences in distribution and plant size, some systematists recognize 2 varieties of brittle pricklypear [9,38,39,50,78]:

Opuntia fragilis var. brachyarthra (Engelm. & Bigelow) Coult, little pricklypear
Opuntia fragilis var. fragilis, pygmy pricklypear

Hybrids: Brittle pricklypear hybridizes with plains pricklypear (O. polyacantha) and grizzlybear pricklypear (O. erinacea) [9].



Brittle pricklypear is state listed as protected in Nevada [73], threatened in Iowa and Wisconsin [36,82], and endangered in Illinois and Michigan [35,53]. Little pricklypear is listed as threatened in Iowa, protected in Nevada, and protected from salvage in Arizona. Pygmy pricklypear is listed as threatened in Iowa, protected in Nevada, and protected from salvage in Arizona [73].


SPECIES: Opuntia fragilis
Brittle pricklypear is widely distributed across North America. It occurs from Ontario south to Texas and west to British Columbia, Washington, Oregon, and California. Brittle pricklypear is rare in Ontario, Iowa, Illinois, Michigan, Washington, and Wisconsin [39,77], and extremely rare or possibly extirpated in California [17,39]. Brittle pricklypear is found further north than any other cactus species in the world, growing in northern Alberta only 4o south of the Arctic Circle [9]. The Flora of North America provides a distribution map of brittle pricklypear. Plants database provides state distributional maps of its varieties.

Varieties: Pygmy pricklypear occurs throughout the general range of brittle pricklypear. Little pricklypear occurs in Colorado, New Mexico, Arizona, and Utah [39].

FRES21 Ponderosa pine
FRES29 Sagebrush
FRES30 Desert shrub
FRES32 Texas savanna
FRES33 Southwestern shrubsteppe
FRES35 Pinyon-juniper
FRES36 Mountain grasslands
FRES38 Plains grasslands
FRES39 Prairie
FRES40 Desert grasslands

STATES/PROVINCES: (key to state/province abbreviations)



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

K010 Ponderosa shrub forest
K011 Western ponderosa pine forest
K016 Eastern ponderosa forest
K017 Black Hills pine forest
K018 Pine-Douglas-fir forest
K019 Arizona pine forest
K023 Juniper-pinyon woodland
K024 Juniper steppe woodland
K031 Oak-juniper woodland
K037 Mountain-mahogany-oak scrub
K038 Great Basin sagebrush
K039 Blackbrush
K040 Saltbush-greasewood
K053 Grama-galleta steppe
K055 Sagebrush steppe
K056 Wheatgrass-needlegrass shrubsteppe
K058 Grama-tobosa shrubsteppe
K063 Foothills prairie
K064 Grama-needlegrass-wheatgrass
K065 Grama-buffalo grass
K066 Wheatgrass-needlegrass
K067 Wheatgrass-bluestem-needlegrass
K068 Wheatgrass-grama-buffalo grass
K069 Bluestem-grama prairie
K070 Sandsage-bluestem prairie
K074 Bluestem prairie
K075 Nebraska Sandhills prairie
K082 Mosaic of K074 and K100
K085 Mesquite-buffalo grass
K086 Juniper-oak savanna
K087 Mesquite-oak savanna

42 Bur Oak
68 Mesquite
220 Rocky Mountain juniper
236 Bur oak
237 Interior ponderosa pine
239 Pinyon-juniper
242 Mesquite

