PARENT SESSION
Contributed Oral Session 33: Grassland and Shrubland Communities
Monday, August 8, 1:30 PM - 5:00 PM, Meeting Room 522 A, Level 5, Palais des congrès de Montréal
Chaos on the Sagebrush Sea: Catastrophe theory and rangeland vegetation change.
Salo, Lucinda*,1, Rhodes, Michael2, Pyke, David2, 1 USGS Forest & Rangeland Ecosystem Science Center, Boise, ID2 USGS Forest & Rangeland Ecosystem Science Center, Corvallis, OR
ABSTRACT- Exotic cheatgrass (Bromus tectorum) produces continuous fine fuels, which increase the frequency and extent of wildfire and have resulted in the loss of millions of hectares of big sagebrush (Artemisia tridentata) in the Intermountain Western U.S. These sites can then become caught in a cycle of cheatgrass and fire, as frequent reburns progressively weaken native perennial species. Outright loss of sagebrush, combined with fragmentation and degradation of remaining stands, threaten sagebrush obligate species such as Greater Sage-Grouse (Centrocercus urophasianus). These changes in vegetation have been described as either linear succession/regression along predictable pathways or as sudden transitions among multiple discrete vegetation states. In examining vegetation change after wildfire, we are tapping local experts' knowledge in addition to characterizing vegetation and soils on Wyoming big sagebrush (A. tridentata subsp. wyomingensis) sites in the northern Great Basin. Both local experts' knowledge and field data indicate that 'site occupancy', or the amount and distribution of perennial plants, is an excellent predictor of cheatgrass canopy cover, our proxy for probability of reburn. We are quantifying site occupancy using gap sizes between perennial plants and are developing indices to describe the distributions of these gaps. These gap distribution indices allow us to describe over 50% of the variability in cheatgrass cover. Catastrophe theory may provide a unifying theory within which we can describe rangeland vegetation dynamics that can be either linear pathways or sudden transitions. This theory provides mathematical models for describing both succession/regression as well as the largely irreversible changes associated with thresholds. Other methods of describing non-linear dynamics may be used to determine threshold values for state transitions to exotic systems.
Key words: wildfire, cheatgrass, Bromus tectorum, Artemisia