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Restoration Through Reintroduction of
Fire and Herbivory
Charles G. Curtin, Arid Lands Project and Malpai Borderlands Group,
Santa Fe, NM
H
istorical writings, old photographs and paintings, and paleocological studies
document that changes typical of desertification have greatly altered the
landscape and vegetation of the arid regions of the southwest within the last 125
years (Hastings and Turner 1965, Cooke and Reeves 1976, Grover and Musick
1990, Bahre 1991). Data from long-term ecological studies and remote sensing
indicate that these changes have continued, and in some cases accelerated, in the
last 20 years (Ray 1995, Betancourt 1996, Brown et al. 1997).
In our studies we investigate how a combination of cattle grazing and fire can be
used to moderate or reverse shrub increases in grassland habitats and how disturbance
processes structure grasslands. In companion studies, reintroduction ofblack-tailed prairie dogs ( Cynomys ludovicianusssp. arizonensis) on the same site are using similar research
protocols to measure the effects of these native herbivores on grassland ecosystem function. The fire and grazing studies are supported by the Interagency Fire Center in Boise,
Idaho, and the Animas Foundation. The studies are complementary to those with the
Jornada Experimental Range. The prairie dog studies are supported by the Animas Foundation, the New Mexico Fish and Game Department, and the Thaw Charitable Trust.
Structure and Dynamics of Arid Ecosystems in
Southwestern North America
Four factors ultimately dominate arid ecosystem processes in southwestern North
America: 1) temporal variation in climate; 2) spatial variation in topography, geomorphology, and soil; 3) fire; and 4) herbivory.
First, unpredictable timing in the amount of rainfall determines the input of the
primary limiting resource: water. The annual precipitation is distinctly bimodal, with winter rains that benefit c3 forbs, shrubs, and trees, and summer rains that benefit c4 forbs
and grasses and CAM succulents. Second, variation in slope, aspect, exposure, geological
parent material, and soil determines the spatial template upon which all ecological processes, including the infiltration and redistribution ofwater, occur. Third, fires, usually
caused by "dry" lightning strikes at the beginning ofthe summer rainy season, are a major
cause of disturbance. And, fourth, grazing by domestic livestock and native species removes biomass and sometimes contributes to ecosystem degradation. While other influences, such as additional human impacts, are important in local circumstances, the above
four factors are generally the most important (Sears 1935, Hastings and Turner 1965,
Bahre 1991). An understanding ofhow they operate and interact will greatly improve our
ability to predict changes in arid ecosystems due to both natural and human -caused environmental variation.
We study factors influencing ecosystem structure, dynamics, and productivity
in the Malpai Borderlands Ecosystem by focusing on how fire and herbivory structure semi-arid grasslands. Given time and resource constraints, it would be impossible to study all variables affected by fire and herbivory. We have chosen to
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Restoration Through Reintroduction of Fire and Herbivory
Curtin
focus on vegetation because vegetation in turn has important effects on overall
biodiversity as well as the Southwest's ranching economy. We will also study small
mammal community composition as an additional measure of biodiversity because: 1) the community composition of small mammals as resource consumers
provides a good bioassay of resource distribution and availability within the ecosystem; and 2) as keystone herbivores, small mammals play a crucial role in directing and aggregating resources with the ecosystem (Brown and Heske 1990).
Additional groups of special interest are birds and reptiles that provide additional
assays of resource abundance and distribution.
Experimental Design
The Gray Ranch is a 130,000 ha working cattle ranch located in the bootheel of
southwestern New Mexico and forms the western boundary of the Malpai Borderlands
Ecosystem. It is in the Mexican Highlands Shrub-Steppe ecoregion. The Gray is covered
by grassy plains from 1200 to 2200 min elevation and by mountains, some ofwhich rise
over 2800 min elevation.
The Animas Foundation has agreed to set aside the 4,483 ha ( 11,000 acre)
McKinney Flats pasture as an experimental area. U ngrazed since at least 1991,
the McKinney Flats pasture is located at an elevation of 1650 m. It contains a
gradient from Plains-Great Basin grasslands (Bouteloua association), to Semidesert grasslands (Bouteloua-Hilaria-Sporobolusassociation), to Chihuahuan Desert
grassland/shrub lands ( Prosopis association).
Given that we are interested in landscape and ecosystem processes, we believe the use
of2,200 acre (916 ha) pastures, encompassing 1 x 1 km study blocks, with 200 x 200m
study areas containing five 150m transects, is an appropriate scale at which to conduct
our study. This scale is large enough to replicate landscape processes and small enough for
the blocks to be comparable, while containing enough separation between vertebrate
populations to have true replication.
