Mechanical abrasion, and not competition for light, is the dominant canopy interaction in a temperate mixed forest

Competition for canopy space is a key process in forest dynamics, but the underlying mechanisms are not well understood in old-growth forests. We combined laser-based canopy structural analysis and length-increment measurements of canopy branches for studying lateral and vertical crown expansion in four temperate broad-leaved tree species (Fagus sylvatica, Fraxinus excelsior, Carpinus betulus, Tilia cordata) in an old-growth temperate mixed forest. Using a 30 m-tall mobile canopy lift for accessing the canopy contact zone between different pairs of species, we were able to analyse the growth dynamics of more than 230 branches in their dependence on local light availability and neighbour species identity. The minimum distance between neighbouring crowns (crown shyness) was also determined for different species pairs. Contrary to expectation, lateral branch increment in the contact zone was not dependent on light availability for three species; a positive relationship existed only for Fraxinus. More than 50 percent of the lateral branches of Fagus, Carpinus and Tilia were broken due to mechanical abrasion at least once within the last six years prior to our measurements (2006-2011). Only the thicker more robust branches of Fraxinus showed minor damages. Mechanical interaction in allospecific neighbourhood was highly asymmetric for the species combinations Fraxinus-Tilia, Fraxinus-Catpinus, Fagus-Tilia, and partly asymmetric in the pair Fraxinus-Fagus (lower damage in the respective first species). Crown shyness was highest in the Fagus-Fraxinus neighbourhood (average minimum distance: 1.0 m), intermediate in Catpinus-Fagus pairs (0.6 m), and low in Fagus-Fagus (0.2 m) and Fagus-Tilia pairs (0 m; direct contact). We conclude that the dynamics of lateral canopy expansion in this mixed forest are mainly determined by mechanical interactions and not by competition for light. Interactions were in many cases asymmetric and depended on the species. This has important implications for forest dynamics modelling and forest management in mixed stands. (C) 2015 Elsevier B.V. All rights reserved.