Southwest Center for Biological Diversity
P.O. Box 742
Silver City, NM 88062
ph: 505-538-0961
fax: 505-538-3540
email: swcbd@igc.apc.org
Southwest Forest Alliance
P.O. Box 9314
Santa Fe, NM 87504
505-983-4609
505-983-2355
May 9, 1995
TO: Charles Cartwright
Regional Forester
FROM: Grazing and Forest Activists
RE: Livestock Grazing is a Major Cause of Forest Health Problems
Dear Mr. Cartwright,
The existence of a forest health crisis has become the favored rhetoric of
the Forest Service and the
timber industry in supporting increased logging in the western United
States. The public is being
inundated with messages to the effect that if a careless spark doesn't bring
on the apocalypse, bark
beetles, disease or mistletoe will. The Forest Service identifies
overstocking caused by its past failure
to recognize the essential role of fire in forest ecosystems as the problem.
Active fire suppression, however, is certainly not the sole cause of forest
health problems. In
particular, the scientific literature abounds with studies demonstrating
that cattle are a major cause of
overstocking and forest health problems. Unless, and until, overgrazing of
the National Forests ends,
no amount of logging will return our forests to health.
Contrary to current Forest Service claims, the agency has for many years
been well aware that fire
suppression is a threat to forest health. As early as 1924 Aldo Leopold,
while working for the Forest
Service in the Southwest, warned that fire suppression was causing unhealthy
overstocking. By 1950,
Harold Weaver, Regional Forester for the U.S. Indian Service in Phoenix,
reported that Leopold's
predictions were already evident upon the land. He warned that if foresters
continued to focus their
attention solely on harvesting old growth ponderosa pine, "the ship of
forest management" would run
aground on the "shoals and reefs" of fire suppression. Unfortunately,
Forest Service policy makers
willfully ignored the multitude of warnings.
We stress the fact that fire suppression was not a problem of ignorance but
of denial because the Forest
Service is repeating this pattern with regard to livestock grazing. With
this letter we bring your
attention to the well documented role of livestock grazing in the current
forest health "crisis." In
addressing the problems facing the nation's forests, it is imperative that
grazing be accorded a
fundamental place in proposed management solutions. Continued denial is
simply not a viable
alternative.
The Science
The Forest Service frequently cites scientific studies showing that
livestock grazing has contributed to
an increase in woody plants and shrubs (including larger plants such as
pinyon and juniper) across the
western United States (see reviews by Arnold 1950; Ellison 1960; Bahre
1991). Also well
documented, but seemingly ignored, are studies showing that cattle grazing
has also significantly
contributed to dense stands of "dog hair" thickets in ponderosa pine
forests. Scientists have found that
livestock grazing causes overstocking of trees by removing grasses which
would otherwise prevent
seedling establishment through a variety of mechanisms including:
competition with pine seedlings for space, water and nutrients;
poisoning of pine seedlings;
acting as a fire vector, allowing low temperature surface fires to spread
widely.
Cows directly facilitate pine invasions by:
fostering the creation of seedbeds by exposing soil and/or by destroying
cryptogamic (biotic) soil crusts;
inducing arroyos which dry out wet meadows and allow pine invasion.
Aldo Leopold (1924) was among the first to recognize that overgrazing
results in reduced fire
frequency. Indeed, some foresters of the day (and in some cases still)
encouraged grazing so that the
herbaceous fuel load would decrease and denuded areas would act as
firebreaks (Leopold, 1924). At
about the same time, pioneering southwest silviculturalist G.A. Pearson
(1923) noted that ponderosa
seedlings were more likely to establish and grow vigorously where herbaceous
vegetation was heavily
grazed than where grass was thick. He noted that the invasion of heavily
grazed areas by pine
seedlings was conspicuous in northern Arizona.
In a series of papers published in the 1940s and '50s, Harold Weaver, whose
experience with the
subject of fire suppression was formulated as Forest Supervisor on the
Colville Indian Reservation in
eastern Washington then later as Regional Forester with the US Indian
Service in Phoenix, built up to
the connection between livestock grazing and ponderosa thickets, stating
that overgrazing was "of great
significance in development of such stands, through breaking up of original
sod cover and preparation
of mineral seedbeds. Such appears to be of particular significance in the
Southwestern forests."
