By Chris Schnepf
One of the first ways we expect climate change to impact forests is with the behavior and effects of forest insects and diseases. To assess that, it is important to monitor forests for evidence of insects and diseases that kill trees. Things to monitor include:
- Which species of tree is being affected?
- Are whole trees turning brown?
- Are tree crowns slowly fading or thinning?
- Is there heavy needle or leaf loss (where are they being lost?) or evidence of insect feeding?
- Are leaves/needles abnormal?
- Are there dead branches?
- Is growth slowing significantly?
- Are conks or mushrooms growing out of the tree’s trunk?
These symptoms can be caused by insects, disease, animal damage, old age, trees competing with each other, normal tree development (e.g. lower branches being shaded out by branches higher in the tree), environmental factors such as drought, or some combination of all these factors. The symptoms may or may not indicate a problem, depending on their extent, what is causing them, and your goals and objectives for the site. These symptoms may be altered as climate changes, so monitoring for them is also an important part of assessing climate change.
But noticing tree symptoms is only the beginning. Ultimately, you also need to determine what is causing the symptoms and whether it is something that could kill the entire tree or other trees. Many of the insects and diseases that affect forest trees also affect landscape trees. You can bring in a sample or photos into the local extension office, for help in identifying the cause. If they are available, a local state forester may also be able to visit the site and help you assess the question. Once you understand symptom causes, these experts can help you explore how those causes could change as temperatures warm or precipitation patterns change.
Identifying an insect or disease that can kill a tree doesn’t mean it is a problem. Most of the organisms that kill trees are native to our forests, and occur at some endemic level most of the time. Ultimately, monitoring dead and dying trees is less important than monitoring forest conditions that favor the insects or diseases. Successful methods to minimize forest insects and disease problems are usually preventative, before problems are noticed. The two most critical characteristics to monitor for forest health are species compositionand stand density (the number of trees per acre, basal area, stand density index, etc.). For example, is a forest heavily composed of species that are highly vulnerable to key insects or diseases common to the area? Does a forest have too many stems per acre (a common invitation to bark beetles)?
In North America, we commonly manage forests in ways that mimic some stage of how forests grew here historically. Foresters commonly focus on the forest conditions that were the most resilient in the face of various insects, diseases, fire, and climate extremes that occurred in an area over the long term (say, the last 500 years). Resilience is a major theme to adapting to climate change, so many foresters naturally view this as the best initial strategy to adapt forests to changing climate.
Healthy forests are not just about healthy trees. Many forest owners value wildlife and biological diversity on their forests, and these can be monitored as well. For example, if a landowner finds unrecognized plants on their property, they can consult publications available locally to identify them or bring a sample of the plant into the local extension office for help in identification. If it is an invasive plant, the landowner is that much closer to recognizing it and removing it to improve forest health. If it is a native plant, they have enriched their understanding of their forest’s biology and ecology.
Monitoring is a critical part of growing healthy forests. It becomes even more critical in a time of changing climate. Monitoring also provides occasion for you to learn more about how your forest is working, which helping you manage more successfully towards your goals.