Into the Woods: The benefits of diversity

By

Ethan Tapper

On the radio this morning I heard a teacher say, “The jobs we are preparing our students for haven’t been created yet.” 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 toward a healthy, productive condition, often while extracting a resource. We are constantly at the disadvantage of time, not fully understanding 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 mature – like climatic variables, timber markets. Often it seems that we are managing the forests of the future with our eye on the past, thinking the current conditions must persist forever. As we look ahead as landowners and natural resource managers, it is imperative that we recognize the “jobs” 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. While in forestry we often consider structural diversity (the abundance of different ages and arrangements of trees in a forest), let’s frame this discussion in terms of species diversity. Also, while the other plants and animals of the forest are also important, let’s talk about the concepts in terms of trees.

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.” By this measure, Chittenden County has some of the most diverse forests found anywhere in the northeastern U.S.

Less well-known is beta diversity, which is the difference between the species composition of two 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 those present in most other areas (high beta diversity). Without recognizing 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, or 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 respecting unique sites.

Managing forests for diversity is important because it helps make forests resilient. Many different species of trees means the forest has a variety of tools to deal with different forms that disturbance may take (or, in keeping with our analogy, the “jobs” it may be called on to serve in the future). Disturbances put forests at risk of soil and nutrient loss, infestations of invasive species and some serious management headaches. Forests with more diversity are better equipped to maintain their overall health in the midst of these destabilizing events.

A prime example is climate change, which will affect our forests in ways we don’t fully understand. As forest managers, our best chance at dealing with this is making sure forests are resilient and can respond to any new challenges that arise.

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 and 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 St., Essex Jct., at 585-9099 or at ethan.tapper@vermont.gov.