Canopy space filling and tree crown morphology in mixed-species stands compared with monocultures

Mixed-species forest stands are well explored in their favourable ecological, economical, and socio-economical functions and services compared with pure stands, but still poorly understood in their structure and functioning. Canopy structure and tree morphology affect the environmental conditions within the stand, the tree growth, and by this most forest functions and services. Here, I review how canopy structure and crown morphology in mixed stands can differ from pure stands and how this depends on the selection of tree species and interactions between them. The focus is on the macrostructure of canopy and crowns derived from the trees' positions, their convex crown hulls, and their space filling with branches. In mixed canopies the sum of the crown projection area, but not the ground coverage by crowns, mostly exceeds pure stands due to multiple crown overlaps. The interspecific differences in crown shape and allometric scaling cause a 'selection effect' when complementary species are combined. In interspecific environment furthermore 'true mixing effects' like intraspecific shifts in size, shape, and inner space filling of crowns may occur. The much denser and more plastic canopy space filling in mixed stands may increase light interception, stand density, productivity, and growth resilience to disturbances. I discuss the relevance of interspecific interactions for forest management, model building, and theory development and draw perspectives of further research into stand canopy and crown structure. (C) 2014 The Author. Published by Elsevier B.V.