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How Healthy is Your Forest?
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Proper management keeps a forest strong and better able to resist pest issues, leading to healthier and more productive forestland.

By David Coyle

Forest health can mean many things. For forest landowners the primary consideration is how the health of the forest affects the ability to manage, monetize, and enjoy property.

Forest health is a function of tree health, but they are not synonymous. A healthy forest can have a few unhealthy trees, and an unhealthy forest can have some healthy trees. A healthy tree has adequate water, nutrients, and sunlight. It can acquire the resources it needs to grow, defend itself from insects and pathogens, and store energy in its roots.

Any condition that limits a tree’s ability to do these things can impair the tree’s health and in turn cause that tree to be stressed. Stress can be devastating; think of your own body.  When you’re stressed, whether it’s from lack of sleep, poor eating habits or any number of factors, you’re more likely to get sick. When an animal is stressed, its immune system is compromised. When a tree is stressed, it’s more susceptible to insects and pathogens. Thus, staying healthy and free from stress is the key to a tree’s continued survival.

Three factors affect forest and tree health the most: weather, people, and pests. Drought is the most common weather event that decreases forest and tree health. During a drought, a lack of water and (usually) warm temperatures can cause trees to weaken. Signs that a tree has been affected by drought can be subtle, like a shedding of needles or leaves. Pine needles might turn yellow, then red, then brown, and fall off. Hardwood leaves might fall without changing color.  Trees shed needles or leaves during drought to cope with decreased resources.

People most often damage forest health by mismanagement. When forests get overcrowded, trees get stressed. This is particularly true in pine production forests. When trees reach pulpwood size, it’s time to thin the stand.

Pests can harm tree and forest health in many ways, but they usually don’t attack unless trees already are stressed. There are certain exceptions, of course, like the emerald ash borer, an invasive insect that is happy to infest and kill healthy ash trees. However, for every devastating emerald ash borer-type pest there are countless others only able to successfully attack trees when trees are stressed. These include many pine beetles and fungi, especially in the southeastern U.S.  In fact, damage from these pests is generally worse during and after droughts.

As a landowner, you probably see many things on your trees or property that might indicate some variety of pests. The challenge is recognizing what is and isn’t worthy of further investigation (maybe even by a professional). Moderate defoliation and dead or dying branches often are noticeable, but rarely detrimental to affected trees. A little defoliation or chewed leaves on hardwood trees usually isn’t cause for concern.

There are thousands of insect species, usually caterpillars (but sometimes beetles) that feed on leaves. Unless you’re in an area with widespread defoliation (i.e. during a gypsy moth or fall cankerworm outbreak) you’re likely to see some defoliation on leaves of many different hardwood tree species. There might be a few holes in the leaf or on the edge of a leaf or the whole leaf may be affected. Whatever the case, as long as the defoliation is scattered and not extensive, the tree probably is going to be fine.

Both hardwood and pine trees are likely to have dying or dead branches. In pines, a dying branch or two usually is not a big deal, particularly on a mature tree. Every branch on a tree must produce a certain amount of food to justify the tree keeping it alive; think of it as a quota. If the branch fails to make its quota on a regular basis, the energy the tree must expend to keep the branch alive begins to outweigh the energy the branch produces. At this point, the tree cuts the branch off from water and nutrients, and the branch begins to die.

Once a branch is dead or dying, it becomes susceptible to insects and fungi. Ips bark beetles especially tend to take advantage of the situation. These little brown pests often attack weak branches on North American pines, but usually damage is confined to a branch. Under normal conditions, in a healthy tree, they don’t spread to the main stem or inflict much damage.

Damaging pest situations rarely arise under normal, non-stressful conditions such as when trees get sufficient water, nutrients, and sunlight. A stressed tree is a susceptible tree, and stressed trees are subject to damage by all sorts of pests. Often, large-scale defoliation occurs in areas of less-than-optimum tree growth, like urban forests or ridgetops, or during drought. These events are easy to spot; caterpillar poop (scientists call it frass) often rains down under the tree in the process of being consumed, and insect bodies and pieces of leaves are usually widespread over the vegetation and ground below. Severe defoliation events can make it look like trees are leafless in the middle of summer. 

In pine stands, the same factors tend to stress trees, especially drought.  Drought-stressed trees are particularly susceptible to attacks from bark beetles. Ips bark beetles usually cause one or a few trees in a group to turn brown. They don’t kill the trees; trees attacked by Ips bark beetles usually are so stressed, they’ve already started dying. The beetles merely push them over the edge.

Southern pine beetles (another type of bark beetle present in the southern U.S.) also are drawn to trees stressed by drought or mismanagement. Unlike Ips beetles, which usually infest only a small number of trees, southern pine beetles are capable of rapid population increases that can overwhelm a tree regardless of its level of stress.

Not all insect damage is cause for concern. Sawdust on the tree trunk or at the base of a tree usually means ambrosia beetles have chewed their way into the wood. These tiny beetles come to the tree after it’s nearly dead. Large wood “shavings” suggest woodboring beetle larvae.  Again, these usually come after the tree is mostly dead. 

Globs of sap, called pitch tubes, might be a nothing to worry about, or might be a big issue. Pitch tubes indicate that bark beetles have chewed their way into the tree when it was still alive. Sometimes, the tree wins this battle, and pushes out more sap than the beetle can handle. In this case, you might see a dead beetle stuck in the pitch.

The position and size of the pitch tube can indicate which kind of beetle did the damage. Smaller pitch tubes all over the tree are generally Ips bark beetles or pine beetles (southern pine beetle, mountain pine beetle, etc.). Large pitch tubes, bigger than a quarter in diameter, are usually from turpentine beetles. If you see pitch tubes, green needles on the ground, and trees in varying stages of death (yellow, red, and brown needles) that’s usually a sign of an active bark beetle infestation. If you see this, it would be a good decision to call a forest health professional to have them evaluate your stand.

One of the best defenses we have against bark beetles is sound forest management: thin trees when necessary, control competing vegetation, and match the right tree species to the land and soil. Taking proper care of your stand will keep it healthy and better able to resist or tolerate pest issues, and will lead to a healthier, more productive forest.

 

David Coyle runs a forest health and invasive species program for Southern Regional Extension Forestry and is a member of the University of Georgia’s Warnell School of Forestry and Natural Resources. 

*This article initially ran in the May/June issue of Forest Landowner.

 

 

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