There is a growing consensus that natural disturbances will intensify under forecasted global climate change. The southwestern United States, for example, is expected to undergo both extended periods of drought and longer wildfire seasons. The temporal response of desert grassland communities to the effects of drought and fire is being examined using a time series of Landsat Thematic Mapper images from 1984-2011 (n = 309 scenes). The study area is the 474 km² Buenos Aires National Wildlife Refuge (BANWR) in Arizona, where refuge managers and surrounding private land owners have focused on repeated, prescribed fire as the primary habitat restoration tool for these grasslands.
We calculated a time series of vegetation cover using the Soil-Adjusted Total Vegetation Index (SATVI). The SATVI was independently calibrated and validated with plot data from multiple long-term grassland monitoring databases in the region. Using a comprehensive fire atlas that contained 25 years of mapped information from 211 prescribed fires and wildfires, we 1) located grassland areas with multiple overlapping fires, 2) correlated periods of fire and drought based on the Palmer Drought Severity Index (PDSI), and 3) stratified our sample areas by fire return and drought periodicity. Within these areas we collected total vegetation cover information using 20m x 50m line intercept plots (n= 91) and calculated cover of bare soil, annuals, native and non-native grasses, woody plants, and overall species richness.
Vegetation sampling and remote sensing results indicate that grass and woody plant cover in areas that received multiple (4) prescribed fires were indistinguishable from unburned areas when those prescribed fires occurred in years when the growing season experienced extremely low precipitation, and ultimately, the positive ecological effects from earlier fire were negated. Areas that had fewer prescribed burns but were followed by average or slightly below average precipitation had higher grass cover and lower woody cover than matched unburned areas. The results provide range managers with information to understand the optimal timing of prescribed burning and fire suppression activities that promote ecosystem resilience and habitat restoration.