EGU21-10443
https://doi.org/10.5194/egusphere-egu21-10443
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Predicted and observed multidecadal variations of tree physiological responses to climate and rising  CO2: insights from tree-ring carbon isotopes in temperate forests.

Soumaya Belmecheri1, R. Stockton Maxwell2, Alan. H Taylor3, Kenneth. J Davis4, Rossella Guerrieri5, David. J.P. Moore6, and Shelly. A Rayback7
Soumaya Belmecheri et al.
  • 1University of Arizona, Laboratory of Tree Ring Research, Tucson, Arizona 85721, USA. (sbelmecheri@email.arizona.edu)
  • 2Radford University, Department of Geospatial Science, Radford, Virginia, 24142, USA. (rmaxwell2@radford.edu)
  • 3The Pennsylvania State University, Department of Geography and Earth and Environmental Systems Institute, University Park, Pennsylvania, 16802, USA. (aht1@psu.edu)
  • 4The Pennsylvania State University, Department of Meteorology and Atmospheric Science and Earth and Environmental Systems Institute, University Park, Pennsylvania, 16802, USA. (kjd10@psu.edu)
  • 5Department of Agricultural and Food Sciences (DISTAL), University of Bologna, Viale Fanin, 44, 40127, Bologna, Italy (rossellaguerrieri@gmail.com)
  • 6University of Arizona, School of Natural Resources and the Environment, Tucson, Arizona, 85721, USA. (davidjpmoore@email.arizona.edu)
  • 7University of Vermont, Department of Geography, Burlington, Vermont, 05405, USA. (shelly.rayback@uvm.edu)

Increasing water-use efficiency (WUE), the ratio of carbon gain to water loss, is a key mechanism that enhances carbon uptake by terrestrial vegetation under rising atmospheric CO2 (ca). Existing theory and empirical evidence suggest a proportional increase of WUE in response to rising ca as plants maintain a relatively constant ratio between the leaf internal (ci) and ambient (ca) partial CO2 pressure (ci/ca). This has been hypothesized as the main driver of the strengthening of the terrestrial carbon sink over the recent decades. However, proportionality may not characterize CO2 effects on WUE on longer time-scales and the role of climate in modulating these effects is uncertain. We evaluated the long-term WUE responses to ca and climate from 1901-2012 CE by reconstructing intrinsic WUE (iWUE, the ratio of photosynthesis to stomatal conductance) using carbon isotopes in tree rings across temperate forests in the northeastern USA. We further replicated iWUE reconstructions at eight additional sites for the 1992-2012 period-overlapping with the common period of the longest flux-tower record at Harvard Forest to evaluate the spatial coherence of recent iWUE variation across the region. Finally, we compared tree-ring based and modelled ci/ca over the 1901-2012 period to examine whether temporal patterns of ci/ca reconstructions are consistent with predictions based on the optimality principle of balancing the costs of water loss and carbon gain.

We found that iWUE increased steadily from 1901 to 1975 CE but remained constant thereafter despite continuously rising ca. This finding is consistent with a passive physiological response to ca and coincides with a shift to significantly wetter conditions across the region. Tree physiology was driven by summer moisture at multi-decadal time-scales and did not maintain a constant ci/ca in response to rising ca indicating that a point was reached where rising CO2 had a diminishing effect on tree iWUE.  The ci/ca derived from tree-ring d13C and the predicted values based on the optimality theory model had similar median values over the 1901-2012 CE period, though with a modest agreement (R2adj = 0.22, p < 0.001). The reconstructed and predicted ci/ca trends were not statistically different from 0 when estimated over the 1901-2012 CE period; however, isotope-based reconstruction of the ci/ca trendshowed distinct multidecadal variation while the predicted ci/ca remained nearly constant. Our results challenge the mechanism, magnitude, and persistence of CO2’s effect on iWUE with significant implications for projections of terrestrial productivity under a changing climate.

How to cite: Belmecheri, S., Maxwell, R. S., Taylor, A. H., Davis, K. J., Guerrieri, R., Moore, D. J. P., and Rayback, S. A.: Predicted and observed multidecadal variations of tree physiological responses to climate and rising  CO2: insights from tree-ring carbon isotopes in temperate forests., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10443, https://doi.org/10.5194/egusphere-egu21-10443, 2021.

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