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Stomata control carbon and water vapour exchange between leaves and the atmosphere, thus it can influence water use efficiency (WUE) and reflect plant adaptation to climate. However, the spatial patterns of leaf stomatal traits and relationships between stomatal trait and WUE across natural communities remain unclear. We measured stomatal density, stomatal size and stomatal area fraction for 737 plant species from nine forests ranging from tropical to cold temperate forests. Stomatal density, stomatal size and stomatal area fraction were all log-normally distributed, and different across species, plant functional groups (trees, shrubs, and herbs), and communities. At the regional scale, variation in stomatal traits was primarily related to species, followed by climate and soil types. The community-weighted mean of stomatal size increased linearly with latitude, whereas those of stomatal density and stomatal area fraction showed humpbacked relationship. The community-weighted mean of stomatal area fraction was correlated with climatic aridity, consistent with the adaptation strategies of plant species to achieve high maximum rates of gas exchange in arid regions when water is available. Further, community-weighted mean of stomatal area fraction was positively correlated with WUE in natural forest communities, indicating that plants have lower stomatal conductance in order to adapt greater aridity conditions. These findings highlight the strong associations of stomatal traits with plant functional group and climate at a regional scale, representing the adaptation strategies of stomatal traits across natural forest communities to climate.

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