|
Wyoming Sagebrush Fire
Active
Periodic fire is a natural part of sagebrush steppe ecosystems in the Great Basin (Pyke 2002). However,. a suite of human-caused factors has dramatically changed fire regimes in many of these systems. Decades of fire suppression and livestock grazing have produced dense, single-aged sagebrush stands (Heady 1988, VVY lnteragency Vegetation Comm. 2OO2) and invasion by cheatgrass (Bromus tectorum) has increased the frequency and extent of fire in disturbed areas (Whisenant 1990), resulting in a cycle of cheatgrass and fire that prevents reestablishment of native perennial species (D'Antonio & Vitousek 1992). Simply reintroducing fire to these systems, even at appropriate temporal and spatial scales, can have disastrous consequences.
The Hart Mountain Antelope Refuge Comprehensive Management Plan (1994) mandates the use of prescribed fire for vegetation management. In sagebrush steppe the goal is restoring the natural variability of these communities and increasing native understory grasses and forbs for wildlife, including pronghorn antelope (Antilocapra americana), pygmy rabbits (Brachylagus idanhoensis), and greater sage grouse (Centrocercus uropha-sianus) (BLM 2002). Although guidelines for sage grouse habitat have been developed (Connelly et el. 2000), the impact of fire on these habitats is not well understood, as there have been few manipulated studies (Rowland and Wisdom 2002).. The lowest and driest portions (~72,000 acres) of Hart Mountain Refuge support Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis)., the least resilient of big sagebrush communities after disturbances such as fire (Hironaka et el. t983). Prescribed bums in these areas have resulted in increases in cheatgrass cover and frequency, accompanied by decreases in cover and frequency of native understory species (Ballard 2002).
Fire surrogate treatments have the potential to thin dense, single-aged Wyoming sagebrush stands and to allow native understory species to increase with lower risk of invasion by cheatgrass than with prescribed fire or wildfire (Connelly et al. 2000). At Hart Mountain Refuge we will compare vegetation change after fire, chemical (Spike herbicide), and mechanical (mowing) treatments with an untreated control area. We will evaluate differences in reductions in sagebrush foliar cover and density, increases in understory grasses
and forbs, and changes in cheatgrass canopy cover among the fire and fire surrogate treatments, compared with the control area.
Our study will also describe thresholds of recovery for native, understory species vs. conversion to cheatgrass dominance after fire and fire surrogate treatments in Wyoming big sagebrush. Identifying these thresholds, will provide guidance for land managers at Hart Mountain Antelope Refuge, and at other FWS lands in the shrub steppe, in meeting management objectives for wildlife, including sage grouse, which require both sagebrush and native forbs for successful, reproduction (Schroeder et al. 1999, Connelly et al. 2000), by restoring dense Wyoming big sagebrush stands that are now at high risk of invasion by cheatgrass following wildfire.
Our overall goal is to test our prediction that fire surrogate treatments can thin dense, single-aged Wyoming big sagebrush stands, which are now at high risk of invasion by cheat-grass following wildfire (Wisdom et al.. 2003), and allow native understory grasses and forbs to increase with a low risk of invasion by cheatgrass. This research will also increase understanding of the impact of fire on sage grouse habitat.
At Hart Mountain Antelope Refuge we will treat patches within a four section (2560 acre) study area with fire, chemical (Spike herbicide), and mechanical (mowing) treatments, and establish an untreated control area. Each treatment will be assigned to one of the four sections.
We will evaluate:
thinning of Wyoming big sagebrush stands by measuring sagebrush density and foliar cover,
increases in native understory by measuring native perennial grass and forb densities,
invasion by cheatgrass by measuring cheatgrass canopy cover.
We will describe:
thresholds of recovery for native, understory species, vs. conversion to cheatgrass dominance by evaluating '"site occupancy'" of native perennial species by measuring plant basal cover and gap size distributions among perennial plants.
We will characterize vegetation using four 25m transects at each of a minimum of 6 sample points in each treatment type, plus control area, before and after treatments. At each transect we will measure:
A. Foliar cover of shrub species and basal cover of all plant species, bare soil, rock, biological soil crust, or litter, from 100 line-point intercept points along the 25m transect. This will allow us to evaluate sagebrush cover and "site occupancy" of preferred species and biological soil crusts, before and after treatments.
B, Sizes of basal gaps among perennial plants along the 25m transect. This will produce a distribution of gap sizes, allowing us to evaluate the potential for soil erosion, water runoff, and invasion by undesirable species, before and after treatments (Herrick et al. 2003). In addition, the frequency distribution of gaps of various sizes can vary among states in state and. transition models (Bestelmeyer 2003).
C. Sagebrush density by height class in a 5-m x 25-m belt transect. This will allow us to evaluate the population structure of sagebrush, before and after treatments.
D. Cheatgrass canopy cover in a 0.5-m x .25-m belt transect. This will allow us to evaluate, the potential for rebum at the site.
E. Densities of all perennial grass, and forb species, from 13 0.5-m .x 0.5-m quadrats along the 25m transect.
F. Soil aggregate stability, determined with a field soil stability kit. This will allow us to evaluate: the level of soil organic matter and resistance to erosion (Herrick et al. 2001).
Metal spikes will mark the start, end, and center points of each transect. This will allow relocating transects with a metal detector. At least three digital photographs will be taken at each transect:
Landscape view taken from the 0 point of tape, with the camera ~1.5m above the ground, and a sign board indicting location and date
Identical landscape view taken from the same location, without sign board
View of soil surface at ~2 m point on tape, taken from the same location
Discriminant function analysis (DFA) will allow us to determine which variables discriminate among responses of each of the treatment areas and the control area and to visualize shifts in the overall structure of vegetation after treatments. Multivariate analysis of variance (MANOMA) will allow us to identify significant differences among treatments and the control area in sagebrush density and foliar cover, native perennial densities, cheatgrass canopy cover, and "site occupancy".
Pyke, David A. - Supervisory Research Ecologist
Phone: 541-750-7334
Email: david_a_pyke@usgs.gov
 |
