Temperate forests are strong carbon sinks that mitigate global climate change. However, the sustainability of this important ecosystem service in the face of ongoing climate change is unclear, and will depend on the adaptability of the ratio of photosynthesis to stomatal conductance (often called the intrinsic water use efficiency, or iWUE). Results from experimental and modeling studies suggest that CO2 fertilization increases plant iWUE, which is an important assumption in many ecosystem models. However, there is increasing evidence that iWUE increases in response to rising vapor pressure deficit (VPD), driven by hotter and drier conditions. The goal of this study is to better understand the effects of both CO2 fertilization and hydrologic stress on iWUE. Given that VPD has risen in recent decades, we hypothesize that the iWUE response to CO2 fertilization may be significantly conflated with response to VPD. We also hypothesize that the sensitivity of iWUE to VPD will be higher for species with a more isohydric water use strategy.
We collected tree cores from two species with different water use strategies, an isohydric species (tulip poplar, Liriodendron tulipifera L.) and an anisohydric species (white oak, Quercus alba L.) growing in Morgan Monroe State Forest in Southern Indiana. Annual iWUE from 1980 to 2012 was derived from latewood stable carbon isotopes, and was compared with annually averaged VPD and atmospheric CO2. We also compared the tree-level trends in iWUE to those derived at the ecosystem-scale by leveraging the >18 year record of eddy covariance flux measurements from the site.
Both CO2 and VPD have increased significantly in the study site over recent decades, and the iWUE of tulip poplar was equally sensitive to the changes in both variables (β of 0.34 and 0.28, respectively). In the anisohydric species (white oak), however, rising CO2 increased iWUE (β of 0.25), but rising VPD did not. These results are consistent with ecosystem-scale data, which show a >2.5-fold increase in iWUE as VPD increases from <1 to >4 kPa, noting that the stand is dominated by isohydric species. Together, our results suggest that care should be taken to disentangle VPD effects from CO2 fertilization effects when detecting iWUE trends in observational time series. They also suggest that isohydric species may mitigate reductions to carbon uptake imposed by stomatal closure during periods of hydrologic stress by increasing the efficiency of their photosynthetic machinery.