When
Speaker: Soumaya Belmecheri, Tree Ring Laboratory, University of Arizona
Date: Wednesday, September 12th, 2018
Time: 3:00-4:00 pm
Location: ENR2, S107
Abstract: The terrestrial biosphere has been responding to rising atmospheric carbon dioxide concentration (ca), through increased plant photosynthesis and reduced stomatal conductance. The resulting enhancement in gross primary production contributes to the strengthening of the land carbon sink, which currently sequesters 30% of total anthropogenic ca emissions. Elevated ca leads to an increase in CO2 concentration within the leaf (ci), and plants adjust their stomatal conductance towards a proportional ratio of ca and ci (i.e. constant ci/ca), resulting in an increase of water use efficiency (WUE- ratio of carbon gain per unit of water loss). Under elevated ca, the increase in WUE has been widely observed in laboratory and ecosystem experiments and in tree-ring records in response to rising ca since 1850 CE. These observations are consistent with recent atmospheric evidence of greater photosynthetic carbon isotopic discrimination by the terrestrial biosphere, however, they contradict analysis of above-canopy CO2 and water vapor fluxes, which showed a larger increase in WUE over the past 20-years requiring trees to close their stomata to preserve a constant ci. While observational, proxy-based measurements and process-based models support a WUE enhancement with rising ca, the magnitude and underlying mechanisms of plant physiological responses to elevated ca remain highly uncertain. The implication of CO2 effect on forests WUE, global evapotranspiration, and continental runoff should therefore be regarded with caution. These findings have significant implications in predicting future terrestrial C sink and thus require exploring whether these responses represent a short-term acclimation or long-term response of plant functional traits to environmental changes. Analyses of tree ring stable C isotope ratios (d13C) since the onset of the industrial period allow examination of ci?regulation in response to rising ca, including the role of climate in modulating leaf-gas exchange strategies. Data suggest that WUE is still recently increasing in most species but that the rate of increase is less than expected and may have reached a plateau in the recent decades. This evidence demonstrates that a broadly conserved suite of functional traits allows woody plants to adapt their leaf gas exchange to elevated CO2.
FLYER