Tree species richness enhances stand productivity while stand structure can have opposite effects, based on forest inventory data from Germany and the United States of America

Background: In recent studies, mixed forests were found to be more productive than monocultures with everything else remaining the same. Methods: To find out if this productivity is caused by tree species richness, by a more heterogeneous stand structure or both, we analyzed the effects of forest structure and tree species richness on stand productivity, based on inventory data of temperate forests in the United States of America and Germany. Results: Having accounted for effects such as tree size and stand density, we found that: (I) tree species richness increased stand productivity in both countries while the effect of tree size heterogeneity on productivity was negative in Germany but positive in the USA; (II) productivity was highest at sites with an intermediate amount of precipitation; and (III) growth limitations due water scarcity or low temperature may enhance structural heterogeneity. Conclusions: In the context of forest ecosystem goods and services, as well as future sustainable forest resource management, the associated implications would be: Tree species richness is vital for maintaining forest productivity. As an optimum amount of precipitation is accompanied by the highest productivity, changes in climatic conditions should be considered when planning. Resource limitations enhance structural heterogeneity, which in turn can have positive or negative effects on stand productivity. Furthermore, we discuss the difficulties encountered when analyzing different national forest inventories and large data sets.