Nature.org talked with Mark White, the Conservancy’s forest ecologist in Minnesota and author of the new study, about what deer like to eat, what a deer-free forest looks likes and why a spruce savanna is even worse than it sounds.
Why did we do this study?
Back in the late 80’s, The Nature Conservancy and the Encampment Forest Association were concerned about the lack of regeneration of white pine and white cedar in Encampment Forest, an old growth forest along the North Shore. We had big canopy trees that were hundreds of years old but there was almost no regeneration out there.
It was pretty obvious white-tailed deer had something to do with that. You could see heavily browsed pines and cedars.
We constructed three half-acre exclosures with 10-foot tall fences to keep deer out. We also identified an equal number of plots of the same size but didn’t fence them off.
What did you find?
After about 7 years we went back and found some pretty significant differences. There were a lot of white pine and white cedar tree seedlings and saplings in the fenced-off areas. And then in 2008, we went back and resampled every tree bigger than an inch in diameter. We were interested in getting a look at the long-term changes, comparing the changes within these fenced areas to the areas that were not protected from browsing by white-tailed deer.
Deer browse on their favorite foods. During the winter, these are white pine and white cedar. During the growing season, they favor a variety of deciduous species like red maple, mountain maple, and ash. Outside the fenced areas, we couldn’t find any regeneration of white pine or white cedar. In fact, very few young trees that could replace the large white pine and cedar were growing outside the fences. Within fenced areas, there were abundant seedlings and saplings, particularly of white pine.
Outside the fences, we observed a shift to dominance by white spruce, a species with little nutritional value and rarely eaten by deer. Balsam fir, which deer only eat under starvation conditions, was also abundant on some sites.
How do the fenced-off areas compare with the unfenced study areas?
You go into an exclosure and you see a whole range of tree species and sizes growing. It’s hard to walk through because the vegetation is so dense.
Outside the fences, you can see a long way. You can walk through without having to push the vegetation apart. You see the canopy trees but on the ground it’s pretty much nothing but grass, spruce and heavily browsed brush. No one can mistake the difference.
What does this mean for Minnesota’s Northwoods?
What we see is a simplification of the forest. Our forests are gradually becoming less diverse and shifting toward non-preferred food species like white spruce and balsam fir.
Another important observation is that the forest protected from deer accumulated twice as much biomass as did the unfenced plots.
So, we see this shift to less diverse forests that produces less wood fiber. These simplified forests are also likely going to be less resilient to forest pests, climate change and disease. For example, white spruce and balsam fir are projected be highly vulnerable to a warming climate.
And since the deer are suppressing most tree species with the exception of spruce and balsam fir, our forests will change over time to more open areas with fewer trees – spruce mainly – and a lot more grasses and sedges. This shift to a more open condition is one of those things we refer to as a state change. It’s a shift in forest structure. Once those grasses become well-established it’s much harder to get trees growing back in there.
What are the implications for people and nature?
For the forest products industry in Minnesota, high deer populations could limit the amount of wood fiber produced by species that are preferred by deer. The same thing is true for how much carbon can be stored in our forests.
More simplified, low-diversity forests will also likely impact forest songbirds and woodland species that utilize all these other tree species. Pine martens, fishers and a number of other small mammals depend upon these later succession hardwood conifer forests which have a lot of dead wood on the forest floor. This would limit their habitat.
Over time, the character of our forests will be different. Deer browsing pressure, along with climate change, and other factors may lead to decreases in species such as white pine, white cedar, paper birch, black ash, balsam fir, and aspen. While spruce may have an advantage by being a non-preferred food source, they will be susceptible to a warming climate.
What’s the answer?
One of the main answers is forest restoration. We have to plant more white pine and white cedar and other native trees and protect them from deer browsing. The other answer is to find out how to manage the deer population so there’s less browsing pressure.
We have to manage our forests in a more balanced way so we can maintain their beauty, diversity and productivity across the landscape.
Watch a slideshow of the Conservancy's forest restoration in Minnesota's Northwoods.
Link to an abstract of the paper in Forest Ecology and Management.