Annual visits to each tree showed that death rates were climbing an average of 3 percent each year, while the rate of new growth didn't change. The scientists conducting the study mapped tree deaths every year, and correlated short-term changes in tree deaths with parallel changes in climate and other potential factors. The stands chosen have never been logged and hadn't burned since the late 1800s. Over the study period, the temperature increased by about 1 degree Celsius. While small trees are most at risk, the death rates have increased for a range of species at just about any elevation, the study by the U.S. Geological Survey says. The increases in tree deaths do seem to coincide with periods of drought states the study. Droughts typically make trees more susceptible to bug infestations and pathogens.

Fir and pine trees carry the biggest risk; giant sequoias were too sparsely located to detect any trends. Climate change can bring about both drought and drenching rain, but in the arid Sierra Nevada, temperatures are warming without a marked increase in precipitation. Another factor impacting trees is higher rain fall elevations in the winter creating less snow pack. This is less beneficial for trees, since the water immediately drains downstream rather than slowly melting over a period of months. In other studies being presented at meetings of the Ecological Society of America in San Jose this week, scientists reported that stress and diebacks have occurred from Alaska to Mexico, affecting more than 20 million hectares (almost 50 million acres) and many tree species since 1997.