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SPECIES:  Ambrosia deltoidea
Triangle bur ragweed flower spike. Image by Patrick J. Alexander, hosted by the USDA-NRCS PLANTS Database.


SPECIES: Ambrosia deltoidea
AUTHORSHIP AND CITATION: Marshall, K. Anna. 1994. Ambrosia deltoidea. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: []. Revisions: On 28 June 2018, the common name of this species was changed in FEIS from: triangle bursage to: triangle bur ragweed. Images were also added. ABBREVIATION: AMBDEL SYNONYMS: Franseria deltoidea Torr. [13] NRCS PLANT CODE: AMDE4 COMMON NAMES: triangle bur ragweed triangle bursage TAXONOMY: The scientfic name of triangle bur ragweed is Ambrosia deltoidea (Torr.) Payne [24]. It is a member of the aster family (Asteraceae). There are no recognized subspecies, varieties, or forms. LIFE FORM: Shrub FEDERAL LEGAL STATUS: No special status OTHER STATUS: NO-ENTRY


SPECIES: Ambrosia deltoidea
GENERAL DISTRIBUTION: Triangle bur ragweed occurs throughout much of the Sonoran Desert.  It extends southward from southwestern Arizona into Sonora and Baja California, Mexico [2,3,16,24,27].
Distribution of triangle bur ragweed in Arizona. Map courtesy of USDA, NRCS. 2018. The PLANTS Database. National Plant Data Team, Greensboro, NC [2018, June 28] [41].
   FRES30  Desert shrub
   FRES40  Desert grasslands


    7  Lower Basin and Range
   12  Colorado Plateau

   K042  Creosotebush - bursage
   K043  Paloverde - cactus shrub

   242 Mesquite

   506 Creosotebush-bursage
   508 Creosotebush-tarbush

Triangle bur ragweed is a dominant or codominant member of the Arizona
Upland Subdivision of the Sonoran Desert [5,8,16].  Although it mostly
occurs in the ecotone between upper and lower bajadas, it also grows on
upper bajadas, in lowland communities dominated by creosotebush (Larrea
tridentata), and in desert grassland communities [3,4,10,16,33].

In the ecotone between upper and lower bajadas, characterized by steep,
gravelly slopes, triangle bur ragweed occurs in a paloverde (Cercidium
spp.)-cacti-mixed scrub series and the desert scrub community type.
Associated species include yellow paloverde (C. microphyllum), saguaro
(Carnegiea gigantea), condalia (Condalia lycioides), ocotillo
(Fouquieria splendens), jatropha (Jatropha cardiophylla), and prickly
pear (Opuntia spp.) [3,8,16].

On upper bajadas, triangle bur ragweed is codominant with jojoba (Simmondsia
chinensis), yellow paloverde, mesquite (Prosopis juliflora), and
ironwood (Olneya tesota) [4].

In desert grassland communities that have been overgrazed, triangle
bur ragweed can be found among other invading perennial shrubs including
corvillea (Corvillea tridentata), yellow paloverde, jojoba, ocotillo,
wolfberry (Lycium spp.), acacia (Acacia spp.), canotia (Canotia
holacantha), velvet mesquite (Prosopis juliflora var. velutina), and
saguaro [33].

Publications listing triangle bur ragweed as a dominant or codominant
species include:

The Natural Vegetation of Arizona [19]
Sonoran Desertscrub [32]
Vegetation of the Santa Catalina Mountains: community types and
  dynamics [20]

Other species associated with triangle bur ragweed but not previously
mentioned are brittle bush (Encelia farinosa), false-mesquite
(Calliandra eriophylla), Berlandier wolfberry (Lycium berlandieri),
hedgehog cactus (Echinocereus engelmanii), ferocactus (Ferocactus
acanthodes), and white ratany (Krameria grayi).


