Wisconsin Nature

Forest Succession: How Tree Communities Form

Wisconsin has millions of acres of forests, divided into two main areas based on soil types and climate. The line of separation is called the Tension Zone, which stretches from northwest to southeast Wisconsin in an S-shape, and delineates vegetative growth separating northern mixed forest communities from southern broadleaf forest communities. It has a large part to play in how forest succession works.

The north is home to mixed deciduous-coniferous forests, which retains a higher percentage of forest cover as a consequence of climate and the land being less suited to agriculture. Southern communities have traditionally consisted of broad-leaved deciduous forests, oak savanna, tamarack swamp, prairies, and open wetlands that have changed greatly in the last 150 years, largely due to agricultural value and human development.

Taken in total, of Wisconsin’s 35 million acres of land, 17 million acres are forest. Of these, 1.7 million acres (4.7 percent of Wisconsin’s land area) are urban forests. Southeastern Wisconsin possesses much of this urban forest, which includes the one you find at Schlitz Audubon.

These forests are always changing. They are shaped by their environment, which can affect what grows. The composition of trees and understory plants is based partly on the history of the land, ecological processes, and available moisture, as well as some measure of unpredictability. Forests are also a competitive environment, with tree species vying for light and space, waiting for an opening in a developed canopy for the chance to grow. This process is governed by the principle of forest succession.

The Ways of Forest Succession

Succession determines which community of plants is likely to replace another community of plants as an ecosystem matures following disturbance. There are two main types of forest succession, primary and secondary.

Primary succession happens when there is sterile substrate, or a lack of soil, in the ground layer. This was the original condition of much of Wisconsin after the last glacier retreated. Plants don’t grow readily in these conditions. The development of soil and soil fertility is a gradual process related to increases in organic matter and biodiversity, and leads to predictable growth stages, called seres.

The process begins on bare ground, without soil. In Wisconsin, lichens and mosses appeared, leading to grasses and sedges (graminoids) and annual flowering plants, and later, graminoids and perennial flowering plants. These make way for shrubs, followed by shade intolerant trees, and finally shade tolerant trees.

Growth Patterns: How Secondary Succession Works

Secondary succession takes place when there is viable soil in the ground. This is a natural process that occurs in an orderly fashion. Compared to primary forest succession, the stages of secondary succession are more compressed in time and the transitions between stages are less distinct.

Secondary forest succession begins when a forest has been cut, or when a forest clearing is caused by a natural disturbance, such as a fire, storm, ice, or disease. These events set the stage for regeneration. Pioneer species, the plants that are first to populate an area, begin to grow. Weeds, grasses, and flowers anchor the soil. This paves the way for shrubs to establish and increase in dominance, promoting shade and allowing the soil to retain moisture.

In this young canopy-less area, light-loving pioneer tree species may sprout, such as aspen and ash. As time passes, these trees grow and block the light for other shade intolerant trees. That’s when shade-tolerant trees like maple and basswood may become established and eventually dominate, propelling the forest towards maturity and eventually forming a climax community, which can perpetuate itself indefinitely without a disturbance. With each disturbance, the forest must heal.

This is a textbook version of succession that doesn’t necessarily happen in urban and suburban areas in Southeastern Wisconsin. Smaller forests proceed through the phases of succession a bit differently. Isolation, fragmentation, and non-native invasive species alter the natural process of succession. In altered communities like an urban forest environment, as we have at the Center, we use the model of natural succession to inform how we manage the forest. We can’t rely on succession to occur naturally.

Disturbed Forests and Invasive Species

When there is a disturbance in the Center’s forest, pioneer species become established in the new light-exposed territory, including gray dogwood, green ash, and wild strawberry. Also to contend with are buckthorn, dame’s rocket, and garlic mustard. These plants are aggressive non-native invasive species that must be managed or they will quickly dominate the land. One reason that invasive species establish successfully is because they lack natural predators to keep them in check. That’s why our conservation team and land steward volunteers work hard to keep them from becoming established.

Forest Stages at the Center

Prior to European settlement, the forest community at the Center was different from a traditional climax community due to our heavy clay soils. The predominant species were red oak and basswood instead of sugar maple. Currently, paper birch, basswood, and green ash are our dominant species. But emerald ash borer is decimating green ash, white ash, and black ash, causing a disturbance in the forest and leaving large openings in the canopy. This means we are losing 40-50 years of succession.

In smaller urban forests like ours, there are also edge effects. These are forest disturbances that occur near the perimeter of the forest and have an impact on the whole. Bigger forests have a larger ratio of interior to edge, so these effects aren’t as significant. Other factors that influence forest growth are geographic location, moisture, and hydrology.

The Center’s Forest Conservation Strategy

Our conservation department must take all of these conditions into account when it manages our stands of forest. We also consider conditions that will be prevalent in the future, including conditions caused by climate change. For several reasons, the Center’s conservation department is working to establish an oak savanna in ash-colonized former agricultural fields at the Center.

The direct restoration of former agricultural fields to dominant pre-settlement forest communities is not feasible. This is because of expected changes to regional climate, and the effects of invasive species on key dominant tree genera. Emphasis is being placed on establishing and restoring regionally native species that can be expected to survive and reproduce to form self-sustaining populations and species assemblages.

Still, the Center possesses the highest quality forest in the area. It presents an incredible opportunity for people to learn about conservation. Woodland habitats are home to great diversity in both urban forests and in northern forests. Wisconsin woodland habitats have a population of nearly 1,800 species of native plants and 657 species of vertebrates. This doesn’t include nonvascular plants and invertebrates.

You’ll find a lot of that wildlife at the Center, making our forest conservation and management programs so important. Schlitz Audubon is a living laboratory for developing effective ways to manage land and wildlife.

Written with contributions from Michelle Allison, Don Quintenz, and Marc White.