Aboveground Primary Productivity in Forest Ecosystems as a Function of Species Diversity and Composition

In the last two decades, several studies have demonstrated that species richness enhances primary productivity. However, much uncertainty still exists about the relationship between biodiversity and ecosystem functioning (BEF) in forests, especially in highly diverse biomes. This is a primary concern for human welfare as forests play a major role in carbon sequestration at global scale and in providing numerous services. On the other hand, BEF research in forests is especially challenging because of the longevity of trees and the di culty to control environmental factors that co-vary with diversity such as stand age and topography. In this thesis, I examine the e ects of tree species richness on aboveground productivity in subtropical China, by using a combination of both experimental and comparative studies. In the rst two chapters, I worked in an ongoing large-scale tree diversity experiment, BEF-China (www.bef-china.de). In a subset of this experiment, over 150,000 tree saplings were selected from a pool of 40 native broad-leaved species and planted in 386 plots, each containing between one to 16 species. In Chapter 1, I tested the e ect of tree species richness on stand productivity (annual increment of stand basal area) by analyzing data of four annual inventories since the establishment of the experiment (ca. 5000 individuals). I observed the gradual emergence of a positive e ect of species richness on stand productivity. Neither spatial heterogeneity nor functional diversity a ected stand productivity. However, the presence of two fast-growing species, Choeropondias axillaris and Schima superba, explained a signi cant proportion of the variability of stand productivity. Furthermore, I observed that at this stage of stand initiation, mortality (as mortality rate or loss of biomass) was not a ected by species richness. In contrast, survivor growth increased with species richness. Thus, this study demonstrates that even during the initial stages of stand development, diversity enhances stand growth. In Chapter 2, I investigated the e ects of tree species richness on leaf production. Leaf production is an important component of primary productivity in early stages of forest succession. During stand initiation light availability decreases, and trees can potentially adjust their leaf production, leaf size and branching patterns to those changes in light regime. Therefore, I expected that trees growing in mixtures would show not only larger stems but also higher leaf production. During a two-year study period (3 and 4 years after the establishment of the experiment), I monitored the growth of ca. 700 trees in 62 plots, encompassing a diversity gradient from one to 16 species. I assessed stem basal area, total number of leaves, leaf emergence rate, leaf mortality rate, leaf size, and branching rate. At the end of my study period, trees growing in mixtures, especially those of C. axillaris, had larger basal area and longer leaves, but species richness had no e ect on any other trait. Thus, this study shows that tree species richness enhances stem growth but not leaf production during stand initiation. The results suggest then that the complementary use of light is not a main driver of increased productivity in mixtures.