Weaver warned that the future of these forests was in jeopardy, asking
"Shall we attempt to reduce the
hazard in the dense stands, or shall we ask for larger and ever larger [fire
suppression] budgets?"
Today's overstocked forests are a result of the Forest Service choosing the
latter.
Cooper (1960) echoes Weaver's concerns, stating that "The overuse and
mismanagement which
followed introduction of livestock into the West produced profound changes,
some of them permanent,
in the plant cover. In particular, overgrazing has often been assigned as
the primary cause of the
overabundance of young pines. Many of the early arguments against reduction
of grazing on the
national forests were based on the premise that heavy grazing made forest
fires much less frequent."
Similarly, Ellison (1960) describes the process whereby grazing allows the
introduction of woody
plants, shrubs, and trees (including ponderosas) into rangelands and
forests. He surmises that the
invasion of grasslands by trees in the broad, irregular transition from
forest to prairie in the Middle
West may have been caused by a factor seldom suggested or studied:
overgrazing by livestock.
While the ecological processes involved in overstocking are relatively
simple, the question of which
variable, grazing or active fire suppression, is the primary cause of
overstocking long vexed scientists
because it was (and still is) difficult to find an ungrazed ponderosa pine
forest for a control plot. In
1923 Aldo Leopold wrote "Whether grass competitors or fire was the principle
deterrent to timber
reproduction is hard to answer because the two factors were always paired,
never isolated. Probably
either would have inhibited extensive reproduction." We know of three field
studies which were able
to isolate the "paired" variables: these studies have identified grazing
as the principle factor in causing
overstocking.
Rummell (1951) studied two neighboring and ecologically similar ponderosa
pine forests in central
Washington. One had been grazed for 40 years while the other had never been
grazed by livestock.
Neither had experienced a fire for at least 125 years. Rummell
characterized the ungrazed forest as
"one of the few relicts of virgin ponderosa pine forest and range" with "an
almost unbelievably lush
mat" of pine grass. It could be, Rummell felt, "a top standard against
which many other pine forest
ranges with similar climate and physiography can be compared."
Significantly, very little pine reproduction was found: only 85 trees per
acre less than 4' dbh were
present. In sharp contrast, the grazed forest had little grass and 3,291
trees per acre less than 4' dbh
(2,033 pines, 1,016 Doug firs, and 242 Western larch).
Rummell concluded that the high density of herbaceous understory vegetation
on the ungrazed forest
contributed substantially to the low tree reproduction rate. Overall, "heavy
grazing of the herbaceous
understory vegetation, rather than exclusion of fire, appeared to be the
prime factor in explaining the
dense tree reproduction" on the grazed forest (emphasis added).
A similar study was conducted in 1983 on two areas in Zion National Park
"almost identical biotically
and environmentally." They had similar fire histories but markedly
different grazing histories. The
Horse Pasture Plateau was heavily grazed until about 1960, while Greatheart
and Church Mesas -
isolated by cliffs and slickrock - were never grazed and approximated
pre-settlement conditions. Both
areas were dominated by Ponderosa pine and Gambel oak, and both had a
similar history of browsing
by large ungulates.
The authors characterize the study area as "uniquely suited to allow
independent assessment of the
relative importance of fire cessation and livestock grazing in the
conversion of savannas to forests."
They state that the visual contrast between the areas was obvious, with
dense thickets of Ponderosa,
Gambel oak, and Rocky Mountain Juniper saplings prevalent on the grazed
plateau and largely absent
on the ungrazed mesas. They note that fire was excluded from all the study
areas but state that
decreased fire frequency alone is not the essential factor needed to cause
these
physiognomic changes. The presence of savanna conditions on Church and
Greatheart
Mesas despite long fire-free intervals is the strongest evidence yet for our
contention....The fact that there were no thickets of "dog-hair" ponderosa
pine on either
mesa despite a comparable absence of fire, implicates livestock grazing as
the critical
factor (emphasis added).