SPECIES: Ambrosia deltoidea
IMPORTANCE TO LIVESTOCK AND WILDLIFE: According to Stubbendieck and others [28], triangle bur ragweed is worthless as forage for livestock. PALATABILITY: Triangle bur ragweed is unpalatable [31].  McAuliffe and others [43] report that it is highly unpalatable to mammalian herbivores. NUTRITIONAL VALUE: NO-ENTRY COVER VALUE: NO-ENTRY VALUE FOR REHABILITATION OF DISTURBED SITES: Triangle bur ragweed may stabilize critical areas such as surface-mined lands in arid regions.  It can be established by direct seeding or with containerized plants [30]. OTHER USES AND VALUES: The paloverde/triangle bur ragweed community is an aesthetically attractive plant community, and it is often preserved as a form of landscaping when urban development occurs [20]. OTHER MANAGEMENT CONSIDERATIONS: Domestic grazing has caused a decrease in grasses and subsequent lack of fuel for burning in the Sonoran Desert.  As a result, bursage (Ambrosia spp.) has increased in the Sonoran Desert [2].  During a 30-year period of protection from grazing, triangle bur ragweed decreased while perennial grasses and palatable shrubs increased [1].  Hessing and Johnson [11] found that removing woody plants, including triangle bur ragweed, contributed to greater herb growth. Because of its dense, low-lying canopy, triangle bur ragweed may protect the seedlings of other species from herbivory.  McAuliffe [17] found that rabbits consumed a greater proportion of seedlings in the open than under triangle bur ragweed. Triangle bur ragweed is the principal nurse plant for saguaro at Organ Pipe National Monument.  Its shade reduces maximum soil surface temperatures, and it also provides a microhabitat with elevated soil nitrogen levels [9].


