One of the largest sources of variation in forest productivity is extremes in climate such as drought. Predicting large scale forest responses to expected changes in U.S. drought patterns require an improved understanding of species-specific growth responses. Here, radial growth measurements were collected for co-occurring Quercus alba (white oak) and Quercus rubra (northern red oak) at 50 sites across the eastern US. Radial growth was standardized by converting raw ring-width measurements to basal area increment (BAI) to create site chronologies. Standardized Precipitation Evaporation Index (SPEI) was used to quantify moisture availability during the growing season (May - October). Pearson correlation analysis was performed between radial growth and monthly SPEI values. The strongest correlation between growth and SPEI was observed for the month of June, thus June SPEI was categorized into bins ranging from very dry, to very wet (less than -2 to greater than 2). Radial growth variation within and between each species was assessed across bins to identify diverging growth patterns. T-tests and ANOVA were conducted on the between species at each bin and within species across all bins, respectively.
Results/Conclusions
Significant growth differences were observed at each SPEI bin. Northern red oak exhibited greater growth across the full range of SPEI values, as well as at each SPEI bin. Additionally, differences between white oak and northern red oak BAI varied among sites, indicating both site and species-specific responses. Results support the hypothesis that differences in growth response to moisture stress exist between white oak and northern red oak. These differences may be influenced by site characteristics. Evidence presented here provides limited support that northern red oak trees and white oak employ different growth strategies under increased moisture stress. If substantiated, growth response differences could partially account for recent mortality and regeneration patterns observed in eastern US forests.