Long-term effects of deer browsing: Composition, structure and productivity in a northeastern Minnesota old-growth forest

Although the immediate impacts of elevated deer (Odocoileus spp.) browsing on forest regeneration have been well documented, few studies have examined the longer term consequences. A deer exclosure experiment was initiated in 1991 in an old-growth northern mixed mesic forest in northeastern Minnesota, and resampled in 2008 to examine changes in composition, structure and productivity. Decades of overbrowsing by white-tailed deer have led to almost complete recruitment failure in size classes >2.5 cm dbh for preferred deer browse species Thuja occidentalis and Pinus strobus in unprotected plots. Other palatable browse species have been severely limited in understory development (Populus tremuloides, Betula papyrifera, Fraxinus nigra). Within exclosures, P. strobus gained in all size classes <20 cm dbh, while F. nigra, B. papyrifera, T. occidentalis all showed significant gains. Non-preferred Picea glauca increased outside exclosures, but has also gained within exclosures. The increase in P. glauca across treatments indicates a long-term legacy effect of preferential browsing. Browsing induced suppression of subcanopy density of preferred species and failure of canopy tree replacement may lead to a more open woodland structure dominated by P. glauca. Browsing pressure may negatively impact productivity, as whole tree biomass in exclosures increased at a rate twice that of unprotected plots. The low biomass levels recorded in 2008 (unprotected: 98.0 mg/ha−1, exclosure: 104 mg/ha−1) are approximately 1/2 of values typically recorded in later successional forests in this region indicating lower productivity may be another longer-term legacy of elevated deer population. Continued high browsing pressure is one of many factors contributing to the restructuring of northern Great Lakes forests away from historical variability conditions towards a novel and more homogeneous forested landscape. These simplified forests may be less resilient to the suite of emerging stressors such as climate change and less able to provide ecosystem services such as carbon storage, biological diversity and forest products. Sustained restoration efforts, along with reductions in deer density will be needed to restore species and structural diversity.

► In unfenced plots, preferred browse species did not recruit into the sapling layer.
► In fenced plots, preferred browse species gained in sapling and subcanopy layers.
► Non-preferred white spruce gained in fenced and unfenced plots.
► Productivity doubled in protected plots compared to unprotected.
► Forests may be shifting to a spruce dominated woodland state.