The Influence of Topoedaphic Factors, Canopy, and Bracken Fern on Distributional Patterns of Plant Species at Spread Eagle Barrens

Abstract

Spread Eagle Barrens, located in eastern Florence County Wisconsin occupies a pitted outwash plain created during the late Wisconsin glaciation. The irregular topography was caused by the collapsing of sediment from proglacial streams deposited on stagnant glacial ice. The formation of a heterogeneous landscape with radical small scale variations in site characteristics resulted. The importance of these site variables on the distribution of plant species in a barrens is largely overlooked in favor of more traditional studies of succession caused by disturbance. In the following study, the importance and influence of microsite variations on distributional patterns of species we,re examined using logistic regression. Predictor variables included: % canopy, nutrient index, % soil organic matter, soil pH, site severity index and slope position. The influence of these gradients were examined in detail for common groundlayer species (greater than or equal to 10% frequency). Three relationships were investigated in greater detail: the importance and nature of the gradient, the shape and position of species responses, and the behavior of the vegetation collectively. Single dimensional gradient models were determined first, while multi-dimensional models were created in a hierarchical procedure based on the significance of single gradient models. In addition to these realized niche responses, the influence of bracken fern frond densities were investigated for effects on distributional patterns and possible skewing of optimal responses of other species. Results indicated that all gradients examined were significant in at least one species, with % canopy being the most important overall. Responses of species probabilities along single gradients were often nonlinear, with both quadratic and cubic functions being common. Across the landscape then, the community organization of species often did not follow theoretical patterns assumed in plant ecology and ordination analyses. In the multi-variable gradient analyses, species responses were often complex in their optimal probability response surface, appearing to segregate niche space as the number of variables (dimensions) increased. It appears that niche shifts, based on logistic models, take place as changes in density of a strong competitor like bracken fern (Pteridium aquilinum) occur. These results suggest that spatial heterogeneity of both abiotic and biotic factors on the landscape are of significant importance in explaining distributional patterns of species. Perhaps, providing evidence of why mosaic patterns of vegetation often occur and their importance to the ecosystem.