Recent drought conditions in the Conterminous United States

Droughts are common in virtually all U.S. forests, but their frequency and intensity vary widely both between and within forest ecosystems (Hanson and Weltzin 2000). Forests in the Western United States generally exhibit a pattern of annual seasonal droughts. Forests in the Eastern United States tend to exhibit one of two prevailing patterns: random occasional droughts, typical of the Appalachian Mountains and of the Northeast, or frequent late-summer droughts, typical of the Southeastern Coastal Plain and the eastern edge of the Great Plains (Hanson and Weltzin 2000). For plants, a reduction in basic growth processes (i.e., cell division and enlargement) is the most immediate 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 makes some forests more susceptible to infestations of tree-damaging insects and diseases (Clinton and others 1993, Mattson and Haack 1987). Furthermore, drought may increase wildland fire risk by impeding decomposition of organic matter and reducing the moisture content of downed woody materials and other potential fire fuels (Clark 1989, Keetch and Byram 1968, Schoennagel and others 2004).