Although ecologists do not define the term “drought” consistently (Slette and others 2019), one definition that is applicable to forests is that a drought is a period of precipitation deficit that persists long enough to deplete available soil water, leading to impacts on trees and other plants; in some cases, these impacts include plant injury or death (Anderegg and others 2012, Hanson and Weltzin 2000). Under this definition, droughts affect most forests in the United States, but their frequency and intensity vary considerably between geographic regions (Hanson and Weltzin 2000). These variations define the regions’ predominant drought regimes. Most forests in the Western United States are subject to seasonal droughts on a yearly basis. By comparison, forests in the Eastern United States usually exhibit one of the following drought patterns: random (i.e., occurring at any time of year) occasional droughts, as usually observed in the Appalachian Mountains and the Northeast, or frequent late-summer droughts, as usually observed in the Southeastern Coastal Plain and the eastern portion of the Great Plains (Hanson and Weltzin 2000).
Koch, Frank H.; Coulston, John W. 2020. Chapter 4 - Drought and moisture surplus patterns in the conterminous United States: 2018, 2016 2018, and 2014 2018. In: Potter, Kevin M.; Conkling, Barbara L., eds. Forest health monitoring: national status, trends, and analysis 2019. Gen. Tech. Rep. SRS-250. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station: 83-102.