101 Bluebunch wheatgrass
102 Idaho fescue
104 Antelope bitterbrush-bluebunch wheatgrass
105 Antelope bitterbrush-Idaho fescue
109 Ponderosa pine shrubland
110 Ponderosa pine-grassland
212 Blackbush
301 Bluebunch wheatgrass-blue grama
302 Bluebunch wheatgrass-Sandberg bluegrass
303 Bluebunch wheatgrass-western wheatgrass
304 Idaho fescue-bluebunch wheatgrass
309 Idaho fescue-western wheatgrass
310 Needle-and-thread-blue grama
311 Rough fescue-bluebunch wheatgrass
314 Big sagebrush-bluebunch wheatgrass
317 Bitterbrush-bluebunch wheatgrass
320 Black sagebrush-bluebunch wheatgrass
322 Curlleaf mountain-mahogany-bluebunch wheatgrass
401 Basin big sagebrush
402 Mountain big sagebrush
403 Wyoming big sagebrush
404 Threetip sagebrush
405 Black sagebrush
406 Low sagebrush
408 Other sagebrush types
412 Juniper-pinyon woodland
414 Salt desert shrub
415 Curlleaf mountain-mahogany
416 True mountain-mahogany
417 Littleleaf mountain-mahogany
501 Saltbush-greasewood
502 Grama-galleta
504 Juniper-pinyon pine woodland
505 Grama-tobosa shrub
601 Bluestem prairie
602 Bluestem-prairie sandreed
603 Prairie sandreed-needlegrass
604 Bluestem-grama prairie
605 Sandsage prairie
606 Wheatgrass-bluestem-needlegrass
607 Wheatgrass-needlegrass
608 Wheatgrass-grama-needlegrass
609 Wheatgrass-grama
610 Wheatgrass
611 Blue grama-buffalo grass
612 Sagebrush-grass
613 Fescue grassland
614 Crested wheatgrass
615 Wheatgrass-saltgrass-grama
701 Alkali sacaton-tobosagrass
702 Black grama-alkali sacaton
703 Black grama-sideoats grama
704 Blue grama-western wheatgrass
705 Blue grama-galleta
706 Blue grama-sideoats grama
707 Blue grama-sideoats grama-black grama
708 Bluestem-dropseed
709 Bluestem-grama
710 Bluestem prairie
711 Bluestem-sacahuista prairie
712 Galleta-alkali sacaton
713 Grama-muhly-threeawn
714 Grama-bluestem
715 Grama-buffalo grass
716 Grama-feathergrass
717 Little bluestem-Indiangrass-Texas wintergrass
718 Mesquite-grama
720 Sand bluestem-little bluestem (dunes)
721 Sand bluestem-little bluestem (plains)
722 Sand sagebrush-mixed prairie
724 Sideoats grama-New Mexico feathergrass-winterfat
727 Mesquite-buffalo grass
729 Mesquite
733 Juniper-oak
734 Mesquite-oak
735 Sideoats grama-sumac-juniper

Brittle pricklypear occurs in a variety of desert, grassland, prairie, and woodland communities. It occurs as a community associate and not a dominant species [9]. Brief descriptions of the common dominants and associates are presented below. More detailed descriptions of plant communities where brittle pricklypear occurs are available in the publications listed at the end of this section.

Brittle pricklypear is commonly found in upland grasslands dominated by various bunchgrasses including blue grama (Bouteloua gracilis), buffalo grass (Buchloe dactyloides), Sandberg bluegrass (Poa secunda), bluebunch wheatgrass (Pseudoroegneria spicata), western wheatgrass (Pascopyrum smithii), and green needlegrass (Nassella viridula) [31,46,55,72,81].

In tallgrass prairies dominated by big bluestem (Andropogon gerardii) and little bluestem (Schizachyrium scoparium), brittle pricklypear occurs but is an uncommon associate [30,81].

Brittle pricklypear is a common associate in a wide variety of habitat types dominated by big sagebrush (Artemisia tridentata) and silver sagebrush (A. cana). It also occurs as an associate in various shrub communities including those dominated by greasewood (Sarcobatus vermiculatus), shadscale saltbush (Atriplex confertifolia), and blackbrush (Coleogyne ramosissima) [14,31,49,72].

In the Sandhills region of Colorado and Nebraska, brittle pricklypear is commonly found in communities dominated by sand sagebrush (Artemisia filifolia), prairie sandreed (Calamovilfa longifolia), sand bluestem (Andropogon gerardii var. paucipilus), hairy grama (Bouteloua hirsuta), and sandhill muhly (Muhlenbergia pungens) [58,62].

