Tree stems majestically hold up the forest canopy and lead our eyes into the sky. The leafy ceiling above gives us comfort supported by those massive woody cylinders.
What about those stems? Forest products are generally considered lumber, partitioned slices of tree stems that are cut to specification and put together into homes, furniture and a zillion other practical uses. Wood provides considerable human habitat. Wood in the forest ecosystem has significant value as wildlife habitat, too.
What is a Tree?
That marvelous living thing that we call a “tree” is mostly, actually dead. The marvelous collective functions of the tree, photosynthesis, conduction of water and nutrients, growth and simple physical support, all add up to “tree”. But is it all “alive”?
Life by definition means cell division and respiration. The only parts of a tree that are truly alive in the biological sense are the cambium layer, root tips and the living parts of the leaves. That’s it. Most of a tree, wood, was once alive, but now functions as tubes for the movement of water up and nutrients down, and support tissue for the photosynthetic surfaces reaching up to compete for sunlight.
Wood is dead tissue. Mostly cellulose, hard to digest for most organisms, but great structure for wildlife habitat; especially after the living defenses of the tree are gone. Once the tree dies, the habitat value of wood takes off.
Remember the three primary needs of wildlife; food, water, cover? Dead wood providers food in the form of insects living in the dead wood, and cover in the form of cavities, crevices or loose bark.
The inside of a tree stem is usually unavailable habitat until primary cavity excavators (woodpeckers) make their nest cavities in the dead stems. Shazam! Suddenly, the inside of a tree stem is cover. And a really great place to rest and raise young.
Approximately 40 percent of forest wildlife species use dead wood for some portion of their life cycle. The list is long. A few of the species that use standing dead trees, or snags, are:
- Pileated woodpecker
- Hairy woodpecker
- Downy woodpecker
- Red-naped sapsucker
- Douglas squirrel
- Long-tailed weasel
- Flying squirrel
Woodpeckers will make new cavities every year, or improve old ones, as a regular part of courtship and nesting behavior. These cavities in dead tree stems are prime real estate. Fledging rates (babies to adulthood) for cavity nesting birds are much higher than rates for ground or cup nesters. Non migratory species (such as the pileated woodpecker) will use cavities for roosting in the non-breeding season. Small birds such as nuthatches will sometimes communally roost in cavities together. Flying squirrels are known to cuddle through cold winter days (they are nocturnal) piled into cavities. Cavities in dead tree stems are some of the best cover available in forest habitats, and are limiting factors for the presence or absence of many species.
When a tree falls and becomes a log, habitat value continues. The log acts in a similar way as a standing dead tree, providing food in the form of insects and fungi. Cover too, particularly in the interstitial spaces provided between layers of rotting wood. Wildlife use of logs is extensive, (and the subject of a future article). For example, many salamanders spend the dry seasons inside of logs, where conditions stay moist all year. Small mammals such as forest mice and voles live in and around rotting logs. Rotting wood feeds the soil with organic matter and nutrients.
Trees provide many habitat functions in the forest. Ironically, once they die, much of their habitat value to forest wildlife species increases. Snags and logs can last many, many years. Attentive landowners know of old dead trees on their property, and value them for the habitat they provide. These dead trees add character, beauty and habitat value to forest land.
Mature living trees present habitat on the surface of the stems. These barky surfaces are good foraging habitat for some species. Small birds, such as brown creepers, glean the tree surfaces for spiders and other insects hiding in the cracks. Dead branches can harbor insects too, and small birds such as chickadees can use these pieces of attached dead wood as important foraging areas.
Forestry activities tend to manage trees for maximum growth, thus producing many live, solid stems. Thinning activities enhances this growth and produce much live tree surface. Deliberate attention to maintenance, and even creation, of dead wood habitat structures can provide significant benefit to wildlife populations.
Wood. It is habitat.
By Ken Bevis, DNR Landowner Assistance Wildlife Biologist
For more information or to schedule a free site visit to your forest land, contact Ken at: Ken.Bevis@dnr.wa.gov