A New Approach to Assess CO2/H2O Exchange: The Chilean Mediterranean Savanna Ecosystem Carbon Assimilation Efficiency

Stomatal conductance has proven to be one of the most important factors for water and carbon exchange between plants and the atmosphere. Stomatal conductance is a key mechanism in which plants try to maximize their CO2 uptake, but also restrict unnecessary water loss through transpiration. As carbon is gained through photosynthesis inevitably leads to transpiration by the leaf, Water Use Efficiency (WUE) is an important gas exchange parameter available to quantify carbon gain as a function of water loss. In general, WUE becomes higher as the stomatal conductance becomes smaller, so in some cases high WUE plants have a poor rate of CO2 uptake. This work presents two methods to estimate stomatal conductance through eddy covariance data collected on a Chilean Mediterranean savanna ecosystem. The first method is a physiological approach based on the Ball and Berry model, and the second method takes the biophysical model proposed by the Shuttleworth and Wallace. In this ecosystem, we have found a poor CO2/H2O exchange, usually related to high WUE values. We propose an alternative WUE index corrected by stomatal conductance. This new index would represent the actual carbon assimilation efficiency as a function of transpiration, but also weighted by stomatal conductance as a proxy of the total net canopy gas exchange.