Functional relationships between leaves and stem across canopy layers in two contrasting clones of Populus nigra L.

Populus nigra L. represents a model system for plant biology and has a productive interest in breeding for short rotation forestry. The growth potential and adaptive capacity of this species are well characterized, but the canopy effect is poorly investigated. We analyzed morphological and functional leaf traits across a multilayer canopy profile in two contrasting clones of this species, 58-861 and Poli, respectively from northern and southern Italy, grown in field plantation. The results revealed how the variation of leaf functional traits was structured within the canopy. The two clones showed differences in leaf morphology and water use efficiency, but organized a similar functional canopy structure along a vertical profile, related to a gradient of light radiation. An acropetal enrichment gradient of carbon stable isotope was found both in leaves and stem wood across a vertical canopy profile and a tight correlation was found between carbon stable isotopes of leaves and of the respective stem section. Such a functional relationship indicates that substrates for stem growth were sourced from leaf assimilates of the closest canopy layer. These results characterize the physiology of black poplar under micro-environmental conditions at intra-canopy scale and contribute to clarify the canopy effect in young trees.