Ungulate winter habitat selection as a driver of herbaceous-layer heterogeneity in northern temperate forests

Concentrated seasonal habitat use by ungulates may amplify their influence on ecosystem processes. For example, white-tailed deer (Odocoileus virginianus) depend on forests of coniferous trees, such as Tsuga canadensis (L.) Carr. (eastern hemlock), for winter cover and forage in the northern portion of their range. We hypothesized that winter habitat use influences summer herbaceous-layer plant communities through cascading indirect effects resulting from the deposition of nitrogenous wastes and browsing of woody plants. Deer use was monitored from 2006-2008 via fecal pellet group counts in 15 T. canadensis stands across the western Upper Peninsula of Michigan. Detailed vegetation and soil sampling was performed during the summer of 2008. Estimates derived from winter inputs of fecal pellets, suggest that overwintering white-tailed deer deposit 3.3 +/- 0.8 kg N ha(-1) yr(-1). Results from nonmetric multidimensional scaling ordinations and permutational ANOVAs of distance to group centroids of ground-layer vegetation and soil attributes indicated that high levels of winter deer-use were significantly associated with greater heterogeneity in ground-layer community composition and relative soil nutrient variability within and between stands. Our results highlight potentially important feedbacks between habitat selection by ungulates, site productivity, and plant community structure in forest ecosystems.