Abstract
Key message Differences in architectural traits of European beech saplings submitted to different levels of competition intensity and neighborhood diversity show a shift toward lateral growth exploration for overtopped trees but less expressed apical control is maintained for beech grown with conifers. Abstract Crown plasticity is a fundamental process to optimize the acquisition of light in forests, where it is often the limiting factor for growth. The processes leading to crown dynamics in response to biotic interaction are controlled by the competitive status of a tree, as well as the species diversity of the neighborhood. Yet, the ontogenic diversity of the different branches leading to crown structure makes it difficult to model crown plasticity. In this study, we used single-image photogrammetry and a semi-automatic topology reconstruction software to map the architecture of 3-year-old European beech trees (Fagus sylvatica L.) grown in pure pots or mixed with Douglas fir (Pseudotsuga menziesii (Mirbel) Franco) or Norway spruce (Picea abies L. H.Karst). Further, we investigated the effect of competition intensity (estimated with height rank) and neighbourhood diversity on architectural traits. To do so, we quantified the mean volume, slenderness, number of branches, branching rate, internode length, and diameter ratio for each stem and branch, up to the order three. Overtopped trees developed fewer branches and biomass but allocated more biomass to branches of a higher order than to the stem. The increase in the ramification of the stem and the diameter ratio of the first-order branches for overtopped trees indicate a shift in the growth strategy and a decrease in apical control. As competition intensity and neighbour identity have opposite effects on stem traits, it seems that the positive effect of plant diversity on growth can be attributed to the intrinsic effects of species identity rather than to competition release.