Tree species diversity enhances individual growth in a temperate forest of central Europe

Background/Question/Methods

Whether biodiversity favors ecosystem productivity and stability has been a central question in ecology over the past decade. Experiments in grasslands suggest that species richness promotes productivity through interspecific complementarity. We know surprisingly little about the biodiversity-ecosystem functioning relationship in forests, however, in part because their size and long life spans make trees are hard to study and cause temporal and spatial heterogeneity. Here we take an original approach by inferring past growth from tree rings in a temperate forest in the Czech Republic, and determine the effects of diversity on individual tree growth and stability. We selected plots to include all possible combinations of four of the most abundant European tree species, and measured diversity as the effective number of species, accounting for both species richness and evenness. We identified and mapped all trees (DBH>10cm) in the 45 plots, and extracted increment cores for 576 trees. Tree cores were measured and cross-dated, and yearly increment series were used to calculate individual growth rates. Data were further analyzed using classical dendrochronological methods and mixed-effect models.

Results/Conclusions

Absolute growth rates decreased with age and varied among species, with Fagus sylvatica growing fastest, followed, in order, by Picea abies, Quercus petraea, and Larix decidua. More importantly, growth rates were positively related to species diversity, which also correlated with structural diversity (i.e. the distribution of tree sizes in the plot). Moreover, the sensitivity of yearly increment to climate varied among species. Together, these results suggest that European tree species are complementary in their use of spatial and temporal niches. The coefficient of variation of growth rate differed among species, independent of diversity. We hypothesize that the diversity effect was overshadowed by stand density, in that competition intensified in dense stands, which inhibited higher growth rates in favorable climate years, and thereby constrained variation in growth rates.