Tree Line Shifts

摘要

Repeat photography is helping reveal when a warmer climate is affecting vegetation-and when it isn't.In the American Southwest, work by Robert Webb, a plant ecologist recently retired from the U.S. Geological Survey in Tucson, Ari- zona, has shown that fewer, shorter freezes are paving the way for more cactus seedlings and certain frost-sensitive shrubs to survive winters. Among Webb's 10,000 paired pho- tographs-some spanning 140 years-are many showing increases in these desert plants.