Vegetation Sampling
A number of scaling and data collection exercises were conducted to determine when sample sizes asymptote and what the minimum necessary sampling
intensities were. We have gone to a sampling regime that entails 40 x 40 em
quadrats set at every meter along the 150 m sampling lines. Frequency data is
compiled from within the quadrats and line-intercept data from each of the four
corners of the quadrate resulting in 600 points per 150m transect. Aerial photography will be used to track the dynamics of larger woody species such as mesquite
(Prosopis spp.), agave, and yucca.
Birds
Preliminary sampling ofbird species in the fall of1998 documented far higher
bird diversities then recorded for most arid grasslands. We are monitoring bird
populations in 500 m disturbance and control plots, yet sampling intensity will
increase from two to four extended surveys per year. These are being conducted
in May and June to determine the abundance and biomass of breeding birds
during the most food-limited and food-critical periods for the endangered
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Restoration Through Reintroduction of Fire and Herbivory
Curtin
aplamado falcon (Falco femora/is). Winter sampling is conducted when diverse
winter flocks are on the site to determine the effects of resource variation on
migratory birds. The May period corresponds with early breeding season of the
falcons and when migratory birds have left. The July period corresponds with
young-feeding/fledgling period of the falcons and is also when resident birds are
coping with reproduction and the most extreme heat and moisture conditions of
the year.
Small Mammals
Twice a year (once in the spring and once in the fall), Sherman traP.s will be placed at
30m. intervals along the 150m transects (240 traps per night). We have found that to
ensure the traps are all picked up by the heat of the day, only one-half of the site should
be trapped at a time. The duration of trapping will be for three days in each location. Due
to relatively high mammal densities and diversities on the site (roughly 12 species on the
site at a given time and two to 10 captures per 200 x 200m sampling area), this approach
is proving effective at recording mammal density and diversity.
Lizards
To monitor lizard populations, we will place pitfall traps at each of the rodent sampling stakes, thereby allowing direct comparison between lizard and small mammal populations. Pitfall traps will be censuses for five days, four times yearly including the late
spring, after adults emerge and become active (early May), in early summer before the hot
dry periods prior to the monsoon (early June), in July after the monsoon (when heat and
drought sensitive species are likely to be active), and in the early September after the yotmg
of the year become active.
Physical Features and Parameters
Aerial photos will be taken of the site during the winter and each winter thereafter to
document changes in woody vegetation. Post-fire photography will also be conducted to
document the spatial dynamics of fire events. Important landscape features such as kangaroo rat mounds will be documented and mapped using GPS to determine spatial and
temporal variations in these nutrient sinks and diversity hotspots.
Pre- and post-fire monitoring will entail measurements of soil chemistry and fuel
loads in different vegetation types. Paint chips around target species will document burn
intensity, and selective marking offocus species groups (grama grasses, yucca, sub-shrubs,
and Coryphanthra cactus) will provide additional information on the effects ofburning.
References
Bahre, C. J. 1991. A legacy ofchange: Historic human impact on vegetation in the Arizona borderlands.
The University ofArizona Press, Tucson.
Betancourt, J.L. Long- and short-term climate influences on southwestern shrublands. In proceedings:
shrubland ecosystem dynamics in a changing environment. J. Barrow, E. McArthur, E. Durant, R. Sosebee, J.
Tausch. Gen. Tech. Rep. INT-GTR-338. U.S. Forest Service, Ogden, UT.
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Restoration Through Reintroduction of Fire and Herbivory
Curtin
Brown, J.H. and E.J. Heske. 1990. Control of a desert-grassland transition by a keystone
rodent guild. Science 250: 1705-1707.
Brown, J.H., T.J. Valone, and C.G. Curtin. 1997. Reorganization of an arid ecosystem in
response to local climate change. Proceeding of the National Academy of Sciences. 94: 9729-9733.
Cooke, R.U. and R.W. Reeves. 1976. Climatic causes and biotic consequences of recent desertification in the American southwest. Oxford: Clarendon Press.
Grover, H.D. and H.B. Musick. 1990. Shrubland encroachment in New Mexico. Climatic
Change 17: 305-330.
Hastings, J.R. and R.M. Turner, 1965. The changing mile: an ecological study of vegetation
change with time in the lower miles of the arid and semiarid region. University of Arizona Press,
Tucson.
Ray, T. W. 1995. Remote monitoring ofland degradatiion in arid/semiarid regions. PhD. Thesis,
California Inst. ofTechnology.
Sears, P.B. 1935. Deserts on the march. University of Oklahoma Press, Norman, OK.
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