Brittle pricklypear occurs in various woodland communities, notably dry ponderosa pine (Pinus ponderosa) communities, dry ponderosa pine-Douglas-fir (Pseudotsuga menziesii) habitat types, pinyon-juniper (P. edulis-Juniperus spp.) woodlands and Gambel oak (Quercus gambelii) thickets [1,51,77,79,80].

Publications that discuss plant communities in which brittle pricklypear occurs are listed below. The list is neither restrictive nor all inclusive.

AZ [13,49]
CA [13]
CO [13,51,58]
ID [18]
MT [1,14,55]
ND [31]
NE [62]
NM [13]
NV [13]
ON [72]
SD [30,81]
SK [33]
TX [13]
UT [13,27]


SPECIES: Opuntia fragilis
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Several florae provide keys for identifying brittle pricklypear [9,32,45,50,80].

Brittle pricklypear is a perennial native mat- or clump-forming cactus, usually 2 to 4 inches (5-10 cm) tall. The clumps or mats often exceed 12 inches (30 cm) in diameter. The succulent stem segments, or pads, are 0.5 to 5 inches (1.2-2.5 cm) wide and range in length from 0.8 to 2 inches (2-5 cm) for pygmy pricklypear and 2 to 2.8 inches (5-7cm) for little pricklypear. Aereoles on the pads give rise to 2 to 7 barbed spines that are 0.5 to 0.8 inch (1.2-2 cm) long for pygmy pricklypear and 0.8 to 1.2 inches (2-3) cm long for little pricklypear. Flowers are solitary, 1.2 to 2 inches (3-5 cm) long and broad. The fruit is a pear-shaped berry, 0.6 to 0.8 inch (1.5-2 cm) long, and is usually spiny. The seeds are glabrous, flattened, oblong to subcircular, and 0.2 inch (5 mm) in diameter. The root system is shallow and fibrous [9,21,50,68].

Brittle pricklypear is extremely tolerant of drought. It avoids drought damage by accumulating water in storage cells that contain mucilaginous materials with a strong water-retaining capacity [37].

Stem succulent

Brittle pricklypear reproduces by seeds, layering, and sprouting from detached stem segments [68].

Breeding system: Brittle pricklypear is monoecious [9].

Pollination: Brittle pricklypear is pollinated by insects [9].

Seed production: Brittle pricklypear does not dependably flower every year in its northerly range, thus limiting seed production in these areas [9,48,79].

Seed dispersal: Seeds of brittle pricklypear are primarily spread when the fruits are eaten by frugivorous birds and small mammals. Fruits also readily attach to the fur and feathers of animals [8,68].

Seed banking: No information is available on this topic.

Germination: Germination rate is reportedly low for seeds of Opuntia species [71].

Seedling establishment/growth: Although the literature reports that brittle pricklypear regenerates by seeds [9], information is lacking on the specifics of seedling establishment and growth.

Asexual regeneration: Asexual reproduction occurs from detached pads which readily root even in the absence of water. The pads are primarily dispersed by attaching to animals by the barbed spines. The pads are also dispersed by gravity and by floating in water during heavy rains or snow melt. In the northerly range of brittle pricklypear, flowering can be rare and the plant may depend wholly on vegetative reproduction [9,48].

Brittle pricklypear can flourish on a great range of sites. It is found at elevations ranging from sea level to 11,089 feet (3,380 m), will grow well on various types of soils under a wide range of moisture regimes, and can survive extremes of both hot and cold temperatures [9,48,80].

Brittle pricklypear is perhaps the most cold tolerant of all the cacti species, being able to survive on sites where the minimum winter temperatures can drop below -58 oF (-50 oC). The cactus avoids freeze damage by rapidly reducing the water content in cells during cold acclimation. The short stature of the plants allows brittle pricklypear to take advantage of the insulating effects of snow and the thermal environment at the soil surface. Brittle pricklypear also is able to withstand temperatures in excess of 131 oF (55 oC) [37,48].

