PS 2-17 - Understory shrub composition in mesic hardwood forests is influenced by microsite heterogeneity in light and soil environments

Abstract: Despite contributing greatly to the diversity of temperate deciduous forests, intermediate-stature shrubs tend to be underrepresented in ground layer herbaceous studies and overstory assessments. Likewise, understory shrubs persist in a forest layer of notable heterogeneity. Treefalls and edge effects produce a mosaic of light availability, while substrate characteristics vary due to underlying geology, microtopography, and uprooting disturbance. When considered with the paucity of community data, there is a considerable gap in our understanding of temperate shrub microsite specificity within temperate forest systems. Given their diversity and importance in providing food and habitat resources, the goal of this study was to characterize understory shrub community composition and microsite specificity within temperate forests systems. Twenty randomly stratified plots (50m²) were surveyed for shrub abundance, light environment, microtopography, and soil analysis at sixteen mature hardwood forests throughout Wisconsin. Soil cores were taken to record the depth of the A1 mineral soil horizon, while A1 soil samples underwent analysis for ratios of sand, silt, and clay. Light environment was characterized using hemispheric photography, while site-level climatic data was derived from twenty-year averages from the Midwest Cli-MATE Database. Nonmetric Multidimensional Scaling (NMDS) of abundance and environmental data utilizing Bray-Curtis dissimilarity allowed for the testing of significant environmental variables along NMDS axes, while community level and species inferences were tested using Generalized Linear Models (GLM) and Generalized Linear Latent Variable Models (GLLVM).

Species richness and Shannon Diversity varied fivefold between sites, with GLLVM’s showing diverging community composition and species cooccurrence between northern and southern forest sites. Multiple regression analyses of NMDS axes determined that percent clay and climatic variables significantly shape understory community composition at the site level, while plot level analyses show soil texture and canopy openness to significantly impact community composition. These results reveal that while climatic and geological factors shape regional community patterns, microsite composition of shrub communities is impacted by both substrate and light dynamics. Density plots of the most abundance species against variables of soil texture and canopy openness qualitatively corroborate these findings. Such species-level analyses highlight the opportunity for future trait-based approaches to further characterize shrub community composition, while providing new insight for land managers to promote a diverse guild of plants that too often get overlooked in the understory.