Forest and Wildlife Extension

Forest Health Notes

A Series for the Non-Industrial Private Forest Landowner

An Overview of Forest Health

Many of us would agree that, whatever our personal objectives when managing a forest, the end result we seek is a healthy, sustainable forest. In a healthy forest, the risk of present or future damage from natural or human-caused agents has been minimized. Forest health has been defined as the condition of a forest when it is:

• Resilient to change
  
• Biologically diverse over a large area (landscape diversity)
  
• Able to provide a sustained habitat for vegetation, fish, wildlife and humans

A healthy forest is made up of healthy trees. Just as humans need a certain combination of food, water and exercise to maintain physical health, trees require certain inputs to maintain their health and growth. If one or more of these inputs is missing or insufficient, the tree experiences stress. Forest managers can help trees by practicing silviculture (from Latin silva, forest) to influence these inputs. The number one health problem in Washington forests is stress caused by having too many trees to the acre, or overstocking. Overstocking causes tree stress because it forces trees to compete with their neighbors for light, water and nutrients. Many silvicultural practices are effective because they reduce the number of trees to the acre and hence the competition for these essential elements. The first requirement for healthy tree growth is light. Plants manufacture their own food by using the sun's energy to convert carbon dioxide and water to a usable food source. Heavy shade, such as is found underneath the closed canopy of the forest, provides insufficient energy for the smaller, less dominant trees to grow very fast. Some species which are called intolerant cannot grow under these circumstances at all. These include pines and larches. Other species such as Douglas-fir, Engelmann spruce and most hardwoods are considered moderately tolerant. They can grow in partial shade. The tolerant species can grow under conditions of heavy shade, although not very fast. These species include grand fir, hemlock, and western redcedar. Silvicultural practices that increase light include thinning and selective harvesting. A thinning operation can cause small, slow-growing (suppressed) trees in heavy shade to receive much more light and to grow much more quickly. This is called a release operation. Harvesting by clearcutting or patch cutting permits enough light to reach the forest floor that the intolerant species such as pine are able to seed in and grow.

The second requirement is water. Trees species vary considerably in their need for water and their tolerance for drought. Shade-intolerant species commonly grow in hot, sunny areas and thus are more resistant to drought. Shade-tolerant species, on the other hand, grow naturally in the cool, moist forest. When drought occurs, which happens frequently in all western states, these shade-tolerant species get more stressed than the shade-intolerant species. A forest manager may try to alleviate this type of stress by thinning the stand. Thinning reduces the total number of trees competing for water and thus can relieve drought stress. However, over-thinning (the removal of too many trees) may increase the amount of sunlight reaching the ground and dry out the area more rapidly. This is especially true on slopes and steep terrain. An individual with ornamental trees
near their home may wish to water them in dry periods, especially if they are trying to encourage rapid growth.

The third requirement is a good nutrient supply. Trees take up minerals through their roots and incorporate them in the developing cells. One of the basic determinants of potential growth of a tree is the level of nutrients available in the soil. Nutrient-poor soils such as serpentine will never produce large trees, but rich soils can't produce large trees if they are overstocked. A forest manager may thin a stand to reduce competition for nutrients. Although it is not always cost-effective, using a fertilizer on forest soils can provide needed tree nutrients.

Light, water and nutrients are essential for tree growth, but a healthy tree is additionally free from agents of stress. Just as an unhealthy person may return to health through natural healing processes and the removal of the organism or situation causing illness, these agents of stress do not necessarily mean the death of the tree. However, depending on the severity, they can kill a tree outright, especially if it has been predisposed by insufficient light, water, or nutrients.

Temperature extremes
Either very high or very low temperatures can cause injury to trees. The problem is worse when the temperature extreme occurs suddenly. Late frosts in spring can kill the emerging shoots, while early frosts in autumn can injure shoots that are still growing. In the winter, very cold temperatures can kill trees outright, or periods when the soil is frozen followed by sudden warming trends can severely stress or kill trees. Additionally, heavy snow can cause breakage of the top or limbs, as can ice storms. In the spring, sudden hot temperatures can kill the emerging shoots. After a thinning or pruning operation, the newly exposed southwest side of the tree may become sunburnt; this is called sunscald. The bark may crack or scale off as it slowly heals,
or the tree may be killed.

Flooding
Too much water can be as much of a problem as not enough. Tree roots need to be able to "breathe;" there are air spaces in soils, even muddy soils. However, during flood conditions these air spaces fill with water and the tree roots can literally drown. Trees that are naturally adapted to riparian, or streamside, areas are usually able to survive a lengthy period of flooding, but most forest trees will be damaged or killed. Local flooding is not simply restricted to storm conditions, but can occur as a result of beavers building dams.

Fire
Fires can be beneficial or highly damaging, depending on their intensity. A low intensity fire can effectively thin a stand by removing younger trees and species that are not fire resistant, such as grand fir and western hemlock. However, trees that remain after a fire may be stressed from charring. Pines that have been scorched are often attacked by red turpentine or other bark beetles, which can finish them off.

Wind
Wind can be a problem in stands that have recently been thinned or are adjacent to clearcuts. The trees in those stands have developed with many trees acting as windbreaks; when nearby trees are removed the trees left may not be strong enough to hold themselves up in a strong wind. Rather than thinning drastically once, a forest manager can thin regularly through the life of a stand, which will permit the trees to grow strong. When preparing to harvest, a forest manager can design small clearcuts that will not expose a great deal of the adjacent forest to the effects of wind.

Insects and diseases
Many insects and diseases can cause stress to the tree; they are more likely to attack if the tree is already under stress from insufficient light, water or nutrients. See other Forest Health Notes in this series for descriptions of specific insects and diseases.

Pollutants
Airborne or waterborne pollutants can negatively impact the growth of a tree. Ozone, so scarce in the upper atmosphere, can be in a higher concentration near the ground. It is particularly troublesome near large cities, where it is produced by automobiles. Other types of air pollution include sulfur dioxide, which is serious in the upper Columbia River Valley, and hydrogen fluoride, which has been a problem near Spokane. All these gaseous pollutants cause foliage damage, thus preventing normal growth.

Logging damage
Heavy equipment in the woods can cause very serious damage. Trees hit by passing equipment or falling trees can receive extensive scarring, which can lead to problems with rot. Additionally, heavy equipment compacts the soil, crushing the air spaces in the soil that tree roots need to breathe. For this reason it is difficult to preserve trees near construction sites, as is often tried on new homesites developed on former forest lands. See Animal and Mechanical Damage, also in this series.

Animal damage
Feeding damage caused by bears, deer, gophers, etc. can put a tree under stress as it strives to heal the damage. Young trees often succumb to pocket gophers, while older trees may have reduced growth for some years as they heal over a bear scar. See Animal and Mechanical Damage, also in this series.

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Forest Health Notes were prepared by Donna Dekker-Robertson, Peter Griessmann, Dave Baumgartner, and Don Hanley, Washington State University Extension. The assistance of Robert L. Edmonds and Robert I. Gara, University of Washington College of Forest Resources, is gratefully acknowledged.

Insect or disease treatments should always be part of an overall forest management planning process. It's a good idea to have a forest management plan for your property. This tool will help you prevent problems and keep your forest healthy. For information on forest stewardship educational programs, contact your local WSU Extension office. For information on technical assistance and financial incentive programs, contact the Washington Department of Natural Resources. The Natural Resources Conservation Service will assist landowners in developing conservation plans.