Environment-driven intraspecific variation shows coordination of functional traits of deciduous oaks among and within different biological levels

Deciduous oaks (Quercus spp.) are distributed from subalpine to tropical regions in the northern hemisphere and have important roles as carbon sinks and in climate change mitigation. Determining variations in plant functional traits at multiple biological levels and linking them to environmental variables across geographical ranges is important for forecasting range-shifts of broadly-distributed species under climate change. We sampled leaves of five deciduous Quercus spp. covering approximately 20° of latitude (~ 21° N − 41° N) and 20 longitude (~ 99° E − 119° E) across China and measured 12 plant functional traits at different biological levels. The traits varied distinctively, either within each biological level or among different levels driven by climatic and edaphic variables. Traits at the organ level were significantly correlated with those at the cellular and tissue levels, while traits at the whole-plant level only correlated with those at the tissue level. The Quercus species responded to changing environments by regulating stomatal size, leaf thickness and the palisade mesophyll thickness to leaf thickness ratios with contrasting degree of effect to adjust the whole-plant functioning, i.e., intrinsic water use efficiency (iWUE), carbon supply and nitrogen availability. The results suggest that these deciduous Quercus spp. will maintain vigour by increasing iWUE when subjected to large temperature changes and insufficient moisture, and by accumulating leaf non-structural carbohydrates under drought conditions. The findings provide new insights into the inherent variation and trait coordination of widely distributed tree species in the context of climate change.