An ecosystem approach to assessing the effects of forest heterogeneity and disturbance on birds of the northern hardwood forest in Michigan's Upper Peninsula

Forested regions such as the Upper Peninsula of Michigan, are considered population sources for area sensitive forest birds which have disappeared from many human-dominated landscapes because of forest loss and fragmentation. Northern hardwood forest landscapes in the Upper Peninsula have been affected by past human activities and continue to be periodically logged. The possibility that anthropogenic activities would over time favor bird species that are tolerant of some levels of forest fragmentation, at the expense of area sensitive species, needs to be evaluated. I developed an ecosystem model for northern hardwood forest landscapes and explored relationships among bird species, and among bird densities, microhabitat variables and landscape level variables to identify criteria necessary to make such an evaluation.; I modeled heterogeneity of the northern hardwood forest at the landscape level in terms of historical change in land use and land type association. I considered period elapsed after last logging to affect vegetation structure at the locality of the bird census station. Habitat relationships among 10 bird species were derived from individual bird species associations with microhabitat variables. A forest fragmentation dimension was evident in habitat relationships among 3 shrub nesting bird species, black-throated blue warbler ( Dendroica cerulescens), American redstart (Setophaga ruticilla ) and chestnut-sided warbler (Dendroica pensylvanica). Forest fragmentation aspects however could not explain density disparities between the 2 conifer associated bird species, blackburnian warbler ( Dendroica fusca) and black-throated green warbler (Dendroica virens). Historical change in land use affected the density of late maturity conifers and forest loss at landscape levels. Logging affected canopy cover and shrub layer development, effects that were local and highly dynamic. Logging was responsible for increased densities of all birds that nest in the shrub-sapling layer in managed forest landscapes but was also associated in some landscapes with a relative greater increase of edge tolerant bird species. Restratification of sampling units based on levels of forest cover within a radius of 1.6 km, length of forest edge within a radius of 500m from bird census station, shrub layer development and canopy opening elucidated what constitutes forest fragmentation effects in the northern hardwood ecosystem. This restratification permitted the quantification of conditions that were differently favorable to 2 edge species and one area sensitive species. The northern hardwood forest is a complex system that is both spatially heterogeneous and dynamic at different scales. Multiple factors modify or aggravate the effects of human activities on bird species that in turn respond differently to different dynamics in the system. Simple assessments of the effects of anthropogenic activities on bird species are inadequate for evaluating the long term maintenance of area sensitive species in the system. The inadequacies of treating the northern hardwood forest as a homogeneous system at equilibrium, and of disregarding differences among forest bird species, relative to their differential sensitivity to forest fragmentation, in current assessments of human activity effects are discussed.