Interspecific differences in drought and pluvial responses for Quercus alba and Quercus rubra across the eastern United States

Quercus is one of the most important genera of plants in North America, but is currently undergoing a decrease in abundance and dominance of North American forest ecosystems. Of the two most common North American Quercus species, eastern white oak (Quercus alba) has experienced a slightly greater decline in dominance compared to northern red oak (Quercus rubra). Changes in Quercus demographics are likely due to multiple factors, but also highlight the importance of understanding the growth-climate relationships of these species under shifting climatic regimes. Therefore, we developed a network of tree-ring chronologies and stomatal conductance measurements where Q. alba and Q. rubra co-occur that spans the longitudinal extent of both species' geographic ranges. We compared growth reductions due to drought conditions, and growth increases due to pluvial conditions, of both species. The drought response (by reductions in radial growth and stomatal conductance) of Q. alba was consistently greater than Q. rubra irrespective of temporal or spatial drought dynamics. Similarly, the pluvial response (relative increase in growth) of Q. alba was greater than Q. rubra. We also found that both species exhibit nonlinear relationships to water availability, where increases in growth during pluvial conditions are less than the decreases in growth due to drought conditions. Thus, we concluded that Q. alba is moderately more sensitive to water availability than Q. rubra, which provides new insight for Quercus isohydricity classification, and may indicate different sensitivities to a changing precipitation regime in North America.