In forests, droughts can cause considerable tree stress, particularly when they coincide with periods of abnormally hot weather, i.e., heat waves (L.D.L. Anderegg and others 2013, Peters and others 2015, Williams and others 2013). Trees and other plants respond to this stress by restricting fundamental growth processes. Photosynthesis, which is less sensitive than other fundamental processes, decreases slowly at low levels of drought stress but decreases more rapidly as the stress becomes more severe (Kareiva and others 1993, Mattson and Haack 1987). Ultimately, prolonged drought stress can lead to failure of a tree's hydraulic system, resulting in crown death and subsequent tree mortality (Choat and others 2018).
Koch, Frank H.; Coulston, John W. 2021. Chapter 4 - Spatial patterns of drought and moisture surplus in the conterminous United States: 2019, 2017-2019, and 2015-2019. In: Potter, K.M.; Conkling, B.L., eds. Forest health monitoring: national status, trends, and analysis 2020. Gen. Tech. Rep. SRS-261. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station: 85-104.