Hetherington & Woodward, 2003). Under reference conditions of low water stress and non-limiting radiation and temperature, the open apertures of stomata lead to unstressed stomatal conductance (g s,u ). During periods of water, light, or temperature stress, stomata close, thus downregulating stomatal conductance. Over the past decades, much attention has been focused on evaluating the reduction of stomatal conductance in response to

The spatio-temporal variation of stomatal conductance directly regulates photosynthesis, water partitioning, and biosphereatmosphere interactions. While many studies have focused on stomatal response to stresses, the spatial variation of unstressed stomatal conductance remains poorly determined, and is usually characterized in land surface models (LSMs) simply based on plant functional type (PFT). Here, we derived unstressed stomatal conductance at the ecosystem-scale using observations from 115 global FLUXNET sites. When aggregated by PFTs, the across-PFT pattern was highly consistent with the parameterizations of LSMs. However, PFTs alone captured only 17\% of the variation in unstressed stomatal conductance across sites. Within the same PFT, unstressed stomatal conductance was negatively related to climate dryness and canopy height, which explained 45\% of the total spatial variation. Our results highlight the importance of plant-environment interactions in shaping stomatal traits. The trait-environment relationship established here provides an empirical approach for improved parameterizations of stomatal conductance in LSMs.