The authors implicated biotic factors as also being important in limiting
pine regeneration. Quoting
studies showing that various native grasses release toxins (allelopaths)
which kill ponderosa pine
seedlings (Jameson 1968; Rietveld 1975), they concluded that a combination
of competition for soil
moisture and allelopathy played an important role. Fire, they say, "may
have been the most important
secondary factor for the maintenance of savanna conditions (emphasis
added)." They go further,
concluding that:
Our findings challenge the widely accepted notion that the high frequency
of fires in
ponderosa pine savanna was the prime cause for the prevention of
succession to denser
stands of Ponderosa pine or to shade-tolerant but fire-sensitive
conifers.... However
important fire may be for management and maintenance of ponderosa pine
communities,
the key factor in the widespread conversion of savanna to forest seems to
have been
livestock grazing (emphasis added).
The study showed that after the removal of livestock from Horse Pasture
Plateau in 1960, the
establishment rate for ponderosa pine dropped precipitously. Importantly,
this indicates that with the
removal of cows, the potential for natural restoration, via a decrease in
tree density, is possible.
The third study (Zimmerman and Neuenschwander 1984) looked at grazed and
ungrazed ponderosa
pine and douglas-fir stands in the foothills of the Bitteroot Mountains of
Idaho. They found the grazed
stands of both species had more young trees. As did the previous authors,
Zimmerman and
Neuenschwander concluded that "livestock grazing was probably the principal
factor in creating and
maintaining conditions that favored increased tree regeneration." They also
predicted that if the grazed
stands did not burn, they could "stagnate, causing reductions in growth
rates and increased
susceptibility to damage from insects and disease." This study is of great
significance in that their
results suggest that if grazing is excluded and cool fires allowed to burn,
the forests can once again
approach their original healthy state.
Conclusion
While this information has been available for half a century, its
implications have not been expressed
in Forest Service policy for any of the Southwest forest types including
ponderosa pine, mixed conifer
or pinyon-juniper (which is not reviewed here because the overstocking in
this case is almost common
knowledge). Indeed, a review of recent Forest Service publications could
find no mention of grazing
as a factor in decreased forest health (Belsky, 1995). It is imperative,
then, that whatever the next
stages of the forest health debate include, the role of cattle as a causal
and perpetuating factor in forest
health problems be addressed, as opposed to relying on an ecologically
damaging salvage program.
While Forest Service and industry calls for logging ring loud and clear,
calls for reduction of cattle
numbers as a way to ensure that another crisis does not arise in the future
are conspicuously absent. If
the right management choices are made, these forests have the ability to
recover from their current
overstocked condition. The right choice is to decrease grazing on our
National Forests.
Moreover, it is clear that grazing is not just a range issue. It is a
silvicultural problem and therefore
must be analyzed in timber sales, salvage sales, juniper pushes and other
silvicultural activities. We
would like a commitment that the Region will analyze the effects of grazing
in the timber context and
not just as an issue to be dealt with in AMPs and term grazing permits. Will
the Region address this
problem in current management strategies designed to counter the forest
health crisis? Will the Region
address grazing as a cumulative effect in timber sales and other
silvicultural projects?
We would appreciate a reply to our queries prior to our next quarterly
meeting, so that at that meeting
we may have a substantial and informed two-way discussion, rather than a
one-way conversation. We
invite you to call if you have any questions. An extensive forest
health/grazing bibliography, which
includes the literature cited, is enclosed.
Sincerely,
Dennis Morgan, Research Associate
Southwest Center for Biological Diversity
Kieran Suckling, Executive Director
Southwest Center for Biological Diversity
Sharon Galbreath, Chairperson
Grand Canyon Chapter, Sierra Club
Samuel Hitt, Director
Forest Guardians
Joannie Berde
Carson Forest Watch
Tom Ribe
Public Forestry Foundation
Tom H. Wootten, Conservation Chair
Mesilla Valley Audubon Society
Mary Lou Jones
Zuni Mountain Coalition
Joseph Feller
cc: Forest Supervisors, District Rangers (via Forest
Supervisors); Forest Researchers; Southwest
Congressional Delegation
Dave Henderson
Southwest Forest Alliance
Charles Babbitt, President
Maricopa Audubon Society
John Talberth, Executive Director
Forest Conservation Council
Jim Powers
Prescott National Forest Friends
Gary Simpson
Northern New Mexico Chapter
Wilderness Watch
Eleanor G. Wootten, Vice President
T & E, Inc.
