Optimal stomatal behaviour around the world

Stomatal conductance (gs) is a key land-surface attribute as it links transpiration, the dominant component of global land evapotranspiration, and photosynthesis, the driving force of the global carbon cycle. Despite the pivotal role of gs in predictions of global water and carbon cycle changes, a globalscale database and an associated globally applicable model of gs that allow predictions of stomatal behaviour are lacking. Here, we present a database of globally distributed gs obtained in the field for a wide range of plant functional types (PFTs) and biomes. We find that stomatal behaviour di ers among PFTs according to their marginal carbon cost of water use, as predicted by the theory underpinning the optimal stomatal model1 and the leaf and wood economics spectrum2,3. We also demonstrate a global relationship with climate. These findings provide a robust theoretical framework for understanding and predicting the behaviour of gs across biomes and across PFTs that can be applied to regional, continental and global-scale modelling of ecosystem productivity, energy balance and ecohydrological processes in a future changing climate. Earth system models (ESMs), which integrate biogeochemical and biogeophysical land-surface processes with physical climate models, have been widely used to demonstrate the importance of land-surface processes in determining climate and to highlight the large uncertainties in quantifying land-surface processes4–6. Within the biogeophysical components of land-surface processes, gs plays a pivotal role because it is a key feedback route for carbon and water exchange between the atmosphere and terrestrial vegetation. Stomata are small pores on leaves whose aperture is actively regulated by plants in response to multiple abiotic and biotic factors, and their conductance is a major determinant of global land evapotranspiration and global water and carbon cycles. Therefore, our ability to model the global carbon and water cycles under a future changing climate depends on our ability to predict gs globally7. Many ESMs at present use an empirical stomatal model to predict gs and, in the absence of information, assume identical parameter values for all non-water-stressed C3 and C4 vegetation. For example, the LPJmodel4 assumes a constant ratio of intercellular to ambient CO2 concentration (Ci:Ca) of 0.8 for all C3 vegetation and 0.4 for all C4 vegetation. The CABLE model8 uses the empirical stomatal model of Leuning9 with two sets of parameter values, one for all C3 vegetation and one for all C4 vegetation. The CLM 4.0 model10 uses the empirical stomatal model of Ball et al.11 with three sets of parameter values, one for C4, one for needle-leaf trees, and a third for all other C3 vegetation. Although there have been previous synthesis studies on plant stomatal conductance and related traits3,7,12,13, we lack a global-scale database and an associated globally applicable model of gs that allows predictions of stomatal behaviour among PFTs and across climatic gradients. For this study, we compiled a unique global database of field measurements of gs and photosynthesis suitable for estimating model parameters. We employed a model of optimal stomatal conductance14 to develop hypotheses for how stomatal behaviour should vary with environmental factors and with plant traits associated with hydraulic function. The optimization premise 8