Forest Stressors and Resilience: Can Woodland Owners and Other Stewards Make a Difference?

The word resilience pops up frequently among forest researchers and managers these days, but what does it mean? Is it just another buzzword or is it an important call to action for the many people who care about forests and depend on their benefits - including the approximately 740,000 woodland owners across the state of Pennsylvania?

The Merriam-Webster dictionary broadly defines resilience as "an ability to recover from or adjust easily to misfortune or change." That definition also characterizes what is meant by forest resilience. Forest researchers and managers widely agree that long-term resilience of our forests depends on having diverse tree species of varying ages, with variation in structure to provide a variety of habitat types across the landscape. In other words, if we try to ensure that our woods, or the landscape in which our woods occurs, have many different species of trees that are native to where we live, and if we have healthy, young and older trees of those species, the broader forest ecosystem retains its ability to function well and to potentially recover faster from whatever pest, disease, weather extreme, or other stressor may come along next.

A core challenge to forest resilience is that we don't often know what stressors lie ahead. The specific stressors that lead to decline of a particular tree species are impossible to predict. For example, in the late 1980s, researchers at the U.S. Forest Service's Northern Research Station began to see a number of dead and dying sugar maples on Pennsylvania's Allegheny Plateau. Collaboratively, forest researchers and managers spent several decades carefully observing and conducting studies of the sugar maple decline, including a study using dolomitic lime to fertilize forest plots. They found that the sugar maple decline was a due to a complex of stressors. The stressor that caused the actual death event was usually Armillaria fungi or sugar maple borer. However, the dead sugar maples had suffered what is called an inciting stressor when they experienced major defoliation by pear thrips, forest tent caterpillar, fall cankerworm, and several other insects. The other contributing factor found was that some sugar maples had a predisposing stressor that made them more vulnerable: they were on sites with lower levels of calcium and magnesium. In short, each stressor made the sugar maples a little more vulnerable to another stressor.

In cases like the decline of individual species like sugar maple, it's hard to come up with solutions before some impacts have already been felt. This is precisely why tree species diversity is so important to overall forest resiliency. We can look to the ongoing dramatic decline of tree species like ash, elm, and hemlock to illustrate how species diversity contributes to resilience. In places where ash, elm, or hemlock has been a dominant species in a woodland, the damage and tree deaths caused by the species-targeted insects (emerald ash borer, hemlock woolly adelgid) and disease (Dutch elm disease, elm yellows) is extensive, leaving woodland stands with many (or all) trees of that species dead within a relatively short time frame. In woodlands where there is a greater number of different tree species, the impact is not quite as devastating - the results being the death of some trees here and there throughout the stand or in small patches.

When we consider the stressors our forests are facing now and the unknown stressors that will certainly occur in the future, it may feel that forest resiliency is a goal beyond our influence. After all, what can we do to combat the devastating effects of pests and disease when these occur across landscapes and regions and will require concerted effort? However, there are ways that woodland owners can have great impact in creating resilient forest ecosystems across the landscape, which will help retain functionality even when species may be lost. Specifically, we should think about two key elements of forest resiliency--1) diverse species to help mitigate the impacts when one tree species gets knocked down by a new insect pest or disease or weather conditions that act as stressors, and 2) establishment of a diversity of tree age classes across a landscape. These well-understood stewardship challenges also present options for woodland owners interested in managing for resiliency. Two key actions woodland owners can take to create a more species and age diverse forest are controlling invasive plants and reducing the impact of deer where pressure is high.

We can help woodland resilience substantially by taking an active role in preventing further spread and impact of invasive plants that interfere with successful establishment and retention of natives. Doing so helps species diversity and helps reduce challenges to native forest regeneration. The relatively rapid spread of invasive plants comes at a direct cost to the long-term survival of our native woodland plants and tree seedlings because they are so highly competitive against native species. Invasive plants are quite successful in reproducing and in competing for light and other resources needed for growth. Dense patches of invasive shrubs reduce both the quantity and quality of light that new seedlings need. For example, look below the umbrella of branches created by an established stand of bush honeysuckle, and you very likely will find a complete lack of seedlings in that dense shade. Many species can tolerate some degree of shade, but all of our native woodland plants (woody and herbaceous) need some exposure to light to help seedlings establish. Openings that bring light to the forest floor are essential in the process of creating age diversity, getting tree seedlings established and able to grow, yet they also create conditions where invasive plants can thrive. Care must be taken when creating openings for light.

Secondly, in areas where deer pressure is high, woodland owners and managers can contribute to forest resilience by helping to reduce the impact of deer on forests--by hunting to reduce the local pressure, and/or by installing fencing or tree cages in areas where a younger age class of trees is desired. It has been long studied and documented by forest researchers and managers that deer browsing is a significant challenge to successful establishment and growth in some of Pennsylvania's woods. Browsing on tree seedlings themselves is one challenge, while another effect is that the composition of the understory is changed as deer have preferred browse. This preferential browsing results, sometimes, in forest understories that are dominated by one or more plant species that deer do not prefer, including New York, bracken, and hay-scented ferns. As the diversity of plants in a woodland understory is negatively impacted by repeated, long-term browsing, wildlife diversity is also affected. Studies by researchers at the U.S. Forest Service's Northern Research Station found that bird diversity was negatively impacted in areas with higher deer densities because the way the deer browse on woodland plants changes not only the species of plants, but the structure of the woodland plant communities as well. Some birds nest on the ground, while others nest in the lower branches of the canopy, and other bird species make nests higher up in the canopy. The change in forest structure due to deer browsing led to a change in bird diversity in these areas. Another result was that less diversity in tree, shrub, and woodland wildflowers also meant there was less diversity in insects--namely the many types of caterpillars that are important food sources for birds. A third legacy of high deer densities is that the number of plants that interfere with successful regeneration was higher in areas with high deer densities. One Forest Service study found a 40% increase in "interfering plants" during a ten-year period on forest plots being studied to understand deer impacts.

Looking at the many ways that deer and invasive plants affect the dynamics of a forest ecosystem, it becomes clear that if those who own and care for our woods take action, we can indeed make a difference in building forest resilience. We can make significant strides toward more resilient woodlands across Pennsylvania's landscape by taking an active role in reducing invasive plants and reducing deer impacts through hunting and/or fencing to protect regeneration. Our active efforts to limit the spread of invasive plants and the impacts of deer will help to attain greater species diversity and help to establish younger trees--both of which are fundamental to the long-term health and resiliency of our woods.

If you have questions or would like assistance in determining the current status of your forest, the PA DCNR Bureau of Forestry has a service forester assigned to each county to assist woodland owners in advancing good forest stewardship on their properties. You can find these professionals on the DCNR website.