Trees in the western U.S. have died off at an increasing rate over the past few decades, finds Phillip J. van Mantgem of the U.S. Geological Survey and his colleagues. They studied various plots in three regions: the Pacific Northwest, California and the continental interior near the Rocky Mountains. The culprit seems to be locally higher temperatures, which decrease the available water and boost the activity of a bark-damaging fungus. As a result, trees on average are younger and smaller—and less able to hold on to carbon.

Annual number of deaths per 1,000 trees in:

1984: 7.1

2001: 11.4

Number of trees in study area in:

1984: 58,736

2001: 47,641

Calculated increase in annual number of deaths per 1,000 trees: 4

Number of years for the tree mortality rate to double in:

Pacific Northwest: 17

California: 25

Continental interior: 29

SOURCE: Science, January 23, 2009