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

[摘要] Spread Eagle Barrens, located in eastern Florence County Wisconsin occupies apitted outwash plain created during the late Wisconsin glaciation. The irregulartopography was caused by the collapsing of sediment from proglacial streams depositedon stagnant glacial ice. The formation of a heterogeneous landscape with radical smallscale variations in site characteristics resulted. The importance of these site variables onthe distribution of plant species in a barrens is largely overlooked in favor of moretraditional studies of succession caused by disturbance. In the following study, theimportance and influence of microsite variations on distributional patterns of specieswe,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 groundlayerspecies (greater than or equal to 10% frequency). Three relationships were investigated in greater detail: theimportance and nature of the gradient, the shape and position of species responses, andthe behavior of the vegetation collectively. Single dimensional gradient models weredetermined first, while multi-dimensional models were created in a hierarchicalprocedure based on the significance of single gradient models. In addition to theserealized niche responses, the influence of bracken fern frond densities were investigatedfor effects on distributional patterns and possible skewing of optimal responses of other species. Results indicated that all gradients examined were significant in at least onespecies, with % canopy being the most important overall. Responses of speciesprobabilities along single gradients were often nonlinear, with both quadratic and cubicfunctions being common. Across the landscape then, the community organization ofspecies often did not follow theoretical patterns assumed in plant ecology and ordinationanalyses. In the multi-variable gradient analyses, species responses were often complexin their optimal probability response surface, appearing to segregate niche space as thenumber of variables (dimensions) increased. It appears that niche shifts, based onlogistic models, take place as changes in density of a strong competitor like bracken fern(Pteridium aquilinum) occur. These results suggest that spatial heterogeneity of bothabiotic and biotic factors on the landscape are of significant importance in explainingdistributional patterns of species. Perhaps, providing evidence of why mosaic patterns ofvegetation often occur and their importance to the ecosystem.

[发布机构] University of Wisconsin-Stevens Point, College of Natural Resources