Tree Snag Ecology

The Ecosystem In and Around a Dead Tree

An Old Oak Tree Snag. Image by Steve Nix

The small image included with this article is an old dead tree snag on my rural property in Alabama. It is a photo of the remains of an old water oak that lived grandly for over 100 years. The tree finally succumbed to its environment and completely died of old age about 3 years ago. Still, its size and rate of deterioration suggests that the tree will be around and influencing my property for a long time yet - and for that I am pleased.

What Is a Dead Tree Snag?

Tree "snag" is a term used in forestry and forest ecology which refers to a standing, dead or dying tree. That dead tree will, over time, lose its top and will drop most of the smaller branches while creating a debris field underneath. As more time goes by, maybe as long as several decades, the tree will slowly be reduced in size and height while creating a viable ecosystem in and underneath the decomposing and falling biomass.

A tree snag's persistence depends on two factors - the size of the stem and the durability of the wood of the species concerned. The snags of some large conifers, such as coast redwood on the Pacific Coast of North America and the largest cedars and cypress of U.S. coastal south, can remain intact for 100 years or more, becoming progressively shorter with age. Other tree snags of species with rapidly weathering and decaying wood - like pine, birch, and hackberry - will break up and collapse in less than five years.

A Tree Snag's Value

So, when a tree dies it still has not completely satisfied its ecological potential and the future ecological value it provides. Even in death, a tree continues to play multiple roles as it influences surrounding organisms. Certainly, the impact of the individual dead or dying tree gradually diminishes as it weathers and further decomposes.

But even with decomposition, the woody structure may remain for centuries and influence habitat conditions for millennia (especially as a wetland snag).

Even in death, my Alabama tree continues to have a tremendous influence on the micro ecology in, around, and under its decomposing trunk and branches. This particular tree provides nesting for a significant squirrel population and raccoons and is often called a "den tree". Its branching limbs provide a rookery for egrets and perches for hunting birds like hawks and kingfishers. The dead bark nurtures insects that attract and feed woodpeckers and other carnivorous, insect-loving birds. The fallen limbs create understory cover and food for quail and turkey underneath the falling canopy.

Decaying trees, as well as fallen logs, may actually be creating and influencing more organisms than a living tree. In addition to creating a habitat for decomposer organisms, dead trees provide critical habitat for sheltering and feeding a variety of animal species.

Snags and logs also provide habitat for plants of higher orders by creating habitat provided by "nurse logs". These nurse logs provide the perfect seedbed for tree seedlings in some tree species.

In forest ecosystems such as the alluvial Sitka spruce-western hemlock forests of the Olympic Peninsula, Washington, almost all tree reproduction is confined to rotten wood seedbeds.

How Trees Die

Sometimes a tree will die very quickly by a devastating insect outbreak or from virulent disease. More frequently, however, a tree's death is caused by a complex and slow process with multiple contributing factors and causes. These multiple causal concerns are typically categorized and labeled as abiotic or biotic.

Abiotic causes of tree mortality includes environmental stresses like flooding, drought, heat, low temperatures, ice storms, and excess sunlight. Abiotic stress is particularly associated in the death of tree seedlings. Pollutant stresses (e.g., acid precipitation, ozone, and acid-forming oxides of nitrogen and sulfur) and wildfire are usually included in the abiotic category but can significantly impact older trees.

Biotic causes of eventual tree death can result from plant competition. Losing the competitive battle for light, nutrients or water will limit photosynthesis and result in tree starvation. Any defoliation, be it from insects, animals or disease can have the same long-term effect. Declines in the vigor of a tree from periods of starvation, insect and disease infestations and abiotic stresses can have a cumulative effect that eventually causes mortality.