Resistance and Resilience in Forests

​Michigan forests provide innumerable benefits. To start, forests help maintain biodiversity and clean air and water. They are also used for hiking, hunting, camping, wildlife observation, and collecting berries and mushrooms. Our forests also feed local economies. How then do we sustain forests ecologically in our quickly changing world? All forests are dynamic; they change in composition and structure over time. Different forests naturally change in different ways by different means over different time frames. As defined before, a “disturbance” is anything that impacts the amount of living material (biomass) in a forest. Jack pine forests, for instance, are as fire-dependent as any forest type in North America. Fire is an essential disturbance for naturally regenerating these forests and providing complexity. Fires shape these forests in dramatic ways every hundred years or so. Fires kill most mature trees and provide for biodiversity. Fires also open up cones so seeds are released. These seeds germinate on the forest floor on which the leaf litter is removed by fire. Not surprisingly, a jack pine rarely lives much more than a hundred years. It is adapted to fairly frequent disturbance. Conversely, our forests of American beech, sugar maple, and eastern hemlock are more stable; natural change occurs in these forests, but less dramatically and over longer time frames. Not surprisingly, trees in this forest type live many hundreds of years. They also commonly grow quite large and become more vulnerable to the effects of wind. “Windthrow” is a natural disturbance and effects not only a single tree, but other trees knocked down as a large tree falls to the forest floor. The downed log provides habitat for many organisms, including male ruffed grouse that “drum” on logs. The “canopy gap” provides an irregular patch of sunlight in the forest. In gaps, saplings and seedlings take advantage of pulses of sunlight and grow more rapidly. Forests regulated by windthrow are structurally complex. While forests have always undergone change of different types and over different time frames, science suggests that forest disturbances are changing in type, return interval, and severity (degree of impact). For instance, many non-native organisms are now competing with native species for space and resources in our forests. Some of these exotic species are causing tree mortality. Oak wilt, Emerald ash borer, beech bark disease, and hemlock wooly adelgid threaten the sustainability of our forests. These new agents of disturbance, and potential changes to fire and wind patterns, require us to reevaluate forest management. “Resistance” is the ability of a forest to remain unchanged when challenged by disturbances. “Resilience” is the ability of a forest to reorganize itself after a disturbance so that it still functions. These two terms can be used as a basis for planning and can guide our forest management actions. In other words, a landowner could have these as specific goals in a forest management plan and can use these to help devise forest treatments. To promote resilience and resistance, early evidence suggests that forest diversity is important. Forest diversity is a product of site conditions of soil and climate, as well as past management activities. To maintain forest diversity, landowners should focus on what will be left behind after logging is done. In other words, think about “desired future condition.” This can include leaving less common tree species in the forest after a treatment. In many of our forests now dominated by oaks, there were once red pine and eastern white pine. If scattered pine trees are still found, these should be retained to provide seed and increase the pine component of the future forest. In cases where no pines or other conifers are found, underplanting seedlings in gaps created by logging would increase diversity. In a similar way, many pine forests historically managed by fire had scattered oaks and aspen. Future management should aim to keep these forests mixed and not solely one species (called a monoculture). When a future disturbance occurs that is not planned for (like a new exotic species), these forests would be provided a greater likelihood of resilience and resistance. A repeated principle in this column has been the importance of forest diversity. While previously discussed in the context of wildlife, tree species diversity is also an important factor in having resilient and resistant forests for the future. Diverse forests theoretically have more potential management options. In the end, however, uncertainty abounds. Understanding what our forests once were like in terms of composition and structure and how they once functioned provides us with ways to understand current conditions and plan for an uncertain future. Greg Corace is the forester for the Alpena-Montmorency Conservation District. For more information, including sources used in this article, Greg can be contacted via email (greg.corace@macd.org) or phone (989.356.3596 x102).