While broad-scale forest die-back in western North America has received attention, the bulk of forests are not in catastrophic decline, but may still be responding to environmental changes. Using a network of old growth forest monitoring plots in the Sierra Nevada of California we show that tree mortality due to stress and biotic causes increased from 1983 – 2004, coinciding with an increasing index of drought (temperature induced increase in water deficit). Demographic trends were widespread: we found increasing mortality across major taxonomic groups (Abies and Pinus) and elevations (although the mortality rate increase was attenuated with increasing elevation). Our findings suggest that these forests are highly sensitive to drought stress and may be poised for a die-back response if future climates continue to feature rising temperatures without a compensating increase in precipitation.