Plasticity of tree architecture through interspecific and intraspecific competition in a young experimental plantation

Abstract It is acknowledged that trees behave remarkably plastic in response to environmental conditions. Even so, knowledge of how tree architecture in pure and mixed stands compare is largely underexplored. Such information is relevant from a fundamental ecological and an applied silvicultural perspective, given the increased attention for mixed species silviculture and the close linkages between tree architecture and high-quality timber production. The main objective of this work was to test the effects of competition, diversity and species identity of neighboring trees on the architecture of five important European tree species (Quercus robur, Betula pendula, Fagus sylvatica, Pinus sylvestris and Tilia cordata) in a temperate plantation before and during canopy closure. Data were collected in FORBIO-Zedelgem, a five-year old tree diversity experiment in Belgium. For 396 trees we measured architectural properties including branchiness, tree height-to-diameter (HD) ratio, branch diameter and branch insertion angle, and we investigated how these properties were shaped in different competitive neighborhoods using mixed regression models. Species showed contrasting architectural responses to neighborhood competition, in line with species life-history strategies. In more competitive environments, trees of Q. robur (slow growing and light-demanding) increased HD ratio and branch insertion angle to optimize light foraging in the upper canopy; trees of B. pendula (fast growing and light-demanding) increased HD ratio and decreased branching following the branch autonomy principle; trees of F. sylvatica (slow growing and shade tolerant) increased branching to improve light uptake under shading and finally, trees of P. sylvestris (fast growing and light-demanding) and T. cordata (slow growing and shade tolerant) were not shaped in response to competition. Diversity and identity of species in a trees’ neighborhood did not contribute to the architectural plasticity, although competitive differences between pure and mixed stands underpinned such effects for B. pendula, with lower branching in the highly competitive monocultures. We conclude that competition between trees, but not diversity, influences the architecture of young plantation trees before and during canopy closure in mixtures. To guide tree architectural development towards high-quality timber, management may have to pay considerable attention to competitive processes already in the juvenile forest stages.