By Bill Cook
MSU Extension Forester and Biologist, retired
Disturbance is disturbing to many people, especially when it comes to forests. Folks like things as they are, or as they think they are. When something changes the visual quality of a forest, the fan gets hit.
As forests age, increases in natural mortality and subsequent disturbance events happen. Michigan forests are growing older and natural mortality is, indeed, on the rise. For the past several years, natural mortality volumes have exceeded harvest volumes.
Disturbance in our northern forests is the ecological driver for regeneration, habitat diversity, and ecological succession. Disturbance is a good thing. It’s a natural thing. Nature employs several tools to create life-giving disturbance. The most well-known might be fire. However, there is also wind, insects, diseases, and beavers.
Forestry, of course, has learned these lessons and mimics disturbance through harvest practices. Forest management is intentional, serving to emplace disturbance that minimizes the downsides to people, and maximizes the positive outcomes. Nature is more random and spontaneous, which often conflicts with human welfare.
Disturbance has both spatial and temporal elements. The monster wildfires of the western states and much of northern Canada create more problems for people than the smaller fires we are used to in the Lake States. Fire management and the underlying causes for wildfire is a series of topics all by themselves.
On the other end of fire spectrum, prescribed burning can be a very helpful tool to increase the health of certain ecosystems and successfully (and more naturally) regenerate others. Jack pine forests are the poster children of a fire-dependent ecosystem. However, several threatened forest types can also be maintained by prescribed burning, especially oak barrens and savannas.
Oak barrens sport scattered groups of trees and individual trees. The canopy is largely open. The regular occurrence of light fire eliminated invading trees and shrubs and favors a suite of grasses and forbs peculiar to oak barrens or, more properly, prairies.
The term “oak opening” originated with pioneers crossing the open prairies where groups of old bur oaks provided some shade among the seas of grasses. These oaks have thick bark that protects the trees from most wildfires. Over time, these oak openings have been overrun with more mesic tree species. However, the discerning observer will see the remnant bur oaks among the “invading” trees species. This might just conjure-up mental images of what these oak openings of yore might have looked like, although the prairie plants are pretty much gone.
Prescribed burning can also help maintain common oak types and red pine. Removing understory competition from shrubs and non-type trees can pave the way for regeneration of oaks and pine. Blueberries love periodic light fires.
Prescribed burning can also be a tool in the battle to reduce the harm done by exotic plants and shrubs.
Much of our forest character was derived from the unnatural disturbances of century-old logging and wildfires. The Lake States used to have fires at the size scale of those we now hear about in the news. The oak forests of northern Michigan frequently occupy sites where pines used to grow. Pines could not survive the repeated wildfires. Oaks could. So, one can argue that most of those oak forests are human origin, not entirely “natural”.
The benefits of prescribed fire are many. However, it’s a tool that requires some care. Of course, a prescribed fire should not escape and grow into a wildfire. This is a risk that can be minimized. Close attention must be given to burn conditions.
More commonly, smoke is a problem when burning occurs too close to residential areas or travel corridors. Wind direction needs to be plotted. Public outreach is a good idea.
This public outreach component can be critical. Some people are understandably afraid of fire, especially if they have houses among forest types that are prone to fire. Yet, strategic prescribed fire will actually lower the chances of a wildfire by reducing fuels. This reduction happens on human terms, rather than later on nature’s terms with higher fuel loads.
More common, but less glamorous, are the myriads of small non-fire disturbances creating gaps in the forest canopy. This disturbance regime is more the style for our sugar maple-dominated northern hardwood types. Single large trees or groups of trees that die leave gaps allowing more light to reach the forest floor. This increased light helps stimulate regeneration and allows saplings to recruit in the canopy.
Beavers “open up” ten to fifteen thousand acres each year. Thunderstorms, high wind events, insect outbreaks, and ever-persistent disease organisms create a patchwork mosaic of age classes and forest conditions.
Combinations of wind, disease, and insect activity cause trees to die. Usually, these are small and isolated events scattered throughout the forest. However, there are a few glorious cyclical insect cycles which can result in thousands of acres of mortality. These events also encourage reproduction and recruitment of new forests.
Foresters and biologists understand the important role of disturbance in forest ecology. Forest management systems are designed to mimic these natural disturbances suited to particular forest types. So, the next time you see “disturbance”, perhaps thinking about regeneration and young forests might be easier than grousing about changes in visual quality.
