Uncertainties in the role of atmospheric CO2 concentration on evaporation: complex interactions with plant physiology, plant coverage and global warming

Abstract:

Here we review uncertainties in the role of atmospheric CO₂ concentrations on plant evaporation (E). Rising atmospheric CO₂ reduces plant stomatal conductance and increases plant water use efficiency (WUE). Given this effect, Earth System Models suggest that under conditions with no water limitation, the increase in atmospheric evaporative demand (AED) may not necessarily translate into increased E; this has important implications for water available in the form of runoff and soil moisture.

However, plant physiology will affect the influence of atmospheric CO₂ on E. If soil moisture is sufficiently available, the CO₂ uptake by plants will likely increase, potentially resulting in greater leaf area (together with longer vegetative periods as a consequence of warming) that would favor the increase of E. Last but not least, atmospheric CO₂ affects root development, making water at deeper levels potentially more accessible by plants. The fact that the rising CO₂ impact on other E components (interception loss and soil and open water evaporation) is only indirectly affected by WUE changes may be exploited in order to disentangle the WUE effects on available water.

The influence of higher temperatures on plant physiology is also highly relevant. Decoupling between photosynthesis and plant E during extreme high temperatures has been reported to increase E and reduce sensible heat during multiday periods at the ecosystem level. Also temperature effects on leaf wax porosity could be accentuated by warming, which may directly enhance E from leaf tissues. These physiological mechanisms would also interact with the projected VPD increase, which may enhance E under high soil moisture, but reduce stomatal conductance and E if soil moisture is limited. The net effect of these mechanisms on water resources is difficult to establish, and it will probably depend on the adaptation of different vegetation species to temperature thresholds, as well as the potential forest dieback and tree mortality. These aspects cannot be easily modeled, but may have strong hydrological implications. For these reasons, assessments of future water resources based on Earth System Models should be considered with caution, especially during dry periods in which these mechanisms are intertwined with the ecosystems response to water limitations.