Brittle pricklypear is most commonly found on rocky, sandy or gravely soils, but can also flourish on silty, loamy, or clayey soils. It is tolerant of salt-affected, alkaline, and solodized (dealkalized) soils [9,80,81]. 

The moisture regimes at which brittle pricklypear can be found are quite varied. For example, in British Columbia, brittle pricklypear occurs on sites ranging from very xeric to hygric [42].

The following table lists reported elevational ranges for brittle pricklypear:

State or province Elevation
AZ 6,500 to 7,500 feet (1,981-2,286 m) [40]
CO 4,500 to 7,500 feet (1,372-2,286 m) [28]
NM 4,500 to 8,000 feet (1,372-2,438 m) [50]
UT 4,495 to 8,415 feet (1,370-2,565 m) [80]
WA 14 to 4,500 feet (4-1,372 m) [77]
BC 738 to 11,089 feet (225-3,380) [42]

Brittle pricklypear is often an early seral species and is shade intolerant. It may persist well on shallow soils of low fertility where other plants are sparse, but may decrease on more fertile sites as taller vegetation becomes established [48,77]. In a study of rock outcrop succession in boreal Manitoba, brittle pricklypear was the 1st of the stress-tolerant perennials to occupy rock outcrops. The successional sequence was: 1) lichens (Parmelia and Cladonia spp.), 2) moss (Grimmia and Hedwigia spp.) mats, 3) vascular annuals, 4) short-lived perennial forbs 5) stress-tolerant, long-lived perennial forbs, 6) deep-rooted perennial grasses, and 7) trees and shrubs. Brittle pricklypear did not persist past stage 4 [23].

Brittle pricklypear flowers from May at low elevations to July at high elevations. Fruits mature 2 to 3 months after flowering and many persist until the following spring [9,25,50].


SPECIES: Opuntia fragilis
Fire adaptations: Thomas [70] lists brittle pricklypear as fire tolerant. Brittle pricklypear is adapted to survive fire by sprouting from the root crown, by layering from old pads that were buried and protected in the litter layer, and by new seedling establishment [69,70].

Fire regimes: Brittle pricklypear occurs in plant communities with a wide range of fire frequencies, from less than 10 years for many prairie and grassland communities, to the 400 years possible for the Colorado pinyon (Pinus edulis) community. As of this writing (2005), fire ecology studies are lacking for brittle pricklypear. The following table provides fire return intervals for plant communities and ecosystems where brittle pricklypear occurs. For further information, see the FEIS review of the dominant species listed below.

