Abstract
Droughts are common in virtually all U.S. forests, but their frequency and intensity vary within forest ecosystems (Hanson and Weltzin 2000). Accounting for the long-term influence of droughts within a region is difficult due to variations in the spatial extent and intensities over a period. Therefore, we created a cumulative drought severity index (CDSI) (Fig. 1) for the United States using weighted monthly frequencies of Palmer Drought Severity Index values reported at climate divisions for the periods 1960 to 1986 and 1987 to 2013 (National Climate Data Center 2014, Palmer 1965). The occurrences of 'severe' conditions (index value of -3.0 to -3.9) receive a weight of 2 and 'extreme' (index value of ≤ -4.0) receive a weight of 3 to reflect the increased magnitude of these events.
Citation
Peters, Matthew P.; Iverson, Louis R.; Matthews, Stephen N. 2014. Spatio-temporal trends of drought by forest type in the conterminous United States, 1960-2013. Res. Map NRS-7. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station. [scale 1:12,000,000].
