When, where, and why does atmospheric CO2 enhance water use efficiency in temperate mixed deciduous forests?
Abstract
Increasing atmospheric CO2 can enhance photosynthesis and intrinsic water use efficiency (iWUE) in trees, resulting in increased woody biomass production and reduced tree sensitivity to water stress. However, these benefits may only be realized when other environmental factors, such as climate or nutrients, are not limiting. Tree response to increasing CO2 is modulated by a variety of environmental variables, including water availability, vapor pressure deficit, atmospheric N and S deposition, and soil fertility. Further, many studies of tree iWUE assess only a single species, making it difficult to assess how CO2 and other change drivers will impact forest carbon and water balance at ecosystem scales. We examined trends in, and drivers of, intrinsic water use efficiency in seven broadleaf deciduous and three evergreen conifer species from seven watersheds spanning a ~6° latitudinal gradient in the Appalachian Mountains of the eastern United States. iWUE chronologies from 1950-2015 were calculated using tree ring 13C time series. Temporal trends were examined for each functional group and species, and linear mixed models including climate, atmospheric N and S deposition, soil base cations (reconstructed using a catchment biogeochemical model), tree size and age, and atmospheric CO2 were used to assess drivers of iWUE. We found that iWUE increased 29% during the study period, coincident with a 28% increase in atmospheric CO2. However, this simple association masks considerable variability in iWUE dynamics among tree functional types and species. We found linear increases in iWUE of broadleaf deciduous trees (slope=0.28 mol CO2 mol H2O yr-1, R2=0.12, p<0.001), while iWUE of conifers plateaued around 1990 (second order polynomial R2=0.56, p<0.001), suggestive of temporarily dynamic gas exchange strategies. Dominant environmental drivers of iWUE varied among species, but iWUE tended to be negatively associated with growing season precipitation and atmospheric deposition. Our findings of nonlinear iWUE trends within species and functional groups, combined with diverse iWUE responses to environmental drivers among species, highlight the need to account for both species composition and environmental factors that modulate tree response to CO2 in projections of forest productivity and water use.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.B44B..02M