Gwen Wardwell, Director
Rio Grande Chapter, Sierra Club
Mike Siedman
Jeff Burgess A P ARTIAL GRAZING/FOREST HEALTH BIBLIOGRAPHY
(I ncluding Literature Cited)
Allen, B.H., and J.W. Bartolome. 1989. Cattle grazing effects on understory
cover and tree
growth in mixed conifer clearcuts. Northwest Science 63:214-220.
Archer, S. and F.E. Smeins. 1991. Ecosystem-level processes. Pages 109-139
in R.K.
Heitschmidt and J.W. Stuth, editors. Grazing management: an ecological
perspective.
Timber Press, Portland, Oregon.
Arnold, Joseph F., 1950. Changes in Ponderosa Pine Bunchgrass Ranges in
Northern Arizona
Resulting from Pine Regeneration and Grazing. J. Forestry 48: 118-126.
Bahre, Conrad Joseph, 1991. A Legacy of Change - Historic Human Impact on
Vegetation of the Arizona Borderlands. Tucson, University of Arizona
Press 231p.
Bahre, Conrad J., 1985. Wildfire in Southeastern Arizona Between 1859 and
1890. Desert
Plants 7(4): 190-194
Baumgartner, D. and J.E. Lotan, eds. 1987. Proceedings of a symposium:
ponderosa pine - the
species and its management. Spokane, Washington.
Belsky, Joy and Dana Blumenthal. 1995. Ecological Assessment of Livestock
Grazing in
Forests of the Western Interior United States. Paper prepared for the
Eastside Ecosystem
Management Project (US Forest Service). Submitted 14 April 1995. The
report is being
condensed for submission to Conservation Biology.
Blackburn, W.H. 1984. Impacts of grazing intensity and specialized grazing
systems on
watershed characterisitics and responses. Pages 927-983 in Developing
strategies for
rangeland management. Westeview Press, Boulder, Colorado.
Blackburn, Wilbert H. and Paul T. Tueller, 1970. Pinyon and Juniper
Invasion in Black
Sagebrush Communities in East-Central Nevada. Ecology 51(5) pp. 841-848
Brawn, J.D. and R.P. Balda. 1988. The influence of silvicultural activity
on ponderosa pine
forest bird communities in the Southwestern United States. IN Bird
Conservation, Vol. 3,
G.A. Jackson, ed. International Council for Bird Protection and the
University of Wisconsin
Press.
Campbell, R.R. 1954. Fire in relation to forest grazing. Unasylva 8:154-158.
Chandler, R.F. Jr. 1940. The influence of grazing upon certain soil and
cliamtic conditions in
farm woodlands. Journal of the American Society of Agronomy 32:216-230
Clary, W.P., 1975. Range Management and its Ecological Basis in the
Ponderosa Pine Type
of Arizona: The Status of Our Knowledge. USDA Forest Service Research
Paper RM-158
35p. Rocky Mt. For and Range Exp. Stn., Fort Collins, Colo. 80521
Cooper, C.F., 1960. Changes in Vegetation, Structure, and Growth of
Southwestern
Pine Forests since White Settlement. Ecol. Monog. 30(2): pp. 129-164
Cottam, W.P. 1976. The impact of man on the flora of the Bonneville Basin.
Research Paper Number 76-1, Department of Geography, University of Utah,
Salt Lake City,
Utah, USA.
Covington, W. W. and M. Moore, 1994. Southwestern Ponderosa Forest
Structure. Journal of
Forestry, January 1994. p. 39-47
Covington, W.W. and M.M. Moore, 1992. Postsettlement Changes in Natural Fire
Regimes: Implications for Restoration of Old Growth Ponderosa Pine
Forests. In Old
Growth forests in the Southwest and Rocky Mountain regions: Proceedings
of a workshop,
p. 81-99. USDA For Serv. Gen Tech. Rep. RM-213. 201p.
