Optimal Stomatal Behaviour Around the World: Synthesis of a Global Stomatal Conductance Database and Scaling from Leaf to Ecosystem

Abstract:

Stomatal conductance (g_s) is a key land surface attribute as it links transpiration, the dominant component of global land evapotranspiration and a key element of the global water cycle, and photosynthesis, the driving force of the global carbon cycle. Despite the pivotal role of g_s in predictions of global water and carbon cycles, a global scale database and an associated globally applicable model of g_s that allow predictions of stomatal behaviour are lacking. We present a unique database of globally distributed g_s obtained in the field for a wide range of plant functional types (PFTs) and biomes. We employed a model of optimal stomatal conductance to assess differences in stomatal behaviour, and estimated the model slope coefficient, g₁, which is directly related to the marginal carbon cost of water, for each dataset. We found that g₁ varies considerably among PFTs, with evergreen savanna trees having the largest g₁ (least conservative water use), followed by C₃ grasses and crops, angiosperm trees, gymnosperm trees, and C₄ grasses. Amongst angiosperm trees, species with higher wood density had a higher marginal carbon cost of water, as predicted by the theory underpinning the optimal stomatal model. There was an interactive effect between temperature and moisture availability on g₁: for wet environments, g₁ was largest in high temperature environments, indicated by high mean annual temperature during the period when temperature above 0^oC (T_m), but it did not vary with T_m across dry environments. We examine whether these differences in leaf-scale behaviour are reflected in ecosystem-scale differences in water-use efficiency. These findings provide a robust theoretical framework for understanding and predicting the behaviour of stomatal conductance across biomes and across PFTs that can be applied to regional, continental and global-scale modelling of productivity and ecohydrological processes in a future changing climate.