DISCERNING THE FOREST FROM THE TREES - AN ESSAY ON SCALING CANOPY STOMATAL CONDUCTANCE

Stomata are major conduits for the diffusion of many trace gases between leaves and the atmosphere. The role of the stomata in controlling gas exchange between the terrestrial biosphere and the atmosphere at the landscape, meso- and global scales has only recently been appreciated. Further advances in modeling trace gas exchange will depend on our ability to provide realistic information on stomatal mechanics at the sub-grid scale of landscape and meso scale models; in other words, information is needed at the canopy scale. This paper describes two approaches for estimating canopy stomatal conductance. These are the 'bottom-up' and 'top-down' scaling methods. The bottom-up method entails computing canopy stomatal conductance by integrating the response of individual leaves to controling biotic and abiotic factors, which are determined by the micrometeorology of the canopy. The top-down approach entails inverting a descriptive stand-level, trace gas exchange model to estimate canopy stomatal conductance. The model is driven with abiotic variables measured at a reference point above the canopy. The strengths and weaknesses of these two approaches are discussed, and recommendations for future research are presented.