Multiple scale spatial dynamics of the moose-forest-soil ecoystem of Isle Royale National Park, MI, USA

Abstract

Moose (Alces alces) make foraging decisions at scales that range from plant stems (centimeters) to home ranges (kilometers). These decisions determine the spatial distribution of forage consumption and the consequent impacts on plant communities and nutrient cycles. From the fine scale changes in the size and density of plant stems to the distribution of plant patches and community assemblages across landscapes, the effects of moose browsing at one spatial scale may amplify spatial heterogeneity at scales that are orders of magnitude larger than the scales at which the interactions originally took place.

In this thesis, I focused on how gradients of moose browsing in two valleys at Isle Royale National Park, MI, USA influenced plant fractal geometry and how such changes to plant geometry feedback to moose foraging across larger previously browsed landscapes. I also examined changes in larger scale patterns of forage availability, plant community composition, and soil fertility in response to recent declines in island-wide moose population density. Increasing moose browsing influenced the geometry of deciduous and conifer species differently. The fractal dimension of bite density, bite mass, and forage biomass of aspen saplings all responded quadratically to increasing moose browsing and were greatest at intermediate browsing rates. In contrast, fractal dimension of bite density, bite mass, and forage biomass of balsam fir all declined steadily with increasing moose browsing. These different responses of plant canopies to increased browsing as well as seasonal changes in bite mass altered the distribution of foraging mechanisms across larger previously browsed landscapes. At the larger scale, recent (2005-2007) landscape patterns of available and consumed browse became decoupled from each other and distributions of available forage, plant species composition, and soil fertility were qualitatively different from patterns documented in the early 1990's. These changes are coincident with and likely driven by recent declines in the island-wide moose population of Isle Royale. Collectively, these two studies suggest that large scale landscape patterns of browse availability, species composition, and soil fertility may emerge from finer scale impacts of browsing on plant geometry and the feedbacks to larger scale foraging decisions that moose make in addition to population density. The long-term dynamics of landscape patterns in boreal forests are therefore dependant on both fine scale foraging decisions and large scale population dynamics.