Why And How Wisconsin's Forests Change Over Time

It is a common misperception that forests stay the same. Woodlands change slowly over many years, and the shifts may be imperceptible to the casual observer. While forests can be conserved, or managed and improved, they cannot be preserved. Where preservation implies no change, such a static state is impossible within woodlands, because even when a forest goes unmanaged, change will move forward within.

One of the main drivers of this change is called forest succession, which describes the natural replacement of plant species in an area over time. For example, abandoned farm fields in northern Wisconsin became a common occurrence in the 1940s and 1950s, when many farmers concluded that the climate and short growing season made growing crops in the region difficult. Forests that were once cleared, stumps and all, for these farm fields subsequently transitioned back into wooded areas through succession.

Variation in shade tolerance between tree species is important to the natural progression of a forest. Tolerance is a term that foresters use to indicate a tree's capacity to develop and grow in the shade of and in competition with other trees. Some tree species, such as sugar maple (Acer saccharum) and balsam fir (Abies balsamea), can successfully grow in very shaded conditions. While forestry professionals might joke that sugar maple can grow in the dark, it is not too far from reality — sugar maple seedlings can grow under the full shade of a mature forest canopy.

Other species, such as aspen and jack pine, need full sunlight for growth and development. Tree species are categorized into five levels of tolerance to shade: very tolerant, like sugar maple; tolerant, like basswood (Tilia Americana); intermediate, such as red oak (Quercus rubra); intolerant, like paper birch (Betula papyrifera); and very intolerant, including aspens (such as Populus tremuloides).

Plant communities taking root in Wisconsin’s abandoned farm fields develop in stages, heralded first by the arrival of the "grasses, forbs, and shrubs" stage. This plant community transitions into the "shrubs and seedlings" stage, where trees initially share but then begin to dominate the site. Tree species eventually overtop and out-compete the forbs and shrubs. During this process, intolerant trees continue rapid height growth while tolerant trees occupy their respective niche in the "sapling/pole" stage.

The next stage is termed "young stand," and is marked by rapid growth, prompting severe tree-to-tree competition that may cause weaker trees to die off. Intolerant trees that fall behind in growth are often forced to cede their growing space to more tolerant species. Although competition continues to dictate the species composition of a forest as it develops into a "mature" stand, both intolerant and tolerant trees may share the main canopy.

The last plant community of forest succession is called the "climax" stage, in which a forest reaches a state of relative stability among its plant community. Tolerant species dominate the site, and the climax species will reproduce successfully under their own shade while intolerant trees fail to reproduce under the shaded conditions of the mature canopy. However, even in a climax stand, the forest continues to change. Individual trees die and the gap created is quickly occupied by other tolerant species waiting for access to growing space and sunlight.

As plant communities change during forest succession, stand structure also changes in a process that is often described in four stages: stand re-initiation, stem exclusion, understory re-initiation and old-growth. Under each of these stages, intolerant tree species are replaced by tolerant tree species. Regular hikes through the same aspen stand over time will likely demonstrate one or more of these structural stages.

The rate of natural forest succession is affected whenever a disturbance such as fire, windstorm, pests or human actions occurs on a site. Natural forest succession slows in relation to the frequency and severity of disturbances, and each can push back succession to an earlier stage.

After a major disturbance, for example, pioneer species such as aspen or jack pine can become established in open areas under full sunlight. Eventually, absent further disturbance, these pioneer species will be replaced by other species that will occupy the site through the successional stages, leading to a plant community of climax species, as observed in the hardwood stands of northern Wisconsin.

Understanding how forests change over time is very important when making forest management prescriptions. In most cases, it is easier to work with the natural progression of forest success than to work against it. Tools like the Field Guide for Forest Habitat Types of Northern Wisconsin by John Kotar can be very helpful.

The climax forest type will be dictated by soil variety, moisture availability and other factors. While old-growth climax forests play an important role in succession, they are commonly misperceived as the best type of forest. Plant and animal communities need forests across all stages of forest succession to be successful, though. Pioneer tree species like aspen cannot exist in the shade of a climax forest, and animals like the ruffedgrouse and yellow winged warbler need early successional forest to survive and thrive. Forests change — it is natural and the process is continuous.

Scott Bowe is a wood products specialist with the University of Wisconsin-Extension, director of the University of Wisconsin-Madison's Kemp Natural Resources Station and a professor in the UW-Madison Department of Forest and Wildlife Ecology. This article is adapted from an item originally published by WXPR.