SPECIES: Ambrosia deltoidea
GENERAL BOTANICAL CHARACTERISTICS: Triangle bur ragweed is a native, drought-deciduous, microphyllous shrub growing up to 1.65 feet (50 cm) tall [24, 29].  Innumerable slender and brittle branches spring up from the base forming a compact, roughly hemispherical crown [27, 35].  Mature triangle bur ragweed crowns contain many dead branches which are shed only as a result of weathering [27]. The branches and young leaves of triangle bur ragweed are tomentulose and resinous.  Leaves become glabrous above with age.  Leaf length is 0.6 to 0.8 inches (1.5-2 cm) or smaller [24,27]. Staminate heads are borne terminally.  Two-flowered, pistillate involucres are clustered below the staminate spikes or on lateral branches.  They produce globose, spiny achenes [24,27]. Triangle bur ragweed possesses one long taproot with well-developed laterals [35].  Triangle bur ragweed roots are slender, brittle, and longitudinally ridged with cork [6].  They are confined to the soil above the caliche hardpan--about 8 to 16 inches (20-40 cm) deep [6,35].  Besides roots arising from the main root, Cannon [6] found 50 adventitious roots about 0.12 inches (3 mm) in diameter which arose from the root crown, growing horizontally.  Filamentous rootlets formed on many roots of triangle bur ragweed after the soil had been moistened by rains, but such rootlets were short lived [6]. Triangle bur ragweed is relatively short lived for a desert shrub.  It has an observed longevity of about 50 years [10,27]. RAUNKIAER LIFE FORM: Phanerophyte REGENERATION PROCESSES: Triangle bur ragweed reproduces sexually.  Seed is produced abundantly, and seedlings establish in open space.  In the Sonoran Desert, the relative abundance of triangle bur ragweed seedlings was some 45 times the density of mature triangle bur ragweed plants.  The density of mature triangle bur ragweed was similar to that of mature creosotebush, but 1,001 triangle bur ragweed seedlings were counted while only 124 creosotebush seedlings were found [17]. Triangle bur ragweed seeds are spiny and are probably dispersed by mammals. One study reports that triangle bur ragweed has very limited ability to sprout after top-kill [18].  No other research describes the ability of triangle bur ragweed to sprout. Triangle bur ragweed reproduces from cuttings with the application of auxin [15]. SITE CHARACTERISTICS: Triangle bur ragweed commonly grows in open spaces [17] on pediments, upper bajada surfaces, and basin floors on a variety of substrates including volcanic, granitic, metamorphic, alluvial, and caliche soils.  There was no significant difference in cover of triangle bur ragweed among volcanic, granitic, metamorphic, and alluvial substrates [21]. Triangle bur ragweed is a frequent dominant on gravel outwashes, bajadas, and the ecotone between upper and lower bajadas [3,17,35].  Triangle bur ragweed occurs almost entirely on very coarse soils that are relatively uniform in composition with little differentiation into horizons [3,14,15], although it occurs rarely on sand [27].  Precipitation percolates rapidly through the soil and drains quickly away along the underlying caliche layer [1,35]. Rainfall averages 4 to 12 inches (100-300 mm) annually with a bimodal distribution [16,25].  Temperatures at Organ Pipe National Monument and elsewhere in the Sonoran Desert fluctuate daily and seasonally.  The mean frost frequency at the monument from 1956 to 1985 was 17 frosts per year (standard deviation=7, range=5-34) [21]. Parker [22] studied the soil characteristics of sites dominated by triangle bur ragweed at Organ Pipe National Monument.  Overall, triangle bur ragweed was most abundant on flat terrain, but its cover was reduced on the lower bajadas and in the western portion of the Monument where pH was relatively high [22].  In the Ajo Mountains on south-facing slopes, sites had low pH and high available magnesium.  In the Blanco Mountains, Senita Basin, and Quitobaquito Hills, available magnesium was low.  In the Bates Mountains, Puerto Blanco Mountains, lower elevations of the Ajo Mountains, and flats in Senita Basin, triangle bur ragweed grew on north-facing slopes in volcanic soil.  In the Sonoyta Mountains, triangle bur ragweed grew on northwest- to northeast-facing slopes.  Leitner [14] found that triangle bur ragweed preferred the cooler, more humid conditions of north-facing slopes at Punta Cirio, Sonora, Mexico, although it occurred on both north- and south-facing slopes.  Triangle bur ragweed grows from 1,000 to 3,000 feet (300-900 m) [13]. SUCCESSIONAL STATUS: Triangle bur ragweed is often the first plant to occupy an open area. Recruitment of other species rarely occurs in the open.  Once established, triangle bur ragweed acts as a nurse plant for other species, providing improved microhabitat and protection from herbivory [17,9]. Most mature triangle bur ragweed plants have an associated perennial. Eighty-two out of 103 triangle bur ragweed plants lacking associated perennials were small (and presumably young)--2 inches (5 cm) or less in root crown diameter.  Only 2 of 101 triangle bur ragweed plants that had other perennials associated with them had basal diameters of 2 inches (5 cm) or less [17]. Succession in the desert is difficult to characterize.  Although changes in relative abundances of species may occur, plants rarely disappear from the ecosystem altogether because severe disturbance is usually lacking [17].  McAuliffe [17] described changes in desert vegetation in terms of community dynamics instead of succession.  For instance, in communities codominated by triangle bur ragweed, triangle bur ragweed colonizes open space and acts as a nurse plant for other perennial shrubs.  Some larger, presumably older individuals within the population bear no evidence of former associations with triangle bur ragweed, probably because they have outlived it.  Triangle bur ragweed continues to colonize other open space.  Relative abundance of species may be altered, but triangle bur ragweed presence is maintained [17]. Triangle bur ragweed is known to establish on overgrazed desert grasslands. Tueller [31] suggested that partial or complete protection from grazing on triangle bur ragweed-invaded rangeland will promote secondary succession. Secondary succession results in the establishment of climax grasses and palatable shrubs and a decrease in unpalatable shrubs such as triangle bur ragweed. When growing among creosotebush and jumping cholla (Opuntia fulgida), triangle bur ragweed occupies a distinct root zone so interspecific competition is minimized [35].  In one study, no roots of neighboring plants grew near triangle bur ragweed roots [6]. SEASONAL DEVELOPMENT: Most new leaves on triangle bur ragweed develop and mature during the winter and early spring [29].  Triangle bur ragweed flowers from February to July [33].  Seeds are produced after both summer and winter rains [27].


