Cooperative Extension System offices receive many calls every year about dying forest trees. A variety of insects and diseases kill trees, but lack of moisture frequently is the root cause of tree problems. Moisture stress can kill a tree outright. More often, however, moisture stress weakens a tree, lowering the tree’s natural defenses to bark beetles and other insects or diseases. Forests growing on drier sites and those growing at dense spacings are especially vulnerable.
It is important for landowners to understand that tree loss relates to the natural thinning process that takes place in forests. Scattered dead and dying trees are a forest’s natural reaction to overcrowded trees. The fittest competitors survive. In drier forest types, natural thinning was aided in the past by periodic surface fires that killed smaller trees in the understory.
The natural thinning process is not always sufficient to keep a forest healthy. On some sites, crowded forests are more likely to stagnate than self-thin. In too-dense forests, insect populations can build up and cause too much tree loss. Also, dense stands of stressed or dead trees can be prone to fire, particularly during drought cycles. For example, in Yellowstone National Park in 1988, large acreages of stressed, overcrowded lodgepole pines were killed by mountain pine beetle and then burned by fires.
The following articles provide more information about thinning of forests and woodlands:
Chris Schnepf, University of Idaho
Bennett, Max, Stephen Fitzgerald, Bob Parker, Marty Main, Andy Perleberg, Chris Schnepf, and Ron Mahoney, 2010. Reducing Fire Risk on Your Forest Property. Pacific Northwest Extension Publication PNW 618.
Emmingham, W.H., and N.E. Elwood, 1983. Thinning: An Important Timber Management Tool. Pacific Northwest Extension Publication PNW 184.
McKenna, James, and Keith Woeste, 2000–2014. Planting and Care of Fine Hardwood Seedlings: Fertilizing, Pruning and Thinning Hardwood Plantations. Series of 20+ publications on caring for hardwoods available at: http://www.agriculture.purdue.edu/fnr/htirc/publications/landowners.html.
Schnepf, Chris, Russell T. Graham, Sandy Kegley, and Theresa B. Jain, 2009. Managing Organic Debris for Forest Health: Reconciling Fire Hazard, Bark Beetles, Wildlife and Forest Nutrition Needs. Pacific Northwest Extension Publication PNW 609.