Impacts of grazing and fire on woody plant cover expansion in semi-arid grasslands of the northern Chihuahuan Desert

Background/Question/Methods

Northern Chihuahuan Desert grasslands are highly managed systems which support rich biodiversity and many endemic species as well as provide a valuable economic resource for cattle ranching livelihoods, with 90% of the grasslands open to grazing. These grasslands are experiencing land-cover modification from woody plant cover expansion, leading to diminished biodiversity and grazing capacity. This research used the Landsat Thematic Mapper (TM/ETM+) record from 1984 to 2008 to map changes in woody plant cover and identify spatial patterns and temporal trends of woody plant cover expansion. Spectral mixture analysis (SMA) was used to quantify the percent of woody plant cover in each pixel; trend analysis was used to track per-pixel changes over the time-series. These results were validated using ground-collected measurements and high spatial resolution aerial photographs/satellite data and compared to the 2002 Multi-angle Imaging Spectro-Radiometer (MISR) woody plant cover product (the only year for which the MISR product is available). Validated change maps were analyzed with maps of grazed areas and burned areas to determine the impacts of grazing and fire on woody plant cover expansion.

Results/Conclusions

The change maps were in good agreement with ground-collected measurements and estimates from high spatial resolution aerial photographs/satellite data, with a Kappa coefficient of 0.8. The woody plant cover map from 2002 was also compared separately with the 2002 MISR product, with a correlation coefficient of 0.77. The trend analysis revealed an increase in woody plant cover ranging between 5-30% over the 25-year period with spatial variability in expansion amounts across the region. Interestingly, un-grazed areas within the region exhibited the greatest increases in woody plant cover. Burned areas exhibited a sudden decrease followed by a rapid rate of increase as woody plants regenerated from root systems. Grazed areas burned at frequent intervals experienced the lowest increases. Results will enable resource managers to better understand the dynamics of woody plant cover expansion within the region and allow scientists to develop new models and applications to answer both natural and societal land management questions and support decision-making processes.