Drought trends in the eastern US: Examining the resilience of our forests
Droughts can influence forest composition directly by limiting water or indirectly by intensifying other stressors that affect establishment, growth, and mortality. Eastern US forests have experienced varying degrees of drought frequency and intensity over the last five decades. Given the uncertainty of future droughts, has the current forest composition in the eastern US been resilient to past trends of drought? We calculated a gridded self-calibrated Palmer Drought Severity Index (PDSI) and examined drought frequency and intensity during the period 1961–2012 and compared spatio-temporal patterns of drought to modeled suitable habitat for 134 eastern US tree species. Each species was assigned to one of three drought-tolerant or -intolerant classes and used to determine whether the forest composition is tolerant or intolerant to drought. Two definitions were used to determine the drought tolerance of the forest within 20 km2 grids; one based on the dominant class among species, and another which considered the relative abundance of all species having suitable habitat within a grid.
Within the eastern US, drought frequency and intensity has been low relative to the western US, while durations were relatively short (<6 months) during the period 1961-2012. The majority of the period experienced near normal conditions based on the self-calibrated PDSI dataset. However, both drought and moist conditions have been increasing since the 1990s. When we characterized forest composition using the dominant species to assign drought tolerance, a greater portion of northern and southern forests were classified as moderately drought-intolerant, while the forests of the central plains were classified as highly drought-tolerant. In contrast, accounting for all species reduced the tolerance of the central plains forests and increased the tolerance of northern and southern forest. Additionally, a greater percentage of the central and northern forests were assigned to a balanced class indicating that both drought-tolerant and -intolerant species comprised between 45–55% of the habitat. Intersecting past drought conditions with tolerance levels of forests revealed that much of the drought-intolerant regions have experienced few and/or low intensity droughts. Knowing where and how severe past droughts have played a role, and how current forests compositions have developed in response to these conditions can help managers plan for future droughts.