Browsing induced regeneration failure: Long-term trends in a northeastern Minnesota old growth forest

Background/Question/Methods There is considerable interest in maintaining and restoring eastern white pine (Pinus strobus L.) and northern white cedar (Thuja occidentalis L.) along Lake Superior’s north shore. Since European settlement, logging, slash fires, and development have simplified these forests in favor of early successional species. This has allowed white tailed deer (Odocoileus virginianus Zimm.) to establish themselves where they were previously absent or scarce. Deer densities along the shore are now so high that they have hampered white pine and cedar restoration. In a remnant old growth white pine and cedar forest, we examined the differences in natural white pine and white cedar regeneration over 15 years between paired plots with and without deer fences.

Differences in browsing between species and height classes in unfenced plots and the effects of a spruce-fir patch on browsing were also examined.

Results/Conclusions We found significant differences in the recruitment of small and large white pine saplings (p=0.042, p= 0.022 respectively) and small and large white cedar saplings (p=0.048, p=0.06, respectively) between fenced and unfenced plots: Fenced plots had significant sapling recruitment while unfenced plots did not. In fact, no white pine or cedar saplings were found in two of the unfenced plots.

In unfenced plots, there were significant differences between species and between height classes in the percentage of available stems browsed over both time and space. White pine were browsed at a significantly (p<0.001) higher frequency (mean=44.4% ± 7.6% SE), than were white cedar (mean=14.9% ± 7.1% SE). Germinants of both species were browsed less frequently than seedlings and, when available, saplings. The spruce-fir patch failed to provide an effective refuge from browsing: no significant differences in recruitment or browsing were detected. Deer browsing appears to be a serious threat to white pine and cedar, even in remnant old-growth stands with plentiful seeds sources; fences appear to be an effective means of protection, but are costly to construct and maintain. Innovative seedling protection and deer reduction strategies are needed.