Community or Ecosystem Dominant Species Fire Return Interval Range (years)
bluestem prairie Andropogon gerardii var. gerardii-Schizachyrium scoparium <10 [43,56]
Nebraska sandhills prairie Andropogon gerardii var. paucipilus-Schizachyrium scoparium <10
bluestem-Sacahuista prairie Andropogon littoralis-Spartina spartinae <10 [56]
silver sagebrush steppe Artemisia cana 5-45 [29,57,83]
sagebrush steppe Artemisia tridentata/Pseudoroegneria spicata 20-70 [56]
basin big sagebrush Artemisia tridentata var. tridentata 12-43 [61]
mountain big sagebrush Artemisia tridentata var. vaseyana 15-40 [4,16,54]
Wyoming big sagebrush Artemisia tridentata var. wyomingensis 10-70 (40**) [74,84]
saltbush-greasewood Atriplex confertifolia-Sarcobatus vermiculatus <35 to <100 [56]
desert grasslands Bouteloua eriopoda and/or Pleuraphis mutica 10 to <100 [52,56]
plains grasslands Bouteloua spp. <35 [56,83]
blue grama-needle-and-thread grass-western wheatgrass Bouteloua gracilis-Hesperostipa comata-Pascopyrum smithii <35 [56,60,83]
blue grama-buffalo grass Bouteloua gracilis-Buchloe dactyloides <35 [56,83]
grama-galleta steppe Bouteloua gracilis-Pleuraphis jamesii <35 to <100
blue grama-tobosa prairie Bouteloua gracilis-Pleuraphis mutica <35 to <100 [56]
blackbrush Coleogyne ramosissima <35 to <100
Rocky Mountain juniper Juniperus scopulorum <35 [56]
wheatgrass plains grasslands Pascopyrum smithii <5-47+ [56,57,83]
pinyon-juniper Pinus-Juniperus spp. <35 [56]
Colorado pinyon Pinus edulis 10-400+ [22,26,41,56]
interior ponderosa pine* Pinus ponderosa var. scopulorum 2-30 [3,6,47]
Arizona pine Pinus ponderosa var. arizonica 2-15 [6,19,64]
mesquite Prosopis glandulosa <35 to <100 [52,56]
mesquite-buffalo grass Prosopis glandulosa-Buchloe dactyloides <35
Texas savanna Prosopis glandulosa var. glandulosa <10 [56]
mountain grasslands Pseudoroegneria spicata 3-40 (=10) [2,3]
oak-juniper woodland (Southwest) Quercus-Juniperus spp. <35 to <200 [56]
blackland prairie Schizachyrium scoparium-Nassella leucotricha <10
Fayette prairie Schizachyrium scoparium-Buchloe dactyloides <10 [75]
little bluestem-grama prairie Schizachyrium scoparium-Bouteloua spp. <35 [56]
*fire return interval varies widely; trends in variation are noted in the species review

Surface rhizome/chamaephytic root crown in organic mantle or on soil surface
Ground residual colonizer (on-site, initial community)
Initial off-site colonizer (off-site, initial community)
Secondary colonizer (on-site or off-site seed sources)


SPECIES: Opuntia fragilis
The aboveground parts of brittle pricklypear are readily killed by even low-severity fire. Some fleshy pads may survive low- to moderate-severity fire when they are partially covered by litter or sheltered within a clump of stems. High-severity fire usually kills the entire plant [10,70].

No additional information is available on this topic.

Following mortality of aboveground tissues, brittle pricklypear grows new pads from buds in the root crown. New plants also develop from surviving pads that readily grow new roots whether the pads are detached or still attached to the parent plant [70].

No additional information is available on this topic.

Repeated fires can greatly reduce populations of Opuntia species. High fire frequency may eliminate brittle pricklypears from a site for many years until new plants reestablish from seeds or pads carried onto the site by birds or mammals [9,70].

In Washington, brittle pricklypear habitat has been greatly reduced due to development and forest expansion resulting from fire exclusion [77].


SPECIES: Opuntia fragilis
Stems, fruits, and seeds of brittle pricklypear may comprise an appreciable portion of the diet of at least 44 species of birds and mammals [8]. For example, a study near Flagstaff, Arizona, showed that brittle pricklypear and twist-spine pricklypear (Opuntia macrorhiza) were major food items for Botta's pocket gophers in winter and spring. Although use was less, the pocket gophers also consistently grazed pricklypears in summer and fall [7]. The pads of Opuntia species can be used as emergency forage for livestock after the spines have been singed off [34].

Brittle pricklypear provides food for cactus-feeding insects including moths, bugs, and beetles. For a list of insect species that graze brittle pricklypear, see [8,15,76].

Palatability/nutritional value: Brittle pricklypear is low in nutritional value for livestock [34].

Cover value: No information is available on this topic.

Brittle pricklypear has been recommended as a native species for roadside recovery plantings in shrublands of Nevada [66].

Humans eat the stems, fruits, and seeds of brittle pricklypear. The stems are usually roasted and peeled before being eaten. Fruits are eaten raw, dried or cooked and are often used to make jellies. Native Americans used the mucilaginous juice from the stems as a fixing agent for paints [68]. Seeds are roasted and ground into flour [8]. Brittle pricklypear has been used medicinally to sooth sore throats and relieve skin irritations [39].

