Petiole length and biomass investment in support modify light interception efficiency in dense poplar plantations.

Leaf architecture, stand leaf area index and canopy light interception were studied in 13 poplar clones growing in a second rotation of a coppice plantation, to determine the role of leaf architectural attributes on canopy light-harvesting efficiency and to assess biomass investment in leaf support tissue. Stand leaf area index (L) varied from 2.89 to 6.99, but L was only weakly associated with canopy transmittance (TC). The weak relationship between TC and L was a result of a higher degree of foliage aggregation at larger values of L, leading to lower light-interception efficiency in stands with greater total leaf area. We observed a strong increase in leaf aggregation and a decrease in light-harvesting efficiency with decreasing mean leaf petiole length (PL) but not with leaf size, possibly because, in cordate or deltoid poplar leaves, most of the leaf area is located close to the petiole attachment to the lamina. Although PL was the key leaf characteristic of light-harvesting efficiency, clones with longer petioles had larger biomass investments in petioles, and there was a negative relationship between PL and L, demonstrating that enhanced light harvesting may lead to an overall decline in photosynthesizing leaf surface. Upper-canopy leaves were generally larger and had greater dry mass (MA) and nitrogen per unit area (NA) than lower-canopy leaves. Canopy plasticity in MA and NA was higher in clones with higher foliar biomass investment in midrib, and lower in clones with relatively longer petioles. These relationships suggest that there is a trade-off between photosynthetic plasticity and biomass investment in support, and also that high light-harvesting efficiency may be associated with lower photosynthetic plasticity. Our results demonstrate important clonal differences in leaf aggregation that are linked to leaf structure and biomass allocation patterns within the leaf.