Influence of species interactions and climate on tree mortality in mixed stands of Pinus sylvestris, Betula spp., and Picea abies

Abstract Tree mortality is a complex process associated with endogenous factors such as tree size, species composition or stand density, which may interact with each other and/or with exogenous factors such as climate. We studied the influence of these factors on background tree mortality in Scots pine, Norway spruce, and downy and silver birch mixed forests located in Central and Northern European continental (Poland) and boreal (NE Poland, Lithuania, Sweden and Norway) regions. Our main objective was to disentangle how species interactions influence background mortality along a gradient of climate conditions. To achieve this, we developed mortality models using a set of 188405 sample trees in 6840 permanent sample plots, taken from the National Forest Inventories of the four countries. Our results indicate that tree mortality and the effects of competition were species-specific. The competition from pine resulted in higher tree mortality for the pine itself rather than for birch or spruce, and the competition from spruce was usually the greatest, causing higher tree mortality in the three studied species. Temperature modified the effects of competition, always magnifying the effects of intra- or inter-specific competition depending on the species. However, the effect of temperature was complex, as the density-dependent and the density-independent mortality varied in different ways with temperature. In general, mixing pine with spruce and birch increases pine mortality, whereas it favors spruce and birch survival, especially at warmer sites. These findings may be useful for the management of these mixtures in the context of climate change.