Figure 6: Comparison of NDVI from Landsat TM in areas used by bighorn sheep in winter (January–April) at high elevations (>3000 m) and low elevations (<2000 m), in above-average and below-average precipitation years; (a) values of NDVI (mean and 95% CI) at high and low elevations pooled across TM scenes, and (b) tradeoff of vegetation condition at low versus high elevations, measured as difference between NDVI at low elevation minus NDVI at high elevation (mean and 95% CI) within each TM scene, Sierra Nevada, California, USA, 1987–1994 and 2002–2009.