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

Abstract

[1] 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.

Climate forcing due to optimization of maximal leaf conductance in subtropical vegetation under rising CO 2

“…Moreover, increased photosynthesis might also increase turnover rates, leading to a more dynamic forest with unchanged biomass and LAI (37). Simulations with a global vegetation model, which takes these considerations into account, indicate that in subtropical forests LAI increases by a maximum of 10% after a doubling of CO 2 (38). This marginal increase in LAI increases canopy transpiration by ≈5%, which is not sufficient to compensate for reduced transpiration at the leaf level.…”

Climate model based consensus on the hydrologic impacts of climate change to the Rio Lempa basin of Central America

“…However, this can be complicated by the direct CO 2 effects, which are the direct responses of vegetation to rising levels of CO 2 (e.g., Wigley and Jones, 1985). Direct effects of CO 2 on vegetation affect evapotranspiration (ET) by two counteracting dynamics: CO 2 -induced stomatal closure (which reduces ET); and photosynthesis stimulation (which increases leaf area index and ET) (Kergoat et al, 2002). In tropical regions, these two direct CO 2 effects have been estimated to be of approximately equal magnitude, effectively canceling each other and leaving the net effect equal to that of warming alone (Levis et al, 2000), or leaving the direct CO 2 contribution small relative to that due to changing climate (Piao et al, 2007).…”

Estimate of extended long-term LAI data set derived from AVHRR and MODIS based on the correlations between LAI and key variables of the climate system from 1982 to 2009

“…Remotely sensed data are a primary source and are essential for monitoring and estimating ecophysiological and biophysical processes Soudani et al 2006;Garrigues et al 2008). Currently, the information obtained from the Advanced Very High Resolution Radiometer (AVHRR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) sensors is used to generate LAI products, and is widely used for vegetation monitoring and estimating global change and climate change (Carlson and Ripley 1997;Huete et al 1997;Kergoat et al 2002;Shabanov 2011, Yuan et al 2011;Liu, Liu, and Chen 2012).…”