Exotic grass invasion in western US deserts alters landscape scale responses to precipitation and temperature through fire

Background/Question/Methods

Invasions of exotic grasses are an emerging threat to ecosystem stability in deserts of the southwestern US. These exotic grasses are increasing the frequency and extent of fire to the detriment of native perennial species. Regional analysis of native and disturbed landscapes before and after disturbance is made possible through legacy data sets of Landsat satellite imagery (primarily Landsat 5TM). We examined pre- and post-fire vegetation responses to precipitation and temperature in the Mojave and Great Basin Deserts using Normalized Difference Vegetation Index (NDVI) time series obtained from more than 350 Landsat Thematic Mapper images covering 28 years. Relationships  between NDVI to precipitation and temperature were assessed.

Results/Conclusions

NDVI responses to precipitation and temperature varied between burn conditions but were stable across similar vegetation communities. Precipitation received within 90 days of sampling was the strongest predictor of vegetation greenness for both burned and unburned landscapes. Burned landscapes were twice as responsive to precipitation received 90 days previous to sampling than unburned areas. Increasing temperature negatively influenced NDVI signal from burned landscapes up to 3 fold more than unburned landscapes. Adjusted r² values for the linear models range from 0.58 to 0.85 indicating relative stability in vegetation responses to precipitation and temperature. These findings are indicative that burned landscapes are being dominated by winter annuals and minimal changes have occurred in plant community composition since burning. NDVI responses to precipitation and temperature can be used to track recovery of burned landscapes as native plant communities become reestablished in burned areas. This is pending no reoccurrence of fire. With the recent loss of Landsat 5 and the impaired data for Landsat 7, the launch of the Landsat Data Continuity Mission (LCDM or Landsat 8) scheduled for 2013 will be valuable for continual monitoring of these altered landscapes.