Changes in Vegetation Cover and Productivity in Yosemite National Park (California) Detected Using Landsat Satellite Image Analysis
The Sierra Nevada of California is a landscape where large forest fires have been suppressed for over a century and future climate warming has the potential to alter vegetation cover and surface water runoff. Remote sensing over the past 25 years can add new data and insights to this climate impact study. To determine if vegetation cover density and productivity had changed significantly across Yosemite National Park (NP) in California (USA) since the mid-1980s, a time-series of Landsat satellite imagery was analyzed. The study design controlled for annual precipitation amounts, elevation gradients, and vegetation age since the last stand-replacing (wildfire) disturbance in a comparison of Landsat data. Landsat image analysis over the past 20+ years showed that consistent increases in the satellite normalized difference vegetation index (NDVI) during relatively dry years were confined to large wildfire areas that burned in the late 1980s and 1990s. Unburned areas of the NP above 2900 m elevation on the Sierra Crest showed extensive decreases of more than -100 units in NDVI between relatively dry years. Several relatively small wildfires that burned in the early 1980s (prior to 1987) were the only areas of the NP that showed notable increases in NDVI (more than +200 units) between relatively wet years. These findings conflict with any hypotheses that NDVI-controlled plant evapotranspiration fluxes and river flows downstream could be markedly altered solely by vegetation cover change over most of the Yosemite NP in coming decades.