Continuity in Aspen Forest Management

In my last two articles, I first mentioned the four foundations of ecologically-based forest management (i.e., context, continuity, complexity, and timing) and then described the importance of context. Now, I’ll address the importance of forest continuity and illustrate the concept using the example of aspen forest management in northern Michigan. Forest continuity is the purposeful retention of features of the pre-harvested forest in the postharvested forest. Forest ecologists refer to these features as “structural elements” and they promote forest biodiversity. Examples of these structural features include large, scattered trees of less common species, dead trees (snags), and logs in different stages of decay on the forest floor. Each of these structural elements provides different habitats (or niches) for a broad-array of organisms, including different species of fungi, plants, invertebrates, and vertebrates (game and non-game species). For example, large trees are fundamentally different than smaller trees in how they look (their form) and what they provide to a forest (their function). Look at a large (often older) tree and observe how the patterns of the crown and bark differ from a smaller tree of the same species (often younger). Not surprisingly, many species of hawks and owls use large trees to build nests since the larger limbs, more open crowns, and resulting notches support them. Large trees are also a requirement for many wildlife species that use cavities, pileated woodpeckers are simply too big to fit into a cavity in a smaller tree. Large trees also have complex patterns to their bark. Run your hand along the stem of a young tree and an older tree and you will note the ridges, valleys, and complexity of the older tree’s bark. Not surprisingly, birds from your bird feeder often store seeds in the bark of older trees and not younger trees. To illustrate the concept of continuity in real-world forest management, think of our aspen forests. There are four species of aspen in northern Michigan: trembling aspen, big-toothed aspen, balsam poplar, and cottonwood. All belong to the genus Populus from which the common term “popple” or “poplar” comes from. Before settlement in the late 1800s, aspen was less abundant in northern Michigan. Human activities during the 20th century, such as the clearing of the land for agriculture and then agricultural abandonment, human-caused wildfires, and the management of aspen for timber and game species led to a significant increase in the dominance of aspen across many landscapes. Currently, many aspen forests are managed by removing all trees (clearcutting). When done in the winter, this type of management encourages aspen to reproduce by below ground re-sprouting of “suckers” that derive from the aspen root system and then grow into what we call trees. However, one stand of aspen often consists of only one “tree”, with the many stems being genetic copies of one another. Most individual aspen trees or clones (the term used for the group of individual stems that are actually genetic copies) are relatively short-lived. Rarely does one find aspen older than 100 years. Yet if one walks into an aspen forest of 10 years of age and one of 70 years of age one would have an entirely different experience. Not only would the older forest consist of fewer stems, those found would be larger in diameter and more complexly shaped. Older aspen forests also tend to have other tree species in them that have seeded in over the decades the forest has taken to develop. The ground flora of the forest would also likely differ significantly between younger and older stand, with more variability and diversity in the older stand. To apply the concept of forest continuity, therefore, some of the structural elements of the older stand can be retained. For instance, one can simply not cut the scattered conifers that may have seeded in, or simply not cut all the dead or dying larger aspen, or actively retain decaying logs on the forest floor. Small patches of untreated areas can also be retained during logging operations and thereby provide all these structural features in small patches. Forests are not agricultural fields and they should not be managed as such. Management actions that simplify forests by removing all structural elements tend to do so to the detriment of biodiversity. Retaining structural elements of pre-harvested forests in post-harvested forests provides for forest continuity and forest complexity. Dr. Greg Corace is the forester for the Alpena-Montmorency Conservation District. Greg has spent the last 25 years publishing forest and wildlife research and conducting forest planning, management, inventory, and monitoring on public and private lands across northern Michigan. For more information, including sources used in this article, Greg can be contacted via email (greg.corace@macd.org) or via phone (989.356.3596 x102).