Spatial variability in carbon- and nitrogen-related traits in apple trees: the effects of the light environment and crop load.

Photosynthetic carbon assimilation rates are highly dependent on environmental factors such as light availability and on metabolic limitations such as the demand for carbon by sink organs. The relative effects of light and sink demand on photosynthesis in perennial plants such as trees remain poorly characterized. The aim of the present study was therefore to characterize the relationships between light and fruit load on a range of leaf traits including photosynthesis, non-structural carbohydrate content, leaf structure, and nitrogen-related variables in fruiting ('ON') and non-fruiting ('OFF') 'Golden Delicious' apple trees. We show that crop status (at the tree scale) exerts a greater influence over leaf traits than the local light environment or the local fruit load. High rates of photosynthesis were observed in the ON trees. This was correlated with a high leaf nitrogen content. In contrast, little spatial variability in photosynthesis rates was observed in the OFF trees. The lack of variation in photosynthesis rates was associated with high leaf non-structural carbohydrate content at the tree level. Taken together, these results suggest that low carbon demand leads to feedback limitation on photosynthesis resulting in a low level of within-tree variability. These findings provide new insights into carbon and nitrogen allocations within trees, which are heavily dependent on carbon demand.

Foliage motion under wind, from leaf flutter to branch buffeting.

The wind-induced motion of the foliage in a tree is an important phenomenon both for biological issues (photosynthesis, pathogens development or herbivory) and for more subtle effects such as on wi-fi transmission or animal communication. Such foliage motion results from a combination of the motion of the branches that support the leaves, and of the motion of the leaves relative to the branches. Individual leaf dynamics relative to the branch, and branch dynamics have usually been studied separately. Here, in an experimental study on a whole tree in a large-scale wind tunnel, we present the first empirical evidence that foliage motion is actually dominated by individual leaf flutter at low wind velocities, and by branch turbulence buffeting responses at higher velocities. The transition between the two regimes is related to a weak dependence of leaf flutter on wind velocity, while branch turbulent buffeting is strongly dependent on it. Quantitative comparisons with existing engineering-based models of leaf and branch motion confirm the prevalence of these two mechanisms. Simultaneous measurements of the wind-induced drag on the tree and of the light interception by the foliage show the role of an additional mechanism, reconfiguration, whereby leaves bend and overlap, limiting individual leaf flutter. We then discuss the consequences of these findings on the role of wind-mediated phenomena.

A double-digitising method for building 3D virtual trees with non-planar leaves: application to the morphology and light-capture properties of young beech trees (Fagus sylvatica).

We developed a double-digitising method combining a hand-held electromagnetic digitizer and a non-contact 3D laser scanner. The former was used to record the positions of all leaves in a tree and the orientation angles of their lamina. The latter served to obtain the morphology of the leaves sampled in the tree. As the scanner outputs a cloud of points, software was developed to reconstruct non-planar (NP) leaves composed of triangles, and to compute numerical shape parameters: midrib curvature, torsion and transversal curvature of the lamina. The combination of both methods allowed construction of 3D virtual trees with NP leaves. The method was applied to young beech trees (Fagus sylvatica L.) from different sunlight environments (from 1 to 100% incident light) in a forest in central France. Leaf morphology responded to light availability, with a more bent shape in well-lit leaves. Light interception at the leaf scale by NP leaves decreased from 4 to 10% for shaded and sunlit leaves compared with planar leaves. At the tree scale, light interception by trees made of NP leaves decreased by 1 to 3% for 100% to 1% light, respectively.