SPECIES: Ambrosia deltoidea
FIRE ECOLOGY OR ADAPTATIONS: Because triangle bur ragweed establishes in open spaces, it survives fires that burn with a mosaic pattern [26]. Although triangle bur ragweed seedlings establish after fire, the literature does not mention whether the seed originates on- or off-site. Fires occur frequently in the desert grassland at the eastern margin of the Sonoran Desert.  Fires in the desert proper, however, are infrequent and generally of low severity because production of annual and perennial herbs seldom provides a fuel load capable of sustaining fire [18]. Fires occur occasionally in the Sonoran Desert, mostly in the Arizona Upland where the biannual rainfall produces relatively lush rangeland [23,26].  Following a moist year, the desert floor is covered with sufficient annual plant fuel and litter to carry a fire [23].  When winter annuals thrive, fire is more likely [2,17].  Exotic annuals may be more productive than native species and fuel more frequent, more severe fires [2]. FIRE REGIMES: Find fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes". POSTFIRE REGENERATION STRATEGY: NO-ENTRY


SPECIES: Ambrosia deltoidea
IMMEDIATE FIRE EFFECT ON PLANT: Triangle bur ragweed is usually top-killed or killed by fire [18,26]. DISCUSSION AND QUALIFICATION OF FIRE EFFECT: NO-ENTRY PLANT RESPONSE TO FIRE: Burning generally decreases bursage (Ambrosia spp.) cover [2,7,18,26,34]. It is not clear whether top-killed triangle bur ragweed sprout after fire. One study found 1 percent of top-killed triangle bur ragweed sprouting 4 years after a fire [18].  No other research describes the ability of triangle bur ragweed to sprout. Triangle bur ragweed produces a large amount of seed, and seedlings establish after fire.  Following fires that occurred in 1974 in south-central Arizona, no triangle bur ragweed sprouted, but several survived in unburned patches.  Density of triangle bur ragweed in postfire year 1 was 275 plants per 1,500 square meters; 3 to 4 years later, density was 49 plants per 1,500 square meters.  Eighty-two percent of all seedlings were triangle bur ragweed [26]. Desert fires reduce perennial plant cover, often for several years [26]. In an Upper Sonoran Desert site on the Tonto National Forest, prefire coverage of perennial plants, including triangle bur ragweed, was 30.7 percent.  Perennial cover immediately after fire was 9.3 percent.  The physical characteristics of the study site, including soil surface albedo, microsite maximum and minimum temperatures, and soil water repellency were not greatly altered [23]. The killed portion of a woody, desert plant such as triangle bur ragweed usually represents many years of growth.  Repeated fires, even when they do not kill woody taxa outright, keep them in a juvenile, nonfruiting stage [12]. DISCUSSION AND QUALIFICATION OF PLANT RESPONSE: A low-severity lightning fire occured in late June of 1979 in the Sonoran Desert near Florence, Arizona.  The prefire vegetative community was typical of the Arizona Upland Subdivision of the Sonoran Desert. The dominant species were triangle bur ragweed, yellow paloverde, creosotebush, saguaro, buckhorn cholla (Opuntia acanthocarpa), hedgehog (Echinocereus fasciculatus), barrel cactus (Ferocactus wislizenii), and prickly pear (Opuntia phaeacantha).  Small patches of vegetation were skipped throughout the burn.  With the exception of these skips, nearly all trees and shrubs were completely top-killed [18]. The density of triangle bur ragweed 19 months after the fire was 540 plants per hectare, compared to 6,790 plants per hectare in the unburned control area.  Percent cover of triangle bur ragweed was 0.7 percent in burned areas and 7.4 percent in control areas.  Ninety-three percent of all triangle bur ragweed plants in burned areas were top-killed; overall mortality of triangle bur ragweed was 92 percent.  By 1981, only 1 percent of top-killed triangle bur ragweed plants were sprouting [18]. FIRE MANAGEMENT CONSIDERATIONS: McLaughlin and Bowers [18] hypothesized that two consecutive wet winters may be required for the development of a fuel load adequate to sustain fire in the Sonoran Desert.  The first wet winter would result in higher production of annuals and the addition of large numbers of seeds to the soil.  The second wet winter would facilitate sprouting of the increased number of seeds and production of enough annuals to sustain fire. Native American Hohokam farmers (A.D. 11150-1350) cleared fields, ditches, and broad patches of the desert by fire.  Hohokam farmers found that burning could increase the variety of plants available to be gathered and mammals to be hunted.  The fossil record shows about 40 percent bursage (Ambrosia spp.) pollen in Hohokam times compared with 74 percent in modern times [2]. Desert fires may create potential soil stability problems [23].


