Impacts of drought on tree mortality and growth in a mixed hardwood forest

The tree and shrub species on a 16-ha watershed in the Coweeta Basin were sampled in 1984 and again in 1991 to determine the effects of drought on tree species composition and basal area growth. Mortality and radial growth were determined for tree species within three community types that represent a moisture gradient from moist to dry: covehardwoods > mixed-oak > oak-pine. Tree mortality from 1984 to 1991 was 20 % and 23 % in the cove-hardwoods and mixed-oak communities, respectively, compared to only 12 % in the oak-pine type. With the exception of Oxydendrwn arboreum and Robinia pseudoacacia, the oaks had higher percentage mortality than any other genus; Quercus velutina had 29 %, 37 %, and 20 % mortality in the cove-hardwoods, mixed-oak, and oak-pine types, respectively; Quercus prinus had 23 % mortality in the mixed-oak type; Quercus coccinea had 36 % mortality in the mixed-oak type; and Quercus marilandica had 27 % mortality in the oak-pine type. Mortality occurred mostly in the small-size class individuals (< 10 cm in diameter) for all species, suggesting that thinning was still an important process contributing to mortality 29 yr after clearcutting. Although growth of Liriodendron tulipifera was much higher than growth of either Quercus prinus or Quercus coccinea, growth in Liriodendron was significantly reduced by the 1985-88 drought and no growth reduction was observed for these two dominant Quercus species during the same time period.