VARIATION IN RADIAL GROWTH-RESPONSES TO DROUGHT AMONG SPECIES, SITE, AND CANOPY STRATA

Radial growth responses to drought were examined in the tree-ring reco rds of six species growing within two locations of differing land-use history and soil moisture characteristics, and in overstory and unders tory canopy positions in northern Virginia. Tree species experienced d ifferential ring-width reductions during or immediately following four severe drought periods occurring from 1930 to 1965 and were influence d by climatic variables including annual and summer temperatures, annu al precipitation, and annual Palmer Drought Severity Index. Relative g rowth comparisons averaged across species before and after drought yea rs indicated that understory trees on dry-mesic sites grew 11% faster after drought compared to predrought rates while mesic site trees in b oth canopy positions grew approximately 4% slower. Superposed epoch an alysis indicated that Liriodendron tulipifera growing on mesic sites e xperienced greater ring-width reductions associated with drought than co-occurring, more drought-tolerant Quercus alba and Q. velutina. On d ry-mesic sites, L. tulipifera also experienced greatly reduced growth as a result of drought but exhibited significant growth increases foll owing individual drought events. Quercus alba was the only species tha t exhibited a consistent, significant ring width decrease associated w ith all droughts on dry-mesic sites. In contrast, Pinus virginiana was least impacted by drought on dry-mesic sites but was much more impact ed by drought on mesic sites, indicating a drought x site interaction for this species. Overstory Carya glabra and Q. alba experienced large r growth decreases during drought on dry-mesic versus mesic sites. Und erstory tree growth reductions did not differ between site types but w ere often significantly larger than overstory responses of the same sp ecies on mesic sites. Following drought, most trees exhibited growth r eductions lasting 2-3 years, although several species experienced redu ctions lasting up to 6 years. The results of this study suggest that t ree rings represent an important long-term proxy for leaf-level ecophy siological measurements of growth responses to drought periods.