Increased biodiversity generally enhances terrestrial ecosystem productivity. While niche-use efficiency is thought to drive the biodiversity-productivity relationship, the mechanisms within niche-use efficiency are not well understood. A potential mechanism for niche-use efficiency is nutrient-use efficiency. To measure nutrient-use efficiency, we calculated nitrogen-resorption efficiencies (NRE) because nitrogen is an important growth limiting nutrient for forest productivity. We used a plantation implemented as a full factorial design that included two levels of competition, implemented as different planting densities (one- and two-meter planting densities), and three diversity levels (monocultures, two-, and three-species plantings) that included three hardwood tree species (northern red oak (<i>Quercus rubra</i>), black cherry (<i>Prunus serotin</i><i>a</i>), and American chestnut (<i>Castanea dentata</i>). For our nitrogen-resorption efficiency data, we found that NRE increased as diversity and planting density decreased, but the magnitude of the response varied among species. This outcome suggests that while increased diversity likely provides a release from intra-specific competition, different combinations of species will play a critical role in shaping biodiversity-productivity relationships. Forest nutrient cycling can also be influenced by herbivory. To address the effects of forest diversity on herbivory rates, we monitored rates of foliar damage along with foliar nitrogen content. To measure foliar nitrogen content, we collected spectral data from early, midseason, and late season foliar samples. To assess foliar damage, we collected and imaged leaves from two canopy positions in order to measure late season foliar area and estimate pre damaged foliar area. We found that diversity and foliar nitrogen content have a positive relationship, and diversity does influence canopy damage but the effects vary among species and density. Upon further analysis, we found that foliar nitrogen content and canopy damage are correlated. Meaning individual trees showed a release from intraspecific competition, which lead to an increase in available nutrients and higher canopy quality, showing that stands with higher canopy quality experienced higher levels of damage.<br>

Numerous studies have evaluated the relationship between biodiversity and productivity, with general trends suggesting a positive relationship. While most studies only use species richness, this study also analyzed how productivity changes with functional diversity and Shannon’s diversity index. Functional diversity gives important context to the examination of biodiversity-productivity relationships due to the direct link between organisms’ functional traits and their role in a given ecosystem. This study used data from the USDA Forest Service Forest Inventory and Analysis (FIA) database collected in Alabama, Louisiana, and Mississippi to estimate plot-level productivity and diversity. Structural equation modeling was used to determine the strength of the relationship between measures of diversity and forest productivity. This study found that species richness had the greatest influence on forest productivity, but it was largely an indirect effect mediated by stand density. This study could be expanded spatially to include more forest types for comparison of trends.