Article #174, August 2011
By Bill Cook
Forests exist because they are able to regenerate using several strategies. Barriers to regeneration can retard or prevent replacement of forest stands. The conditions through which forests regenerate change over time.
When glaciers covered what was to become Michigan, there were no trees. There were forests prior to glaciation and forests after glaciation, but probably not quite the same forests. As glaciers retreated, tree species gradually migrated northward from refugia in the south. Some species advanced relatively quickly. Others are more recent arrivals to Michigan. Seed dispersal strategies play an important role in tree migration.
Since the last glaciation, Michigan experienced a series of climate changes. These changes significantly altered the mix and distribution of species, and even the presence of forested lands. The forests that were here during settlement by Europeans and Americans happened to be a snapshot in time. It was a fortunate snapshot in the history and development in each of the Lake States. But that snapshot is only an arbitrary benchmark upon which to compare the forests of today. Our forests are not the same and will not likely be that way ever again. Conditions have changed.
We need to look more to the future than lament or glorify a particular point in the past.
Trees reproduce by three methods. All trees have flowers and fruits, with variable degrees of frequency, abundance, and success. Some trees can reproduce from root suckers or stump sprouts. A few trees can reproduce via layering, which is when a stem comes into contact with the soil and then spontaneously produces roots at that point of contact.
Tree species also have different life strategies. Some grow like crazy but live short lives. Some are more conservative, putting more energy into longevity. And some range in between the two extremes. Of course, there is a wide range of seed types, different seasons of fruition, and many dispersal strategies.
On one end of the life strategy spectrum, aspen is an example of a sun-loving, fast-growing, light-seeded species. It also has a short lifespan and has adapted to the violent death of entire stands. While seeds will germinate on sunny sites with exposed soil, more commonly aspen will sucker from the roots after wildfire or windstorm kills wide swaths of forest. The young trees require warmed soil and full sun exposure. Anything less hampers regeneration success. Many once-common catastrophic, stand-replacing events, such as wildfire, have been largely eliminated from our landscape. That’s why foresters prescribe clearcutting for aspen. Clearcutting provides the conditions under which species such as aspen can be maintained into the future.
On the other end of the life strategy spectrum, sugar maple, Michigan’s most common tree species, is adapted to fairly shady conditions and can bide its time in the understory, waiting for large trees to die, leaving a gap in the forest. Sometimes, a small group of trees die, creating a larger gap. In either case, the understory trees then race to fill that gap. That’s why foresters prescribe selection harvesting in northern hardwood stands. The trees selected are not random, but carefully spaced and evaluated. Northern hardwood research has a long history. The post-harvest stand will be in much better condition than the pre-harvest stand. Too often, non-professional “selection harvesting” selects the wrong trees and the stand is degraded.
All forest management systems achieve at least two of the following three objectives; 1) regeneration, 2) production of wood products, and 3) improving stand conditions.
When barriers to regeneration happen, forests are threatened. Converting a forest to a strip mall or retirement home are obvious examples of halting regeneration and obliterating future forests on those sites. Invasive species, such as buckthorn or garlic mustard, can prevent or reduce tree regeneration. Other exotics can selectively eliminate or diminish specific tree species. Think of emerald ash borer, Dutch elm disease, chestnut blight, and oak wilt. Ecological imbalances can create regeneration failure. Overbrowsing by white-tailed deer is a common and well-documented example. Native Pennsylvania sedge falls in the same category. Climate change may give a regenerative edge to a different set of tree species over time.
Promoting forest regeneration is a primary function of forest management and a primary focus in research-based, professional timber harvesting. In the Lake States, nearly all regeneration is through natural means, assisted by manipulating environment conditions to enhance tree regeneration. Few stands are now regenerated through planting. Planting is an expensive alternative used in only certain situations. Forest practices in other regions of the country are much different, of course. These regions have different forest ecologies, as well as different histories, economies, and cultural perspectives. But in the Lake States, we tend to do things naturally, because it works.
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Bill Cook is an MSU Extension forester providing educational programming for the Upper Peninsula. His office is located at the MSU Forest Biomass Innovation Center near Escanaba. The Center is the headquarters for three MSU Forestry properties in the U.P., with a combined area of about 8,000 acres. He can be reached at firstname.lastname@example.org or 906-786-1575.
by Bill Cook, Forester/Biologist, Michigan State University Extension, 6005
J Road, Escanaba, MI 49829
906-786-1575 (voice), 906-786-9370 (fax), e-mail: email@example.com
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