Spatial and temporal trends in water-use efficiency across U.S. forests: integrating tree ring stable C and O isotopes with eddy covariance data
Abstract
Understanding relations among forest carbon (C) uptake and water use is critical for predicting forest-climate interactions. Water use efficiency (WUE), the carbon (C) gain per unit of water (H2O) loss through transpiration, is the key physiological trait linking C and H2O cycling in forests, and allowing to monitor ecosystem productivity in response to climate change. Stable C isotope composition (δ13C) in tree rings has been extensively used to assess the changes in the tree-level intrinsic WUE (i.e., iWUE - photosynthesis, A/stomatal conductance, gs) in response to climate and anthropogenic forcing (e.g., increase in atmospheric CO2 and nitrogen deposition) over the last century for several forest ecosystems worldwide. At the forest ecosystem level, WUE (WUEe) is obtained as ratio between Gross Primary Productivity (GPP) and evapotranspiration (ET), derived from the eddy covariance measurements. Very few studies compared the two approaches, most of them to date have focused on within-site comparisons. Moreover, most studies examining the influence of climatic factors on tree-WUE have focused on water-limited ecosystems in the Southwest, while much less is known about the dynamics of WUE for mesic forests in the Eastern US. In this study, we compared the two methods across a range of eight to eleven forested Ameriflux sites and climate in the U.S. Furthermore, we examined whether species-specific physiological mechanisms facilitated a better understanding of the ecosystem fluxes. We will present 30-year δ13C (and derived iWUE) and δ18O tree-ring chronologies and foliar isotopes obtained from two dominant species at each site. Spatial (across sites) and temporal trend of tree WUE will then be compared to ecosystem WUE as obtained from eddy covariance data. Relationships between δ13C and δ18O will be explored to elucidate the species-specific physiological mechanisms underlying variation in iWUE. Moreover, drivers of the changes in WUE at the two scales (i.e., tree and ecosystem) will be evaluated and discussed in relations to findings from previous studies. Finally, we will explore the relationship between patterns of leaf internal CO2 (ci)-regulation in response to rising atmospheric CO2, which is one of the major causes of disagreement between the tree and ecosystem level approaches.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.B11B0457A
- Keywords:
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- 0426 Biosphere/atmosphere interactions;
- BIOGEOSCIENCESDE: 0428 Carbon cycling;
- BIOGEOSCIENCESDE: 1813 Eco-hydrology;
- HYDROLOGYDE: 1818 Evapotranspiration;
- HYDROLOGY