Brittle pricklypear may increase in response to heavy grazing. In a Sandberg bluegrass grassland in southern British Columbia, brittle pricklypear was the dominant herbaceous cover species in heavily grazed pastures [72]. However, researchers are not sure if brittle pricklypear populations respond to a reduction in the preferred forage species or if grazing animals simply aid brittle pricklypear's spread and establishment by transporting the pads on their bodies [12].

Brittle pricklypear is susceptible to damage by several insects including the cochineal scale, the cactus bug, and several species of cactus borers [8,15,76].

Opuntia fragilis: References

1. Arno, Stephen F. 1979. Forest regions of Montana. Res. Pap. INT-218. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 39 p. [340]

2. Arno, Stephen F. 1980. Forest fire history in the Northern Rockies. Journal of Forestry. 78(8): 460-465. [11990]

3. Arno, Stephen F. 2000. Fire in western forest ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-vol. 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 97-120. [36984]

4. Arno, Stephen F.; Gruell, George E. 1983. Fire history at the forest-grassland ecotone in southwestern Montana. Journal of Range Management. 36(3): 332-336. [342]

5. Arno, Stephen F.; Wilson, Andrew E. 1986. Dating past fires in curlleaf mountain-mahogany communities. Journal of Range Management. 39(3): 241-243. [350]

6. Baisan, Christopher H.; Swetnam, Thomas W. 1990. Fire history on a desert mountain range: Rincon Mountain Wilderness, Arizona, U.S.A. Canadian Journal of Forest Research. 20: 1559-1569. [14986]

7. Bandoli, James H. 1981. Factors influencing seasonal burrowing activity in the pocket gopher, Thomomys bottae. Journal of Mammalogy. 62(2): 293-303. [55074]

8. Bare, Janet E. 1979. Wildflowers and weeds of Kansas. Lawrence, KS: The Regents Press of Kansas. 509 p. [3801]

9. Benson, Lyman. 1982. The cacti of the United States and Canada. Stanford, CA: Stanford University Press. 1044 p. [1513]

10. Benson, Lyman; Walkington, David L. 1965. The southern Californian prickly pears--invasion, adulteration, and trial-by-fire. Annals of the Missouri Botanical Garden. 52: 262-273. [5267]

11. Bernard, Stephen R.; Brown, Kenneth F. 1977. Distribution of mammals, reptiles, and amphibians by BLM physiographic regions and A.W. Kuchler's associations for the eleven western states. Tech. Note 301. Denver, CO: U.S. Department of the Interior, Bureau of Land Management. 169 p. [434]

12. Berry, Joni. 1977. Effects of grazing pressure on Opuntia populations. Proceedings, South Dakota Academy of Science. 56: 271-272. [5169]

13. Brown, David E. 1982. Great Basin conifer woodland. In: Brown, David E., ed. Biotic communities of the American Southwest--United States and Mexico. Desert Plants. 4(1-4): 52-57. [535]

14. Brown, Ray W. 1971. Distribution of plant communities in southeastern Montana badlands. The American Midland Naturalist. 85(2): 458-477. [546]

15. Burger, Jutta C.; Louda, Svata M. 1994. Indirect versus direct effects of grasses on growth of a cactus (Opuntia fragilis): insect herbivory versus competition. Oecologia. 99(1-2): 79-87. [55072]

16. Burkhardt, Wayne J.; Tisdale, E. W. 1976. Causes of juniper invasion in southwestern Idaho. Ecology. 57: 472-484. [565]

17. CalFlora. 2005. The CalFlora Database: Information on California plants for education, research and conservation, [Online]. Berkeley, CA: CalFlora (Producer). Available: http://www.calflora.org/ [2005, October 28]. [42048]

18. Collins, P. D.; Harper, K. T. 1982. Habitat types of the Curlew National Grassland, Idaho. Provo, UT: Brigham Young University, Department of Botany and Range Science. 46 p. [Editorial draft]. [663]