Covington, W.W. and L.F. DeBano. 1988. Effects of fire on pinyon-juniper
soil. IN
Management of Southwestern Natural Resources, USDA Forest Service.
Covington, W.W. and S.S. Sackett. 1988. Fire effects on ponderosa pine
soils and their man-
agement implications. in Management of Southwestern Natural Resources, USDA
Forest Ser-
vice.
Currie, P.O. 1987. Grazing in ponderosa pine forests. Pages 193-200 in D.
Baumgartner, and
J.E. Lotan, eds. Proceedings of a symposium: ponderosa pine - the species
and its
management. Spokane, Washington.
Ellison, Lincoln, 1960. Influence of Grazing on Plant Succession of
Rangelands. The
Botanical Review V26(1) Jan-Mar 1960 p. 1-78
Evans, R.A. 1988. Management of Pinyon-Juniper Woodlands. Gen. Tech. Rep.
INT-249.
USDA-FS.
Faulk, O.B. 1970. Arizona: A Short History. University of Oklahoma Press,
Norman, OK.
Filip, G.M. 1994. Forest Health decline in central Oregon: a 13-year case
study. Northwest
Science 68(4): 233-240.
Fitzgerald, R.D., R.J. Hudson, & A.W. Bailey. 1986. Grazing preferences of
cattle in
regenerating aspen forest. J. of Range Manage. 39(1):13-18.
Fitzgerald, R.D. and A.W. Bailey. 1984. Control of aspen regrowth by
grazing with cattle. J.
of Range Manage. 37(2):156-158.
Fleischner, Thomas L. 1994. Ecological Costs of Livestock Grazing in
Western North
America. Conservation Biology 8(3): 629-644.
Galbraith, W.A. and E.W. Anderson. 1991. Grazing history of the Northwest.
Rangelands
13(5):213-218.
Graham, Rusell T, James Kingery and Leonard A. Volland. 1992. Livestock
and Forest
Management Interactions. in Silvicultural Appraoches to Animal Damage
Management in
Pacific Northwest Forests. GTR PNW-287. USDA Forest Service, Pacific
Northwest
Research Station, Portland, OR.
Gruell, G.E. 1985. Fire on the early western landscape: an annotated record
of wildland fires
1776-1900. Northwest Science 59:97-104.
Habeck, J.R. 1990. Old-growth ponderosa pine-western larch forests in
western Montana:
ecology and management. Northwest Environmental Journal 6:271-292.
Hall, F.C. 1976. Fire and vegetation in the Blue Mountains - implications
for land managers.
Proceedings of the Tall Timbers Fire Ecology Conference 15:155-170.
Hall, F.C. 1994. Historical and present conditions of the Blue Mountain
forests. Blue
Mountain Natural Resource News. U.S. Forest Service, Pacific Northwest
Research Station,
Portland, Oregon. March: 1-2.
Halloin, L. 1991. Plantation grazing - a feasibility review. Washington
State Department of
Natural Resources, Pullman, Washington.
Harrington, G.N. 1991. Effects of soil moisture on shrub seedling survival
in a semi-arid
grassland. Ecology 72(3):1138-1149.
Harrington, M.G. and S.S. Sackett. 1988. Using fire as a management tool
in Southwestern
ponderosa pine. IN Management of Southwestern Natural Resources, USDA
Forest Service.
Harris, D.R. 1966. Recent plant invasions in the arid and semi-arid
Southwest of the United
States. Annals of the Association of American Geographers 56:408-422.
Harris, G.A. 1991. Grazing lands of Washington State. Rangelands 13(5):222-227.
Harris G.A. and M. Chaney. 1984. Washington Sate grazing land assessment.
Washington
State Cooperative Extension. Pullman.
Hastings, J.R. and R.M. Turner. 1965. The Changing Mile. University of
Arizona Press,
Tucson.
Heitschmidt, R.K. and J.W. Stuth, eds. 1991. Grazing management: an
ecological perspective.
Timber Press, Portland, Oregon.
Hill, R.R. 1917. Effects of grazing upon western yellow pine reproduction
in the National
Forests of Arizona and New Mexico. USDA Bulletin 580.
