Impact of doubled CO₂ on global‐scale leaf area index and evapotranspiration: Conflicting stomatal conductance and LAI responses

Abstract

Current increase in atmospheric CO 2 is expected to modify both climate and plant function, thereby impacting plant structure and gas exchange. We investigate the effects of doubled CO 2 on leaf area index (LAI) and evapotranspiration (ETR) using a global vegetation model for present‐day and doubled‐CO 2 conditions. The model assumes that adaptation of plants to the local climate leads to an equilibrium LAI, which depends on resource availability (minimizing water stress, canopy carbon cost and self‐shading). The model compares reasonably well with remote sensing estimates of LAI. Four doubled‐CO 2 simulations are designed to investigate the role of climate, CO 2 ‐induced stomatal closure, enhanced photosynthesis, and a combination of these effects. These simulations show that plant physiological responses to doubled CO 2 are potentially more important than climate changes for LAI, and equally important for ETR. In addition, even the sign of the simulated changes in LAI and ETR varies with the assumptions on plant responsiveness to CO 2 . A reduction of stomatal conductance moderates or cancels the water losses caused by a warmer and drier climate, but photosynthesis stimulation counteracts this stomatal effect, especially in the mid‐to‐high latitudes, because of enhanced LAI. Experimental evidence of LAI and ETR response to CO 2 has been reviewed and compared to the different simulations. On the basis of this confrontation we argue that plant response to CO 2 doubling may have a relatively small net impact on global ETR and may cause a moderate increase of LAI. Tree stomata may be less responsive to CO 2 than was previously assumed, and stimulated plant growth partly cancels the water savings caused by stomatal closure. Understanding the responses of plant canopies to CO 2 is therefore critical for land surface hydrology in a CO 2 rich world.