Long-term droughtiness and drought tolerance of eastern US forests over five decades

• Recent CDSI values for the eastern US have decreased since the 1960–1986 period.
• Forests were defined and mapped as drought-tolerant, -intolerant, balanced, or mixed.
• Forests’ tolerance levels were balanced to somewhat drought-tolerant/-intolerant.
• More near normal conditions occurred in eastern US forests during the period 1961–2012.

Droughts can influence forest composition directly by limiting water or indirectly by intensifying other stressors that affect establishment, growth, and mortality. Using community assemblages of eastern US tree species and drought tolerance characteristics assessed from literature, we examine recent drought conditions in relation to the spatial distribution of species and their tolerance to drought. First we calculate and compare a cumulative drought severity index (CDSI) for the conterminous US for the periods 1960–1986 and 1987–2013 using climate division Palmer Drought Severity Index (PDSI) values and a gridded self-calibrated PDSI dataset. This comparison indicates that drought conditions in the East tend to be less frequent and generally less severe than those in the West, and that the West has had a large increase in CDSI values in the latter period. Then we focus on the past and potential future role of droughtiness in eastern forests, which are relatively more diverse than western forests but have individual species that are uniquely affected by drought conditions. We found that eastern US forests tend to be relatively balanced in the composition of drought-tolerant and -intolerant species and that drought conditions are relatively uncommon in the East. Understanding the composition and distribution of drought tolerance levels within forests is crucial when managing for the impacts of drought (e.g., managing for survival), especially given the expected rise of drought in the future.