“Why have so many of my trees died?” Forest landowners across Tennessee have been recently asking themselves this question. It seems like a simple question, but the answer is more complex.
WHAT MAKES A FOREST UNHEALTHY?
Your forest is more than just a group of trees growing in the same area—it’s a sophisticated system of trees, shrubs, fungi, bacteria, and wildlife cooperating and competing for limited resources. Space, nutrients, water, and sunlight all affect the health and vitality of each organism in the forest and the forest overall. When any of these resources are out of balance, like when a drought reduces available water in the forest, your trees become more vulnerable to diseases and pests.
Due to the effects of the climate crisis, environmental stressors like droughts, storms, and floods are becoming more and more common. And with the increased global trade of goods like lumber and crops, new invasive insects, bacteria, and fungi are being introduced to our environment with alarming regularity. Often, the local tree species have no natural defenses against these invaders.
Foreign plants are an issue as well. The infamous kudzu vine is an example of a plant that is perfectly adapted to dominate certain environments—and it was brought to the U.S. intentionally for erosion control! But seemingly minor invasive plants can also disrupt the balance of nature. Privet was brought to the U.S. from Europe to be used for landscaping and hedges but has escaped into natural areas where it forms thickets that prevent native seedlings from reaching adulthood by depriving them of sunlight, space, water, and nutrients.
WHY DO TREES DIE?
Natural causes
Trees are like everything else on the planet—they don’t live forever. We are lucky to have some very long-lived species in Tennessee like the white oak, sweetgum, and black gum, which are believed to live up to 650 years. However, all living things will eventually succumb to the effects of time. Some tree species, like sassafras, typically don’t live longer than 30 years, and red maple will typically only live to about 100 years old. While this may sound like a long time, consider that if your forest was already mature 20 or 30 years ago, those trees may be approaching the natural end of their life.
Competition and predation
Very few trees survive to a great age in the wild. Many seedlings and saplings are eaten by deer, trampled by bears, dislodged from the soil during storms, or sucked dry by native insects. Height helps to protect a tree’s precious leaves from most threats and expose them to more sunlight, but being tall comes with complications. More towering trees must transport water a longer distance and need more nutrients and a strong root system to stay structurally sound. This means they need space to grow their roots laterally for support and downward to access water. This can prove impossible if competing trees densely surround the current root system, especially in places with shallow soil like mountainsides or steep hollows. If a tree cannot supply the nutrients and water needed to support photosynthesis taking place 90 or 100 feet up from the ground, it will not survive.
Compound factors
As trees grow closer together and compete more heavily for resources, a game of inches develops. A branch that loses access to sunlight because of a competing tree can become vulnerable to insects, fungi, and bacteria. If a tree cannot protect itself from an infection in its limb by self-pruning or compartmentalization, it may cause a downward spiral. And if there is a drought or flood, the compounding effect of multiple stresses can shock the tree, inhibiting its ability to produce food—almost always resulting in death. Mechanical damage can occur when nearby limbs and stems fall and rub off bark from a tree. In the case of older trees that are no longer thriving, this kind of damage can be impossible to repair, making the tree vulnerable to infection.
FOREST HEALTH VS. TREE HEALTH
So far, we have mostly looked at reasons why an individual tree may die. But, how can we tell if a whole forest is dying? A few individuals leaving the population will happen in a typical year, but widespread mortality can be a sign of failing forest health.
ENVIRONMENT
Over the past five years, Tennessee has had several severe droughts, especially in the late growing season and early spring. We saw the exceptional effects of these droughts during the 2016 fall fire season and again in October of 2019. On top of that, many parts of the state were battered by strong winds and tornadoes in 2020 and again in early 2021. Flooding has also been a problem for low-lying areas, causing tree roots to suffocate from lack of oxygen and plenty of moisture for fungi to spread. All in all, it’s been a difficult half-decade for Tennessee’s forests.
ATTACKERS
On top of these extreme weather conditions, we also have a growing list of invasive plants, insects, and diseases spreading into our state. Invasive pests and diseases like hemlock wooly adelgid, emerald ash borer, and sudden oak death threaten the continued existence of keystone species. Meanwhile, invasive plants like Multiflora Rose, Tree of Heaven, Royal Paulownia, and Bradford Pear, among many others, are outcompeting native plants for habitat and resources.
This combination of factors has put significant stress on many trees and forests across Tennessee. And you can safely bet that none of these stressors are going away anytime soon. The forests of tomorrow will not look the same as the forests of today in Tennessee.
HOW CAN YOU PROTECT YOUR FOREST?
Luckily, you aren’t powerless in this situation. You can act to protect your trees and shape the future of your forest. Take steps to increase age and species diversity in your forest, like planting young native trees in the new openings where older trees die. Cut back invasive weeds to reduce their impact on native plants, or even treat them with chemical herbicide if you can safely do so. Watch for signs of poor health like cankers, yellowing leaves, or slow growth and remove sick trees early before infection can spread. You can also ask a professional forester from the state’s Division of Forestry to look at your forest and advise you on any concerns or management techniques that apply.