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
Empirical Evidence for Catastrophic Shifts in Sagebrush Steppe.
Washington-Allen, Robert*,1, 2, West, Neil1, Ramsey, R. Douglas1, 1 Utah State University, Logan, Utah, USA2 University of Tennessee-Knoxville, Knoxville, TN, USA
ABSTRACT- Plant-herbivore models have predicted catastrophic shifts in shrubland or grassland dominance in relation to high-levels of drought and grazing in sagebrush steppe rangelands. Time series of satellite imagery in conjunction with contemporaneous land management and climatic data can be used to retrospectively assess the relative effects of herbivory, fire, and intense droughts on the spatial characteristics of vegetation in drylands. Life-form composition, clumping, and fragmentation metrics were used to characterize a 26-year time series of life-form maps of a commercial ranch in northeastern, Utah. This thematic time series was derived from standardized anniversary dry season Landsat scenes from 1972 to 1997. Time series and power law analyses were used to quantify trend and detect thresholds in metric response. First-order difference regression was used to relate a regional measure of soil water availability, herbivory, and fire to the landscape metric time series. Grass cover was dominant, but has been declining since 1972, possibly due to the influence of the higher livestock numbers from 1891 to the late 1960s. Shrub cover has crossed a threshold towards fragmentation and cover dominance after 1991. This change was accompanied by decreased contagion of landscape patches, an increase in bare ground cover, patch size, and number within 1 to 5 years. Changes in life-form composition were significantly correlated with grazing and were coincident with multiple droughts from 1987 to 1989, particularly the Great North American Drought of 1988. However, recovery towards mean conditions was observed during the very strong El Niño event of1997.
Key words: catastrophe, drylands, remote sensing, thresholds