SPECIES: Ambrosia deltoidea
FIRE CASE STUDY CITATION: Marshall, K. Anna., compiler. 1994. Effects of prescribed and wildfires on triangle bur ragweed on the Tonto National Forest, Arizona. In: Ambrosia deltoidea. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: []. REFERENCES: Cave, George Harold, III. 1982. Ecological effects of fire in the upper Sonoran Desert. Tempe, AZ: Arizona State University. 124 p. Thesis. [7]. Patten, Duncan T.; Cave, George H. 1984. Fire temperatures and physical characteristics of a controlled burn in the upper Sonoran Desert. Journal of Range Management. 37(3): 277-280. [23]. SEASON/SEVERITY CLASSIFICATION: summer/low-severity STUDY LOCATION: The study was located in Bulldog Canyon in Tonto National Forest, Arizona, at 33 degrees 15 minutes north latitude and 111 degrees 33 minutes west longitude. PREFIRE VEGETATIVE COMMUNITY: The prefire vegetative community was typical of the Upper Sonoran Desert paloverde-cactus-shrub association (Cercidium spp.-Opuntia spp. and Carnegiea gigantea-Ambrosia spp.).  Plants occupied about one-third of the total ground cover.  Triangle bur ragweed (A. deltoidea) occupied about 15 percent of the total ground cover.  Annual forbs and grasses in this association are abundant after winter and heavy summer rains, providing enough fuel to carry a fire. TARGET SPECIES PHENOLOGICAL STATE: At the time of the fire, triangle bur ragweed would have been flowering and/or fruiting. SITE DESCRIPTION: Topography is flat except for one small, dry wash bisecting the site.  The soil is composed of sandy loam argids with a desert pavement surface.  Elevation at the site is 1,485 feet (450 m). The burn site is adjacent to an area burned by wildfire on April 26, 1980.  The 1980 wildfire may have been more severe since both 1979 and 1980 were years of above average precipitation, and the standing herbaceous vegetation was probably relatively lush.  The study compares the prescribed fire and the wildfire sites. FIRE DESCRIPTION: The fire was ignited on June 12, 1981.  Conditions were typical for summer months in the Upper Sonoran Desert.  Air temperatures ranged from 104 degrees Fahrenheit (40 deg C) in the shade to 132.8 degrees Fahrenheit (56 deg C) 0.4 inches (1 cm) above the unshaded soil surface. Relative humidity remained at 29 percent during the fire.  Mean air movement for the duration of the fire was low at 0.001 meters per second.  Mean wind velocity for gusts was 2.75 meters per second.  Mean soil moisture percent in the upper 2 inches (5 cm) of soil was 0.61 percent in open areas and 0.80 percent in shaded areas.  Litter fuel averaged 143.3 grams per square meter. FIRE EFFECTS ON TARGET SPECIES: In 1981, 1 year after the wildfire and immediately following the controlled burning, the density of triangle bur ragweed was greater on the wildfire site than on the prescribed fire site, mainly because of the establishment of triangle bur ragweed seedlings on the wildfire site. Postfire cover measurements were nearly the same. Triangle bur ragweed density was reduced by 82 percent on the prescribed fire site immediately after the controlled burning.  The prefire density of triangle bur ragweed on the prescribed fire site was 6,275 plants per hectare.  Immediately after the controlled burning, triangle bur ragweed density was 1,141 plants per hectare.  One year later, triangle bur ragweed density had not changed significantly. FIRE MANAGEMENT IMPLICATIONS: NO-ENTRY


