Effect of tree species mixing on the size structure, density, and yield of forest stands

Abstract

An increasing number of studies provide evidence that mixed-species stands can overyield monocultures. But it is still hardly understood, how the overyielding at the stand level emerges from the tree, canopy, and size structure. Analyses of 42 triplets with 126 mixed and mono-specific plots in middle-aged, two-species stands of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies [L.] Karst.), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and European beech (Fagus sylvativa L.) in Central Europe revealed that mixed-species compared with mono-specific stands can have (1) higher tree numbers, higher right skewness and kurtosis of the size distribution, higher inequality of tree sizes, and thereby higher stocking densities and sum of crown projection areas, (2) growth–size relationships with stronger size asymmetry of growth and higher inequality of size growth, and (3) higher stand productivity coupled with higher maximum stand density, canopy space filling, and size asymmetry. These differences depend on the species assemblage. They suggest a deeper entrance of light into the canopy as well as a higher light interception and light-use efficiency as main causes of the overyielding and overdensity. We discuss implications for research and silviculture and draw conclusions for designing and managing resource-efficient production systems.