Competitions and trade-offs in the eco-hydrologic response to CO2 and warming

Abstract:

Many studies find that elevated CO2 enhances water availability (by closing plant stomata) and/or vegetation productivity or greenness (by increasing water-use efficiency.) However, many other studies do not find evidence for the above response(s). In addition, many studies find that warmer temperatures are increasing plant water stress, particularly during droughts, leading to increased plant mortality. Here, I present a framework that aims to reconcile these results. I argue that, as a rule, elevated CO2 always either enhances water availability, or vegetation greenness - but that it does not always enhance both, since there is a trade-off between them. Further, I argue based on leaf gas exchange that the effect of air warming alone is not sufficient to cancel these effects of elevated CO2. Ironically, though, where elevated CO2 enhances vegetation greenness, warming will dangerously increase plants' vulnerability to drought stress. I also review recently discovered plant responses related to leaf temperature which may alter some of the above conclusions.