Recent incidence of sudden aspen decline has increased interest in understanding and predicting aspen mortality in the Western United States. Previous research has highlighted drought as a major contributing factor to sudden aspen decline. However aspen sensitivity to drought is likely to differ between forest stands and individual trees. Factors such as competition, tree size, and disease are likely to mediate tree responses to climate. David Bell was part of a team including scientists from the U.S. Geological Survey and University of Wyoming that examined patterns of aspen mortality across the Western United States. They found that although mortality rates increased with summer temperatures, mortality rates also were influenced by the structure, productivity, and age of the stand, as well as by individual tree size and incidence of disease. These results highlight the importance of local forest dynamics and biotic interactions (e.g., disease) in shaping geographic patterns in tree mortality. The scientists are developing aspen mortality risk maps for the Southwestern United States to be distributed to forest managers in the region.