Adjustment of leaf anatomical and hydraulic traits across vertical canopy profiles of young broadleaved forest stands

No evidence of coordination between leaf hydraulic function and stomatal characteristics was observed in young hybrid aspen stands attributable to the near-anisohydric behaviour and the fast-growing nature of this species. Within forest canopies, there are environmental gradients that enhance water losses in the direction of treetops. Therefore, the foliar morpho-anatomical plasticity, which allows plant acclimation to environmental variations, is of great importance, but empirical information about the relationship between leaf structural and functional traits is inconsistent, varying with ecological requirements and the life strategy of the species. In this study, leaf hydraulic conductance, anatomical structure, and stomatal morphology were investigated in young hybrid aspen (Populus tremula × P. tremuloides) stands to elucidate the coordination between structural and hydraulic traits within the canopy. Stomatal size and density demonstrated opposite trends with respect to leaf location, leading to a relatively uniform spatial distribution of potential gas exchange capacity in the canopy. The hydraulic system of branches was adjusted to maximise the potential water supply in the midcanopy, containing nearly half of the trees’ total photosynthetic surface. Most vascular traits in both the petioles and laminae demonstrated stronger development towards the treetop that should enhance the water supply of the upper foliage, which is exposed to higher irradiance and atmospheric evaporative demand. Small increases in the total vein density expressed per unit lamina area in the upper canopy could not compensate for substantial leaf thickening, resulting in decreased vein density expressed per unit lamina volume and increasing the resistance of the leaf extravascular pathway. No solid evidence of coordination was found between leaf hydraulic function and stomatal characteristics attributable to the near-anisohydric behaviour and the fast-growing nature of this species.