Humphrey, R.R. 1956. History of Vegetation Changes in Arizona. Arizona
Cattlelog 11:32-35.
Humphrey, R.R. 1958. The desert grassland: A history of vegetational
change and an analysis
of causes. Botanical Review 24:193-252.
Humphrey, R.R. 1987. 90 Years and 535 Miles: Vegetation Changes Along the
Mexican Border. Albuquerque. University of New Mexico Press.
Irwin, L.L., J.G. Cook, R.A. Riggs, and J.M. Skovlin. 1994. Effects of long
term grazing by
big game and livestock in the Blue Mountains forest ecosystems. General
Technical Report
PNW-325. U.S. Forest Service, Pacific Northwest Research Station, Portland,
Oregon.
Jameson, D.A. 1968. Species interactions of growth inhibitors in native
plants of northern
Arizona. Forest Service Research Note RM-113, Rocky Mountain Forest and Range
Experiment Station, Fort Collins, CO, USA
Johnson, C.G. Jr., R.R. Clausnitzer, P.J. Mehringer, and C.D. Oliver. 1994.
Biotic and abiotic
processes of eastside ecosystems: the effects of management on plant and
community
ecology, and on stand and landscape vegetation dynamics. General Technical
Report PNW-
GTR 322. U.S. Forest Service, Pacific Northwest research Station,
Portland, OR.
Karl, M.G. amnd P.S. Doescher. 1993. Regulating competition on conifer
plantations with
prescribed cattle grazing. Forest Service 39:405-418
Koehler, D.A., S.D. Thomas, H.D. Russell, and J.A. Mastel. 1989.
Multiresource management
of ponderosa pine forests. General Technical Report RM-185. U.S. Forest
Service, Rocky
Mountain Forest and Range Experiment Station, Fort Collins, CO.
Krueger, W.C. and A.H. Winward. 1974. Influence of cattle and big game
grazing on
understory structure of a Douglas fir-ponderosa pine-Kentucky bluegrass
community. Journal
of Range Management 27(6):450-453.
Larson, M.M. and G.H. Schubert. 1969. Root competition between ponderosa
pine seedlings
and grass. General Technical Report RM-54. U.S. Forest Service, Rocky
Mountain Forest
and Range Experimental Station, Fort Collins, CO.
Laudenslayer, W.F., H.H. Darr, and S. Smith. 1989. Historical effects of
forest management
practices in eastside pine communities in northeastern California. Pages
26034 in A. Tecle,
W.W. Covington, and R.H. Hamre, technical coordinators. General Technical
Report RM-
85. U.S. Forest Service Forest and Range Experimental Station, Fort
Collins, Colorado.
Lehmkuhl, J.F., P.F. Hessburg, R.L. Everett, M.H. Huff and R.D. Ottmar.
1994. Historical and
current forest landscapes of eastern Oregon and Washington. Part 1:
Vegetation pattern and
Insect disease hazards. General Technical Report PNW-GTR-328. U.S. Forest
Service,
Pacific Northwest Research Station. Portland, OR.
Leninger, W.C., and S.H. Sharrow. 1989. Seasonal browsing of douglas-fir
seedlings by sheep.
Western Journal of Applied Forestry. 4(3):73-76.
Leopold, A. 1924. Grass, brush, timber and fire in southern AZ. Journal of
Forestry 22:1-10.
Madany, M.H. 1981. Land use-fire regime interacts with vegetation
structure of several
montane forest areas of Zion National Park. Thesis. Utah State University,
Logan, Utah.
Madany, M.H. and N.E. West, 1983. Livestock Grazing-Fire Regime
Interactions within
Montane Forests of Zion National Park, Utah. Ecology, 64(4). pp 661-667.
Martin, J.W. and R.M. Turner. 1977. Vegetational change in the Sonoran
Desert region,
Arizona and Sonora. Journal of the Arizona Academy of Sciences.
McPherson, G.R. and H.A. Wright. 1989. Effects of cattle grazing and
Juniperuus pinchotii
canopy cover on herb cover and production in western Texas. American
Midland Naturalist
123(1):144-151.