SPECIES: Ambrosia deltoidea
REFERENCES:  1.  Blydenstein, John; Hungerford, C. Roger; Day, Gerald I.; Humphrey, R.        1957. Effect of domestic livestock exclusion on vegetation in the        Sonoran Desert. Ecology. 38(3): 522-526.  [4570]  2.  Bohrer, Vorsila L. 1992. New life from ashes II: A tale of burnt brush.        Desert Plants. 10(3): 122-125.  [18805]  3.  Bowers, Michael A. 1988. Plant associations on a Sonoran Desert bajada:        geographical correlates and evolutionary source pools. Vegetatio. 74:        107-112.  [4408]  4.  Brooks, William H. 1978. Jojoba--a North American desert shrub; its        ecology, possible commercialization, & potential as an introd. into        other arid regions. Journal of Arid Environments. 1: 227-236.  [5162]  5.  Burgess, Tony L.; Bowers, Janice E.; Turner, Raymond M. 1991. Exotic        plants at the Desert Laboratory, Tucson, Arizona. Madrono. 38(2):        96-114.  [15362]  6.  Cannon, William Austin. 1911. The root habits of desert plants.        Washington, DC: The Carnegie Institution of Washington. 96 p.  [5003]  7.  Cave, George Harold, III. 1982. Ecological effects of fire in the upper        Sonoran Desert. Tempe, AZ: Arizona State University. 124 p. Thesis.        [12295]  8.  Daniel, Thomas F.; Butterwick, Mary L. 1992. Flora of the South        Mountains of south-central Arizona. Desert Plants. 10(3): 99-119.        [19896]  9.  Franco, A. C.; Nobel, P. S. 1989. Effect of nurse plants on the        microhabit and growth of cacti. Journal of Ecology. 77: 870-886.  [9766] 10.  Goldberg, Deborah E.; Turner, Raymond M. 1986. Vegetation change and        plant demography in permanent plots in the Sonoran Desert. Ecology.        67(3): 695-712.  [4410] 11.  Hessing, M. B.; Johnson, C. D. 1982. Disturbance and revegetation of        Sonoran Desert vegetation in an Arizona powerline corridor. Journal of        Range Management. 35(2): 254-258.  [4320] 12.  Humphrey, Robert R. 1963. 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Fort Collins, CO: U.S. Department of        Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment        Station: 15-19.  [16036] 27.  Shreve, Forrest; Wiggins, Ira L. 1964. Vegetation and flora of the        Sonoran Desert. Stanford, CA: Stanford University Press. 1575 p.  [4595] 28.  Stubbendieck, J.; Hatch, Stephan L.; Hirsch, Kathie J. 1986. North        American range plants. 3rd ed. Lincoln, NE: University of Nebraska        Press. 465 p.  [2270] 29.  Szarek, S. R.; Woodhouse, R. M. 1976. Ecophysiological studies of        Sonoran Desert Plants. II. Diurnal photosynthesis patterns of Ambrosia        deltoidea and Olneya tesota. Oecologia. 26: 225-234.  [4319] 30.  Thornburg, Ashley A. 1982. Plant materials for use on surface-mined        lands. SCS-TP-157. Washington, DC: U.S. Department of Agriculture, Soil        Conservation Service. 88 p.  [3769] 31.  Tueller, Paul T. 1976. Secondary succession, disclimax, and range        condition standards in desert scrub vegetation. In: Hyder, D. N., ed.        Arid shrublands--Proceedings of the third workshop of the United        States/Australia rangelands panel; 1973 March 26 - April 5; Tucson, AZ.        Denver, CO: Society for Range Management: 57-65.  [3790] 32.  Turner, Raymond M.; Brown, David E. 1982. Sonoran desertscrub. In:        Brown, David E., ed. Biotic communities of the American        Southwest--United States and Mexico. Desert Plants. 4(1-4): 181-221.        [2375] 33.  Whitfield, Charles J.; Anderson, Hugh L. 1938. Secondary succession in        the desert plains grassland. Ecology. 19(2): 171-180.  [5252] 34.  Whysong, Gary L.; Heisler, Michael H. 1978. Nitrogen levels of soil and        vegetation in the upper Sonoran Desert as affected by fire. In: Hyder,        Donald N., ed. Proceedings, 1st international rangeland congress; 1978        August 14-18; Denver, CO. Denver, CO: Society for Range Management:        697-699.  [3990] 35.  Yeaton, Richard I.; Travis, J.; Gilinsky, Ellen. 1977. Competition and        spacing in plant communities: the Arizona upland association. Journal of        Ecology. 65: 587-595.  [4193] 36.  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] 37.  Garrison, George A.; Bjugstad, Ardell J.; Duncan, Don A.; [and others].        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] 38.  Kuchler, A. W. 1964. Manual to accompany the map of potential vegetation        of the conterminous United States. Special Publication No. 36. 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