Why Do Trees Die?

January 15, 2006

Guest article written by John Phillips, a Certified Arborist and a Registered Consulting Arborist.

In the most simplistic sense, trees die when they run out of energy. Energy is needed for all of a tree's processes, including growth, reproduction and defense. This energy is produced primarily in the leaves as a product of sunlight, water and minerals. When the supply of energy is less than optimal, trees are more vulnerable to debilitation from a wide variety of other organisms. Certain types of insects and fungi are predominant among these. For a tree to decline and ultimately die, there are usually many species of these organisms involved over time.

In the western U.S., the most common cause for lowered energy levels is probably soil water deficits. Except for a few locales, much of this area has been subjected to long periods of subnormal rainfall. This condition plays a big part in predisposing trees to insects such as bark beetle and wood borers. Other factors that may contribute to an inadequate supply of water include changes in stand density and soil capacity as a result of past forest practices. Many areas that were cut post WWII have many more trees per acre than before. These trees not only have to cope with the increase in competition for water but a reduced supply where soils have been eroded or otherwise depleted.

The increase in forest density is also a result of fire suppression over the last 150 years. Most of our forests have developed over the last 10,000 years (or so) in the presence of periodic fire. This affected not only the number of trees, but also the composition of the forest in general. Trees in forests do not grow strictly as individuals. They are associated with other plants and animals, many of which form beneficial relationships. There is much about these relationships that is not well understood yet, but it is probably safe to say that they have been affected by past forest management practices.

Our forests of today are sorting themselves out. They are being subjected to conditions they did not evolve with. Where high energy levels can be maintained, survival will be most likely.

John Phillips, is a Certified Arborist and a Registered Consulting Arborist based in Willits in Mendocino County, CA USA.