Most forests periodically experience drought to some degree. Drought is a natural occurrence, and forests are adapted to survive periodic droughts. However plants grow more slowly during droughts, and droughts can make forests more susceptible to certain insects and more vulnerable to fire.
Climate data are collected by the National Climate Data Center (NCDC). With these data the NCDC calculates the Palmer Drought Severity Index (PDSI) monthly by climate division. The PDSI is based on total rainfall, the rainfall periodicity, and soil characteristics. The NCDC archive contains monthly estimates of PDSI from 1895 to the present.
The total number of months of drought over the period from 1895 through 2003 was calculated for each ecological region of the conterminous United States using the NCDC data (National Climate Data Center 1994). The drought occurrence over these 109 years served as a historical account or reference point for each ecological region. These historical accounts were then put on a 10-year basis and compared to the number of months of growing season drought from 1994 through 2003. Drought deviation (fig. 1) was calculated as the difference between the expected number of months of drought over a 10-year period (from the historical data) and the actual months of drought over this recent 10-year period.
The decade 1994 through 2003 was evaluated and the results are shown in figure 1. The negative values (greens) indicate less drought than expected; the positive values (browns) indicate more drought than expected. More than the expected amount of drought occurred across much of the Western U.S., while most of the Eastern U.S. experienced close to the expected amount of drought or less than the expected amount from 1994 through 2003; exceptions in the East were the Florida coastal lowlands, Blue Ridge Mountains, and southern New Jersey area.
Figure 1. Drought deviation for 1994 through 2003 by Bailey's ecoregion section (Bailey 1995, McNab and Avers 1994). Gray areas are nonforest. Forest cover is derived from Advanced Very High Resolution Radiometer (AVHRR) satellite imagery (Zhu and Evans 1994). (Map from Forest Health Monitoring 2004 National Technical Report) (Data source: National Climate Data Center).