Are These the Last Days for Door County’s Boreal Forests?

By Lee E. Frelich

Door County is a world-class ecotourism destination. The Niagara Dolostone that forms the peninsula and presence of Lake Michigan allow coexistence of many unique habitats. These include cliffs, alkaline wetlands and sand dunes, which support an amazing level of biodiversity.

Perhaps the most prominent ecological feature of the peninsula is the boundary, or ecotone, between two of the world’s great biomes: temperate forest (with oak, maple, basswood, beech, cherry, ash, yellow birch and hemlock) and boreal forest (with spruce, fir, pine, cedar and paper birch). Having both biomes essentially doubles the number of forest species – trees, plants, birds, insects, mammals – that thrive in Door County, compared to other equally sized areas with one biome. Unfortunately, due to climate change, the boreal biome in Door County is endangered.

To understand why, we need to know more about why the boreal forest exists and what makes it unique. First, in most of the world, the ecotone between boreal and temperate forests runs west to east, separating the cold boreal forest of the north from more moderate temperate forests of the mid latitudes.

In Door County, however, this ecotone runs down the middle of the peninsula, south to north, with boreal forest lakeside and temperate forest bayside. Second, most boreal forests – for example Minnesota’s Boundary Waters – grow in areas with winters cold enough to kill temperate tree species.

Door County is further south than the usual boreal latitudes, and has mild winters nowhere near cold enough to kill temperate species. Its boreal forests are present solely due to cool summers caused by upwelling of cold water in Lake Michigan along the east side of the peninsula. By my estimation, the summer temperature gradient from bayside to lakeside across the Door Peninsula is about the same as moving from sea level to the top of a mountain 3,000 feet high. A similar summer temperature gradient and temperate to boreal forest transition occurs in the Adirondack Mountains of New York.

In Door County lower elevations are cooler in summer, because cool air brought in by the lake breeze during summer is relatively humid, and settles into the low lakeside elevations, less than 30 feet above Lake Michigan.

Whether Door County’s boreal forests will survive in the future depends on keeping the surface temperature of Lake Michigan cool. However, Great Lakes surface temperatures have been rising along with our warming climate.

When the climate warms, things go wrong for boreal tree species. They can’t use a longer growing season as efficiently as temperate species like sugar maple and red oak. Heat and drought stress lessen their ability to fend off insects and diseases. Seed production falls and seedlings have a difficult time competing with the larger seedlings of temperate species. If this situation persists, the boreal canopy trees will be replaced by temperate species over several decades, during the normal process of death and regeneration that occurs in all forests. Changes from boreal to temperate forests accompanied a warming climate – due to natural cycles – during the retreat of the glaciers in the southern Midwest from 15,000 to 10,000 years ago. This is documented by fossil tree pollen deposited in bogs and lake bottoms with layers of sediment that can be dated.

What has happened can happen again. Research carried out by my students shows that the warming that has already occurred is allowing temperate tree seedlings to invade the understory of boreal forests across the lake states of Michigan, Wisconsin and Minnesota. This contrasts with the situation 60 years ago, summarized by University of Wisconsin-Madison ecologist John T Curtis in his classic 1959 book The Vegetation of Wisconsin, when there was no evidence of temperate invasion into boreal forests.

I witnessed this pattern of change during a hike in Newport State Park in August 2016. Boreal forests that were free of temperate species in the 1970s when I wrote Vascular Plants of Newport State Park, Wisconsin, now have sugar maple saplings growing side-by-side with boreal balsam fir and white spruce.

The question now is whether the recent warming trend will continue and eventually be large enough in magnitude to exclude the boreal forest from Door County. If so, an entire biome would be lost to us and probably the entire Lake States Region. The bottom line is that, although maple and oak forests are lovely, to preserve the unique beauty of our boreal forest and its phenomenal plant and animal life, we must limit the magnitude of future warming by substantially reducing the burning of fossil fuels.

We need to act now to preserve our unique Door County boreal forest for our grandchildren and their children!

Lee E. Frelich is director of the University of Minnesota Center for Forest Ecology. He received a Ph.D. in Forest Ecology from the University of Wisconsin-Madison in 1986. Dr. Frelich has authored numerous scholarly articles together with 25 graduate students and 180 scientists from 23 countries. His research has been featured in the news media, including The New York Times and National Geographic. He has provided consulting services on forest management to a number of government agencies, including the Army, Air Force, U.S. Forest Service and National Park Service.