Trimming influences tree light interception and space exploration: contrasted responses of two cultivars of Fraxinus pennsylvanica at various scales of their architecture

International audienceKey messageComplementarities and/or synergies among different scales of tree architecture enable to achieve different functions simultaneously and/or optimize one function. After trimming, tree reaction occurred in all the scales.AbstractTrees are modular organisms within which each scale has attributes enabling them to optimize different functions. Among these functions, space exploration and light interception are strong determinants of tree productivity and survival. By modifying the crown shape, including foliage dispersion in the 3D space and carbon allocation, tree trimming (i.e. the removal of the higher part of the crown) may largely influence how trees fulfill the space exploration and light interception functions. In this study, we used field measurements of tree annual shoots combined with tree reconstruction from Terrestrial Laser Scanning data to analyze how different scales of the tree architecture complement each other to fulfill different functions simultaneously or combine to fulfill one single function more efficiently. We used morphological and functional variables measured at the annual shoot, axis, and tree scales to compare the architectural and functional strategies of two Fraxinus pennsylvanica cultivars and observe how trimming modifies these strategies. Results show that the two cultivars exhibit different strategies. One cultivar tended to optimize the space exploration function at both the annual shoot and axis scales and to optimize the light interception by displaying a large leaf area at the tree scale. The other cultivar tended to optimize light interception efficiency at both axis and tree levels. In both cultivars, trimming improved light interception efficiency and increased investments into space exploration. Nevertheless, after trimming, the two cultivars maintained their main architectural and functional strategy

Linking ice accretion and crown structure: towards a model of the effect of freezing rain on tree canopies

International audienceBackground and Aims: Despite a longstanding interest in variation in tree species vulnerability to ice storm damage, quantitative analyses of the influence of crown structure on within-crown variation in ice accretion are rare. In particular, the effect of prior interception by higher branches on lower branch accumulation remains unstudied. The aim of this study was to test the hypothesis that intra-crown ice accretion can be predicted by a measure of the degree of sheltering by neighbouring branches.Methods: Freezing rain was artificially applied to Acer platanoides L., and in situ branch-ice thickness was measured directly and from LiDAR point clouds. Two models of freezing rain interception were developed: ‘IceCube’, which uses point clouds to relate ice accretion to a voxel-based index (sheltering factor; SF) of the sheltering effect of branch elements above a measurement point; and ‘IceTree’, a simulation model for in silico evaluation of the interception pattern of freezing rain in virtual tree crowns.Key Results: Intra-crown radial ice accretion varied strongly, declining from the tips to the bases of branches and from the top to the base of the crown. SF for branches varied strongly within the crown, and differences among branches were consistent for a range of model parameters. Intra-crown variation in ice accretion on branches was related to SF (R2 = 0·46), with in silico results from IceTree supporting empirical relationships from IceCube.Conclusions: Empirical results and simulations confirmed a key role for crown architecture in determining intra-crown patterns of ice accretion. As suspected, the concentration of freezing rain droplets is attenuated by passage through the upper crown, and thus higher branches accumulate more ice than lower branches. This is the first step in developing a model that can provide a quantitative basis for investigating intra-crown and inter-specific variation in freezing rain damage