The response of ecosystem water-use efficiency to rising atmospheric CO 2 concentrations: sensitivity and large-scale biogeochemical implications.

Ecosystem water-use efficiency (WUE) is an important metric linking the global land carbon and water cycles. Eddy covariance-based estimates of WUE in temperate/boreal forests have recently been found to show a strong and unexpected increase over the 1992-2010 period, which has been attributed to the effects of rising atmospheric CO ₂ concentrations on plant physiology. To test this hypothesis, we forced the observed trend in the process-based land surface model JSBACH by increasing the sensitivity of stomatal conductance (g _s ) to atmospheric CO ₂ concentration. We compared the simulated continental discharge, evapotranspiration (ET), and the seasonal CO ₂ exchange with observations across the extratropical northern hemisphere. The increased simulated WUE led to substantial changes in surface hydrology at the continental scale, including a significant decrease in ET and a significant increase in continental runoff, both of which are inconsistent with large-scale observations. The simulated seasonal amplitude of atmospheric CO ₂ decreased over time, in contrast to the observed upward trend across ground-based measurement sites. Our results provide strong indications that the recent, large-scale WUE trend is considerably smaller than that estimated for these forest ecosystems. They emphasize the decreasing CO ₂ sensitivity of WUE with increasing scale, which affects the physiological interpretation of changes in ecosystem WUE.
(© 2016 Max Planck Institute for Biogeochemistry New Phytologist © 2016 New Phytologist Trust.)