Results of a new study tie forest "greenness" in the western United States to fluctuating year-to-year snowpack extent. The results show that mid-elevation mountain ecosystems are the most sensitive to rising temperatures and to changes in precipitation and snowmelt. University of Colorado-Boulder scientist Noah Molotch and colleagues used satellite images and ground measurements to identify the threshold at which mid-level forests sustained by moisture change to higher-elevation forests sustained by sunlight. A paper reporting the results was published September 9 in the journal Nature Geoscience. Molotch is the lead author. Co-authors are Ernesto Trujillo of the University of Colorado-Boulder and Ecole Polytechnique Fédérale de Lausanne in Switzerland; Michael Golden and Anne Kelly of the University of California, Irvine; and Roger Bales of the University of California, Merced. "The research demonstrates yet another complexity in the response of mountain ecosystems to increasing temperatures," says hydrologist Tom Torgersen, program director in the National Science Foundation's Division of Earth Sciences, which funded the research. "High-elevation mountain forests are typically temperature-stressed and low-elevation mountain forests are often water-stressed.