19. Cooper, Charles F. 1961. Pattern in ponderosa pine forests. Ecology. 42(3): 493-499. [5780]

20. Eyre, F. H., ed. 1980. Forest cover types of the United States and Canada. Washington, DC: Society of American Foresters. 148 p. [905]

21. Flora of North America Association. 2004. Flora of North America: The flora. [Online]. Flora of North America Association (Producer). Available: http://www.fna.org/FNA. [36990]

22. Floyd, M. Lisa; Romme, William H.; Hanna, David D. 2000. Fire history and vegetation pattern in Mesa Verde National Park, Colorado, USA. Ecological Applications. 10(6): 1666-1680. [37590]

23. Frego, Katherine A.; Staniforth, Richard J. 1986. Vegetation sequence on three boreal Manitoban rock outcrops and seral position of Opuntia fragilis. Canadian Journal of Botany. 64(1): 77-84. [55073]

24. Garrison, George A.; Bjugstad, Ardell J.; Duncan, Don A.; Lewis, Mont E.; Smith, Dixie R. 1977. Vegetation and environmental features of forest and range ecosystems. Agric. Handb. 475. Washington, DC: U.S. Department of Agriculture, Forest Service. 68 p. [998]

25. Gleason, Henry A.; Cronquist, Arthur. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. 2nd ed. New York: New York Botanical Garden. 910 p. [20329]

26. Gottfried, Gerald J.; Swetnam, Thomas W.; Allen, Craig D.; [and others]. 1995. Pinyon-juniper woodlands. In: Finch, Deborah M.; Tainter, Joseph A., eds. Ecology, diversity, and sustainability of the Middle Rio Grande Basin. Gen. Tech. Rep. RM-GTR-268. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station: 95-132. [26188]

27. Harper, Kimball T. 1959. Vegetational changes in a shadscale-winterfat plant association during twenty-three years of controlled grazing. Provo, UT: Brigham Young University. 68 p. Thesis. [45366]

28. Harrington, H. D. 1964. Manual of the plants of Colorado. 2d ed. Chicago: The Swallow Press, Inc. 666 p. [6851]

29. Heyerdahl, Emily K.; Berry, Dawn; Agee, James K. 1994. Fire history database of the western United States. Final report. Interagency agreement: U.S. Environmental Protection Agency DW12934530; U.S. Department of Agriculture, Forest Service PNW-93-0300; University of Washington 61-2239. Seattle, WA: U.S. Department of Agriculture, Pacific Northwest Research Station; University of Washington, College of Forest Resources. 28 p. [+ appendices]. Unpublished report on file with: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT. [27979]

30. Higgins, Jeremy J.; Larson, Gary E.; Higgins, Kenneth F. 2001. Floristic comparisons of tallgrass prairie remnants managed by different land stewardships in eastern South Dakota. In: Bernstein, Neil P.; Ostrander, Laura J., eds. Seeds for the future; roots of the past: Proceedings of the 17th North American prairie conference; 2000 July 16-20; Mason City, IA. Mason City, IA: North Iowa Area Community College: 21-31. [46489]

31. Hirsch, Kathie Jean. 1985. Habitat classification of grasslands and shrublands of southwestern North Dakota. Fargo, ND: North Dakota State University. 281 p. Dissertation. [40326]

32. Hitchcock, C. Leo; Cronquist, Arthur. 1973. Flora of the Pacific Northwest. Seattle, WA: University of Washington Press. 730 p. [1168]

33. Hulett, G. K.; Coupland, R. T.; Dix, R. L. 1966. The vegetation of dune sand areas within the grassland region of Saskatchewan. Canadian Journal of Botany. 44: 1307-1331. [43303]

34. Humphrey, Robert R. 1960. Forage production on Arizona ranges. V. Pima, Pinal and Santa Cruz Counties. Bulletin 502. Tucson, AZ: University of Arizona, Agricultural Experiment Station. 137 p. [4520]

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