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Zusammenfassung:
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 CO2 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 (gs) to atmospheric CO2
concentration. We compared the simulated continental discharge, evapotranspiration (ET),
and the seasonal CO2 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 CO2 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 CO2
sensitivity of WUE with increasing scale, which affects the physiological interpretation of changes in ecosystem WUE.
