OOS 51-3 - Consequences of tree community composition for light stress, spectrally derived leaf traits, and ecosystem function

Tree diversity matters for ecological processes that contribute to emergent properties of ecosystems. We are researching the biological mechanisms for how community-level interactions influence relationships between diversity and function across biological scales. One critical driver of individual and community-level functioning is light. Light availability drives photosynthetic activity, but can also induce stress in light-sensitive species. In this study, we conducted measurements in plots with different species richness and functional diversity and composition, including the angiosperm:gymnosperm ratio.

We hypothesize that tree species with functionally different neighbor tree species tend to have greater access to resources and are exposed to less abiotic stress relative to trees in monocultures, contributing to increased growth and productivity of forest communities over time. We also hypothesize that species-specific traits determine how the species responds physiologically to the local light environment created by neighbor trees.
The study was conducted in a tree biodiversity experiment including 12 species: 8 angiosperms and 4 gymnosperms. We measured local light environment, chlorophyll fluorescence, leaf-level spectra, gas exchange, soil nutrient content. Metrics of growth and survival were collected. Multispectral and LiDAR data of tree plots were obtained using drones. Partial least squares regression models were used to predict leaf traits including foliar chlorophyll content, cellulose, specific leaf area, N, K, and Ca. Correlation matrices of the various traits were made for each focal species.

Analyses show that angiosperm and gymnosperm species typically create significantly different local light environments. Correlations between traits shifted (1) between species and (2) between diversity treatments. We observed significant increases in both foliar N content and quantum efficiency for focal species between (1) monocultures and bicultures, and (2) monocultures and 12-species polycultures. We also observed a significant positive correlation between specific leaf area and net nitrogen mineralization, and a significant negative correlation between foliar cellulose content and net nitrogen mineralization.

From this study, there is evidence for composition and diversity-mediated trends of light availability, light-induced stress, and foliar chemistry. These variables in turn contribute to belowground nutrient cycling, growth, and survival of focal species in different community compositions. Spectrally derived trait metrics are useful in creating a more robust understanding of how leaf chemistry is linked with leaf-level, whole-plant, and community-level function in the context of different community compositions.