Sierra Nevada forests have died back at an increasing rate in the past 25 years, and scientists have thrown out all possible explanations but an increase in the temperature.
Federal scientists linked warming to the increasing rate of death in ponderosa pine, white fir, red fir, Jeffrey pine and other conifers studied in Yosemite and Sequoia national parks. The rate of death has doubled over that time.
While some trees have shown a positive response to global warming conditions -- buoyed by increased carbon dioxide in the atmosphere, and in some cases additional moisture -- the affected forests are in dry areas that receive little rain or snowfall.
The increased temperature increases evapotransporation, making the trees more susceptible to problems associated with water shortages.
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