Virtually all U.S. forests experience droughts, although the intensity and frequency of the droughts vary widely between, as well as, within forest ecosystems (Hanson and Weltzin 2000). Generally, forests throughout the Western United States are subject to annual seasonal droughts, while forests in the Eastern United States can be characterized by one of two predominant patterns: random, occasional droughts (in the Appalachian Mountain region and the Northeast) and common late-summer droughts (in the Southeastern Coastal Plain and near the eastern edge of the Great Plains) (Hanson and Weltzin 2000). In terms of impacts, a reduction in basic growth processes, i.e., cell division and enlargement, is the most immediate plant response to drought; photosynthesis, which is less sensitive than these basic processes, decreases slowly at low levels of drought stress, but begins to decrease more sharply when the stress becomes moderate to severe (Kareiva and others 1993, Mattson and Haack 1987). Drought stress also makes some forests more susceptible to infestations of tree-damaging insects and diseases (Clinton and others 1993, Mattson and Haack 1987). Furthermore, by impeding decomposition of organic matter and reducing the moisture content of downed woody materials and other potential fuels, drought may substantially increase wildland fire risk (Clark 1989, Keetch and Byram 1968, Schoennagel and others 2004).