Miller, R.F. amnd W.C. Krueger. 1976. Cattle use on summer foothill
rangelands in
northeastern Oregon. Journal of Range Management 29(5):367-371.
Mitchell, John E. and Duane R. Freeman. 1993. Wildlife-livestock-fire
interactions on the
North Kaibab: A Historical Review. General Technical Report RM-222. USDA
Forest
Service Rocky Mountain Forest and Range Experimental Station, Fort Collins,
CO. 12p.
Morgan, Penelope, 1994. Dynamics of Ponderosa and Jeffrey Pine Forests in
Flammulated,
Boreal and Great Gray Owls in the U.S.. Forest Service General Technical
Report RM-253,
Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO, USA
O'Laughlin, J., J.G. MacCracken, D.L. Adams, S.C. Bunting, K.A. Blatner, and
C.E. Keegan.
1993. Forest health conditions in Idaho. Report Number 11. Idaho Forest,
Wildlife and
Range Policy Analysis Group, Moscow, Idaho.
Pearson, G. A. 1923. Natural reproduction of western yellow pine in the
Southwest. U.S.
Dept. Ag., Bull 1105. 143 pgs.
Pearson, G.A. 1933. A twenty year record of changes in an Arizona pine
forest. Ecology
14:272-285.
Peet, R.K. 1988. Forests of the Rocky Mountains. Pages 63-101 in M.G.
Barbour and W.D.
Billings, editors. North American terrestrial vegetation. Cambridge
University Press. New
York, New York.
Pieper, R.D. and R.D. Wittie. 1988. Fire effects in Southwestern Chaparral
and pinyon-
juniper vegetation. IN Management of Southwestern Natural Resources,
USDA Forest
Service.
Rassmussen, D.I.. 1941. Biotic communities of the Kaibab Plateau, Arizona.
Ecological
Monographs 11:229-275.
Rietveld, W.J. 1975. Phytotoxic grass residues reduce germination and
initial root growth of
ponderosa pine. Forest Service Research Note RM-153, Rocky Mountain
Forest and Range
Experiment Station, Fort Collins, CO, USA
Roath, L. and W.C. Krueger. 1982. Cattle grazing and behavior on a forested
range. Journal of
Range Management 35(3):332-338.
Rummell, Robert S. 1951. Some Effects of Livestock Gazing on Ponderosa
Pine Forest and
Range in Central Washington. Ecology 32(4)594-607.
Savage, M., and T.W. Swetnam. 1990. Early 19th century fire decline
following sheep
pasturing in a Navajo ponderosa pine forest. Ecology 32(4):594-607.
Schwan, H.E. 1949. Influence of grazing and mulch on forage growth. Journal
of Range
Management 2:142-148.
Skovlin, J.M., R.W. Harris, G.S. Strickler, and G.A. Garrison. 1976. Effects
of cattle grazing
methods on ponderosa pine-bunchgrass range in Pacific Northwest. Technical
Bulletin 1531,
U.S. Forest Service, Pacific Northwest Research Station, Portland, Oregon.
Smith, D.W. 1967. Efects of cattle grazing on a ponderosa pine-bunchgrass
range in Colorado.
Technical Bulletin 1371. U.S. Department of Agriculture, Washington, DC.
Stein, S.J. 1988. Explanations of the imbalanced age structure and
scattered distribution of
ponderosa pine within a high-elevation mixed coniferous forest. Forest
Ecology Management
25:139-153.
Swetnam, T.W. and C.H. Baisan, 1995. (in press) In: C.D. Allen (ed.),
Proceedings of the 2nd
La Mesa Fire Symposium, Mar 29-31, 1994, Los Alamos N.M. National Park Service
Technical Report Series
Tiedemann, A.R. and H.W. Berndt. 1972. Vegetation and soils of a 30-year elk
and deer
exclosure in central Washington. Northwest Science 46(1):59-66.
Tisdale, E.W. 1950. Grazing of forest lands in interior British Columbia,
Journal of Forestry
48:856-860.
Tuomey, J.W. 1891. I. Notes of Some of the Range Grasses of Arizona.
University of
Arizona Agri
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