Successional Ecology in Northern Wisconsin

For a long time in successional ecology, scientists accepted Frederic Clements’ idea of climax stage. This theory states that an ecosystem reaches a final stage of succession and once this stage is reached, it is maintained through ecological equilibrium. Now, ecologists no longer adopt this theory into practice because disturbances prevent ideal environments and different species react differently to different types of disturbances (Gibson 1996). Instead, they classify species as early successional or late successional. This approach will help explain the history of vegetation in northern Wisconsin. To start, it is imperative to know that this region was completely altered by human activities around 100 years ago. During what is known as the Great Cutover, this area experienced millions of acres of deforestation due to a thriving lumber industry in Wisconsin (Whitney 1987). This heavy logging along with frequent fires resulted in a large reduction in hemlock that had previously dominated the forest. With less woody material on the floor of the forest, hemlock seedlings did not have as many opportunities to prosper and could only survive in isolated patches. These patches, however, also coincided with areas of shelter for white-tailed deer in the winter. The hemlocks, therefore, experienced an even greater reduction (Mladenoff and Stearns 1993). After the Great Cutover and frequent fires, this area was perfect for an early successional species, like the Bigtooth Aspen, to thrive. Aspens’ fast growth rate, clonal reproductive capabilities, and shade intolerance makes it a great candidate to be an early successional species ((Pastor and Bockheim 1984). Therefore, it makes sense that aspen would completely dominate this area after it was cleared by human activity and fires.

Clements’ theory would suggest that the area would look the same 100 years down the road. However, today, there are three separate forests made up of different compositions. This is due to the types of disturbances that occurred during this time frame in different sections of this region. In one section, the aspen forest is still an aspen forest today. This suggests that this area experienced crown fires. Crown fires are large and wipe out almost all of the vegetation in an area (Ryan 2002). This would replicate the conditions following the Great Cutover that was ideal for aspens. Aspen seeds also are dispersed by the wind and therefore can initiate new growth across nearby areas. These areas do not have a lot of herbaceous species in the understory because aspen grow fast but do not live very long. This area was likely impacted by a crown fire recently and has not had time to display growth other than the early successional species.

Another area went from aspen to sugar maple-dominated. This area probably has not experienced any major disturbance and fits pretty well with Clements’ theory of a self-replicating climax stage. Because aspen have short lives, when they die they create gaps in the canopy allowing for a shade-tolerant species like the sugar maple to release (Pastor and Bockheim 1984). These areas also promote higher biodiversity in the understory as more time has allowed other species to grow in the absence of a disturbance and more dead biomass falls to the forest floor (Pastor and Bockheim 1984). This dead biomass provides more sources for nutrient cycling as well as an environment helpful for herbaceous species. Sugar maples are quite fire intolerant due to their thin bark. So, in areas that experience ground or surface fires, sugar maples would die and allow a different late successional species, one with thicker bark and higher tolerance to fires, eastern white pines, to dominate (Peterson and Squiers 1995). Because the aspens die and fires occur at varying times, gaps are created at varying times. This uneven dying of aspens and releasing of either sugar maples or white pines cause differing vertical structures and therefore differing amounts of understory biomass between and within the three forests (Berger and Puettmann 2000).

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Gibson, David J. “Textbook Misconceptions: The Climax Concept of Succession.” The American Biology Teacher 58, no. 3 (1996): 135–40.

Mladenoff, David J., and Forest Stearns. “Eastern Hemlock Regeneration and Deer Browsing in the Northern Great Lakes Region: A Re-Examination and Model Simulation.” Conservation Biology 7, no. 4 (December 1, 1993): 889–900.

Pastor, John, and J. G. Bockheim. “Distribution and Cycling of Nutrients in an Aspen-Mixed-Hardwood-Spodosol Ecosystem in Northern Wisconsin.” Ecology 65, no. 2 (1984): 339–53.

Peterson, Chris J., and Edwin R. Squiers. “Competition and Succession in an Aspen-White-Pine Forest.” Journal of Ecology 83, no. 3 (1995): 449–57.

Ryan, Kevin. “Dynamic Interactions between Forest Structure and Fire Behavior in Boreal Ecosystems.” Silva Fennica 36, no. 1 (2002).

Whitney, Gordon G. “An Ecological History of the Great Lakes Forest of Michigan.” Journal of Ecology 75, no. 3 (1987): 667–84.