Spatial and temporal changes in WUE:
linking tree and ecosystem scale measurements at AmeriFlux forestED siteS
SOURCE of funding
Water-use efficiency (WUE), the carbon (C) gain per unit of water loss through transpiration, is a key physiological parameter linking C and water cycles. Its definition and quantification vary depending on the scale of investigation (leaf, tree or ecosystem), the time resolution (seconds, seasons, and years), and the measurement methods used (leaf gas exchanges, δ13C, and eddy covariance). The measure of δ13C in foliar and tree ring samples provide an estimate of intrinsic water-use efficiency, (iWUE, the ratio between photosynthesis, A, and stomatal conductance, gs) at the tree level. Whereas, the ecosystem water use efficiency, WUEe, is derived as the ratio between, gross primary production, GPP and evapotranspiration, ET, both of them obtained by eddy covariance (EC) data. Although the approach of integrating tree-scale (e.g., using leaf or tree ring δ13C) and ecosystem-scale (e.g., based on EC data) measurements to assess WUE is not new, most studies to date have focused on within-site comparisons of the methods applied to forests (e.g., Belmecheri et al., 2014; Scartazza et al., 2014). Analyses that compare both methods applied at the same site, across a range of forest type, and a broad climate gradient are less common. Reconciling the two approaches (leaf and tree-ring isotopes vs. and ecosystem fluxes) and assessing which factors drive changes in WUE (across sites and over time) was the overall aim of the study I conducted during my postdoc at the University of New Hampshire, in Scott Ollinger and Heidi Asbjornsen's group.
1- Spatial changes in WUE. Our analysis at 11 AmeriFlux sites spanned leaf and ecosystem scales and included foliar δ13C, δ18O, and %N measurements; eddy covariance estimates of GPP and ET; and remotely sensed estimates of canopy %N. We found that GPP, ET, and WUEe scaled with remotely sensed-derived canopy %N, even when environmental variables were considered, and discussed the implications of these relationships (particularly canopy %N and ET) for forest-atmosphere-climate interactions. We observed opposing patterns of WUE at leaf and ecosystem scales and examined uncertainties to help explain these opposing patterns. Nevertheless, significant relationship between C isotope-derived ci/ca and GPP indicates that δ13C can be an effective predictor of forest GPP. Finally, we showed that incorporating species functional traits—wood anatomy, hydraulic strategy, and foliar %N—into a conceptual model improved the interpretation of Δ13C and δ18O vis-à-vis leaf to canopy water-carbon fluxes. For this part of the study I worked very closely with Lucie Lepine and Jinfeng Xiao at UNH.
2- Temporal changes in WUE and mechanisms involved. There has been a lively discussion in the literature over the last 5 years, trying to reconcile the scales and approaches for the estimate of WUE and its physiological mechanisms, particularly the response of gs to CO2, often based on the comparison of independent dataset. We compared the two methods (tree-ring isotopes and EC) and investigated this discrepancy by focusing on 8 of the 11 AmeriFlux forest sites included in the study described above, spanning from moist to dry climates. Moreover, we we also compared observed trend in WUE with prediction from the optimal carbon-water balance (Wang et al. 2017). Finally we looked at drivers of changes in δ13C-derived iWUE for the last three decades and physiological mechanisms involved, with particular reference to changes in gs as assessed by tree-ring δ18O values. While developing this part of the study, I worked very closely with Soumaya Belmecheri, Beni Stocker, and Katie Jennings (UNH). Other collaborators included PIs at the 8 AmeriFLux sites included in the study. Forest WUE has been increasing over time, but the underlying physiological mechanism is different according to the tree species and the environment: contrary to earlier findings, increases in A were widespread, while gs reduction was mostly restricted to forests with limited moisture. Our results suggest that under a 15% increase in atmospheric CO2 (from 1982 to 2012) trees were conservative (in their water use) in moisture-limited sites, but they were not elsewhere (including forests in the northeastern U.S.) so to maintain or increase growth. Check out our paper published in PNAS for more details!