Consequences of Biodiversity in Tree Diversity Experiments

Abstract

This dissertation reports on four studies that explore the consequences of changes in tree biodiversity for three ecosystem processes (growth, leaf herbivory and disease, and leaf decomposition) in tree-dominated ecosystems in eastern Minnesota, USA. In Chapter 1, I present the Forests and Biodiversity (FAB) experiment and assess the role of taxonomic, phylogenetic, and functional diversity in this experiment on stem growth of 12 species. Over the third to fifth years of the experiment, trees with more diverse neighborhoods produced more biomass than trees in less diverse neighborhoods. This complementary overyielding effect was associated with species richness (taxonomic diversity) and was better predicted by tree diversity at larger (12 m2) rather than smaller (0.25 m2) spatial scales. I also measured three forms of invertebrate herbivore damage and two forms of disease damage on leaves of nine FAB species; results from this study are presented in Chapter 2. I assessed the consequences of diversity for damage across four spatial scales. Herbivory and disease responded to a variety of metrics of community diversity and these effects were species-specific. Damage, regardless of what kind, was better predicted by community structure and diversity at small spatial scales (1-4 m2) than large scales (9-16 m2). Chapter 3 consists of the presentation of results from the Biodiversity in Willows and Poplars (BiWaP) experiment, in which both the genetic diversity and species diversity of three Salicaceous species was manipulated, and tree growth and herbivory were measured. Diversity did not have a consistent effect on productivity because one dominant species suppressed hetrospecific neighbors. In contrast, specialist gall formation was best predicted by genetic identity and genotypic diversity suppressed leaf mining. Finally, through a separate litterbag decomposition experiment designed in parallel to FAB and presented in Chapter 4, I measured the consequences of leaf chemical diversity for decomposition over two years. When litter from multiple species were mixed, it did not lose mass, cellulose, or lignin differently than would be expected based on monoculture. But more labile carbon fractions (soluble contents and hemicellulose) decomposed more slowly in more functionally diverse litter mixes.