Tree diversity affects chlorophyll a fluorescence and other leaf traits of tree species in a boreal forest

An assemblage of tree species with different crown properties (leaf life span, leaf morphology, light requirements, branching models and distribution of foliage) creates heterogeneous environments at the canopy level in relation to light availability, wind speed and water vapor pressure, which affect photosynthesis and transpiration. This heterogeneity can be expected to be reflected in changes of the functional leaf traits of trees, with special reference to those associated with light interception and use, photosynthesis and transpiration. The effects of tree species diversity (species richness and composition) of forest stands on photosynthetic properties (assessed with chlorophyll a fluorescence – ChlF – analysis) and functional leaf traits of trees were investigated in a boreal forest in Finland. The following leaf traits were assessed: specific leaf area (SLA), nitrogen content (mass-based, N) and carbon isotope composition (δ13C). Needles of Picea abies (spruce) and Pinus sylvestris (pine) and leaves of Betula pendula (birch) were sampled in monospecific and mixed forest stands. Different responses among the species were observed in relation to species richness and species composition of the stands. The functionality and efficiency of the photosystems were positively influenced by the number of species in the birch stands, whereas spruce showed higher efficiency of the photosystems in monospecific stands. Pine showed different photosynthetic responses when it coexisted with spruce or birch. The presence of birch in the forest had a positive effect on the efficiency of photosynthetic electron transport in pine (expressed with the I-P phase of the ChlF transient) that was positively correlated with an increase of δ13C. These effects may be related to high light availability at the canopy level in mixed stands with birch, due to the less dense canopy of birch trees. The results showed that in mixed stands, the functional leaf traits of forest tree species are influenced by species richness and species composition of the stand on the basis of the light requirements of the coexisting species. Multivariate statistical showed that a combination of ChlF parameters and leaf traits are able to individuate homogeneous clusters of tree species assemblage in pine and more partially in birch.