Stomata are integral to regulating the global fluxes of carbon and water between the atmosphere and biosphere. Increased levels of atmospheric CO2 reduce stomatal aperture, allowing maintained or increased rates of carbon assimilation while simultaneously reducing water loss through transpiration. The resulting increases in water use efficiency (WUE) may translate into increased productivity. Yet, the extent to which eastern U.S. tree species have experienced increased WUE over recent decades, and whether increases in WUE translate into increased growth is of critical importance for better constraining forest responses to environmental change. To address this, we developed chronologies of growth and intrinsic water use efficiency (iWUE) for four common eastern U.S. tree species (Juniperus virginiana, Picea rubens, Quercus rubra, Liriodendron tulipifera) spanning 10 study locations in the eastern U.S. Across all species, we collected over 300 increment cores for growth analysis, from which 75 were randomly selected for stable carbon isotope analysis to develop annually resolved chronologies of iWUE for the period 1960 – 2015. We first identified long-term trends in both growth and iWUE, then examined the relationships between growth and iWUE for each species, as well as the influence of environmental factors over the observed trends.
Results/Conclusions
During 1960 – 2015 study period, each species exhibited unique trends in growth and iWUE. Across all sites, the basal area increment (BAI) of a mature Liriodendron tulipifera increased by 0.32 cm2 yr-1. This increase in BAI was accompanied by a 16% increase in iWUE, resulting in a weak, but significant correlation between BAI and iWUE (Tau = 0.20, p = 0.02). Similarly, an average Quercus rubra increased in basal area by 0.33 cm2 yr-1; however, this was accompanied by no change in iWUE, suggesting no link between BAI and iWUE (Tau = -0.14, p = 0.13). Similarly, Picea rubens experienced a dramatic decline, then recovery of growth during the 1960 – 2015 study period, during which time iWUE increased by 32%, resulting in no clear relationship between BAI and iWUE (Tau = -0.12, p = 0.20). Lastly, Juniperus virginiana BAI consistently increased by 0.13 cm2 yr-1 during the study period, accompanied by a 20% increase in iWUE (Tau = 0.46, p < 0.01). Together, these data provide evidence of contrasting trends in growth and iWUE of commonly occurring eastern U.S. tree species, and highlight changes in physiology at the leaf level (i.e. iWUE) are not consistently reflected in growth (i.e. BAI).