Shoot structure and growth along a vertical profile within a Populus-Tilia canopy

We investigated shoot growth patterns and their relationship to the canopy radiation environment and the distribution of leaf photosynthetic production in a 27-m-tall stand of light-demanding Populus tremula L. and shade-tolerant Tilia cordata Mill. The species formed two distinct layers in the leaf canopy and showed different responses in branch architecture to the canopy light gradient. In P. tremula, shoot bifurcation decreased rapidly with decreasing light, and leaf display allowed capture of multidirectional light. In contrast, leaf display in T. cordata was limited to efficient interception of unidirectional light, and shoot growth and branching pattern facilitated relatively rapid expansion into potentially unoccupied space even in the low light of the lower canopy. At the canopy level, T. cordata had higher photosynthetic light-use efficiency than P. tremula, whereas P. tremula had higher nitrogen-use efficiency than T. cordata. However, at the individual leaf level, both species had similar efficiencies under comparable light conditions. Production of new leaf area in the canopy followed the pattern of photosynthetic production. However, the species differed substantially in extension growth and space-filling strategy. Light-demanding P. tremula expanded into new space with a few long shoots, with shoot length strongly dependent on photosynthetic photon flux density (PPFD). Production of new leaf area and extension growth were largely uncoupled in this species because short shoots, which do not contribute to extension growth, produced many new leaves. Thus, in P. tremula, the growth pattern was strongly directed toward the top of the canopy. In contrast, in shade-tolerant T. cordata, shoot growth was weakly related to PPFD and more was invested in long shoot growth on a leaf area basis compared with P. tremula. However, this extension growth was not directed and may serve as a passive means of avoiding self-shading. This study supports the hypothesis that, for a particular species, allocation patterns and crown architecture contribute as much to shade tolerance as leaf-level photosynthetic acclimation.