Our Vermont Woods

By: Ethan Tapper, Chittenden County Forester

When I turned on the radio this morning, I heard a teacher say: “the jobs we are preparing our students for haven’t been created yet.” As a tree-brained forester, this statement struck me as a good analogy for thinking about the importance of ‘diversity’ in forest management. We forest managers make decisions that alter forested ecosystems in the long term, attempting to steer them towards a healthy, productive condition, often while extracting a resource. We are constantly at the disadvantage of time, without a clear idea of what the world may look like in the distant future when the trees we are growing become mature. Many things can change in the course of the 100 to 120 years that it takes a sugar maple to fully mature; climatic variables, the influence, and roles of humans, timber markets. Often it seems that we are managing the forests of the future with our eye on the past, thinking that the conditions which currently exist must persist forever. As we look towards the future as landowners and natural resource managers, it is imperative that we recognize that the “jobs” that we are preparing our forests for haven’t been created (or at least defined) yet, and so our best bet is to encourage the growth of healthy, resilient, complex ecosystems, with as high a degree of diversity as is possible.

In an ecological context, diversity refers to the abundance of different conditions that exist in a given area.  In forest management, we usually split this term up into species diversity and structural diversity. While wildlife, plants and other considerations are always at the forefront of our minds, let’s talk about these concepts in terms of trees, for simplicity’s sake.

The most common understanding of species diversity is Alpha Diversity, which is simply the number of species present on a given site. Weighing Alpha Diversity might lead us to declare that a forest with a wide variety of tree species is “very diverse.” In this sense, Chittenden County has some of the most diverse forests found anywhere in the northeastern United States.

Equally important, but less well-known, is Beta Diversity, which is the difference between the species composition of two different sites. For example, there might be a forest that supports only a few species (low Alpha Diversity) but these species are unusual or distinct from most other sites (high Beta Diversity). Without a recognition of Beta Diversity, we would only value areas used by many species, and ignore many unique sites.

When Alpha and Beta Diversity combine, they form Gamma Diversity, which is the species diversity across the entire landscape. Landscapes with high Gamma Diversity contain many different species (high Alpha diversity) while representing many different types of environments (high Beta Diversity). In forest management, this is what we are typically aiming for: encouraging a wide array of species while also respecting unique sites.

Finally, structural diversity is the arrangement of the trees across the landscape, regardless of species. A landscape lacking structural diversity would look like a park or a tree plantation: trees all sharing a similar height, regularly arranged across the landscape. Structurally diverse landscapes contain pockets of young, old and middle-aged forest, and areas where many age classes of trees are mixed together. A structurally diverse forest is likely to also have high species diversity, because of the many different growth conditions that a structurally diverse landscape creates.

Managing forests for structural and species diversity is important because these conditions help make forests resilient. This means that the forest has a variety of tools to deal with different forms that disturbance may take (or, in keeping with the analogy from the beginning of this piece, the “jobs” it may be called on to serve in the future). For instance, if some exotic pest were to wipe out one tree species, and your woodlot was mostly comprised of that species, your woodlot would suddenly be extremely destabilized. Similarly, if all trees in your woodlot were the same canopy height, a wind event might destroy the overstory, removing all your trees at once. Both of these situations put your forest at risk of soil and nutrient loss, infestations of invasive species and some serious management headaches. Forests with more species and structural diversity stand a better chance of maintaining their overall health, responding vigorously to these destabilizing events.

Managing for diversity also provides habitat conditions for the largest number of species possible. There are wildlife species that require a dense understory, those that require a tight canopy, those that require canopy openings, and many that require a combination of these (and other) conditions in order to feed, move, and reproduce. Providing a diverse array of conditions gives us the best chance at maintaining habitat for as many species as possible and maintaining these habitats over time as forests mature, disturbance strikes, and conditions in the forest change.

Finally, I would be remiss if I did not mention the importance of diversity for coping with the effects of climate change. Climate change is on a course to seriously alter the composition of our forests in ways and on a time frame that we do not fully understand. As forest managers, our best chance at dealing with climate change is to make sure that they are healthy and able to respond to the unforeseen challenges that will arise. Managing for all types of diversity is the best way to do that.

Trees don’t have “jobs,” at least not in the way that we define the term. The “job” of a tree is to survive, grow, and reproduce. But even these basic tasks may become more and more difficult for our trees individually and for our forested communities in the future. The jobs that we are preparing our forests for haven’t been created yet, and so we need to prepare them as best we can for the unknown. Our best hope for that lies in the encouragement of forest health and diversity. 

Ethan Tapper is the Chittenden County Forester for the Vermont Department of Forests, Parks and Recreation. He can be reached at his office at 111 West Street, Essex Junction, at (802)-585-9099, or at ethan.tapper@vermont.gov.