We like to keep things ship shape in our urban environment. We sneer at the cluttered yard of a neighbor. We are offended by the abandoned lot, overgrown with blackberries. We abhor unsightly blemishes on our trees, like deadwood. In short, we often don’t like habitat.
I’m writing to make a case for deadwood, a case for snags. Some of you just said to yourself, “what’s a snag?” A snag is term for a standing dead tree and unfortunately, we’ve been taught that snags are dangerous and that a tree isn’t pretty unless it’s clean of deadwood.
In a healthy and natural setting, dead and dying trees may make up to 20% of the forest, provide homes and food for many other species. The Washington Department of Fish and Wildlife says that West of the Cascades, 39 species of birds and 14 species of mammals depend on tree cavities for their survival. The Hill doesn’t have quite that many, but we’ve got quite a few, including several species of woodpeckers, swallows, and chickadees, as well as Red-breasted Nuthatches, Bewick’s Wrens, and Barred Owls, who all need cavities to nest in. Mammal wise, several species of bats, raccoons, and non-native Eastern gray squirrels rely on cavities for various parts of their lives.
However those cavities don’t just appear out of nowhere in live wood. Deadwood, whether on a fully or partially dead tree, is where a bird or mammal will be able to create a cavity. Only there will the wood be soft enough to excavate. On bigleaf maples, which are often multi-stemmed or have multiple leaders, this might just be one length of wood that has naturally died off. On a Douglas fir or Western redcedar, a dead top, killed by lightning or wind might provide suitable conditions. A tree that is in the process of dying will often have multiple spots soft enough for birds to dig into. Most birds typically just use cavities for nesting, but in cold weather they may become a place to sleep as well. Mammals will use cavities both as sleeping chambers and places to rear young (often they rely on old bird cavities instead of creating their own).
In the city, where trees are close to buildings and human esthetics are the rule, we tend to want to get rid of dead material. This doesn’t have to be. Dangerous snags and deadwood should obviously be dealt with, but when the option is there, we should consider leaving it be. Alternatively, an arborist can reduce the height of a snag to a safe level, or create a so called “wildlife tree” out of a tree that would otherwise be removed. No matter how many nest boxes we put up, we can’t completely meet the demands of animals looking for homes, nor can me mimic the other ecological roles deadwood takes on.
The concept of a “dead” tree and “dead” wood is a misnomer anyway. A piece of material from a tree may no longer have live layers that transfer water and food, but it still has a long ecological role ahead. Besides being home to vertebrates, a dead or dying trees are also home to a bevy of invertebrates, fungi, molds, bacteria, and even other plants (as in the cornerstone of our Pacific Northwest Temperate Rainforests, nurse logs), which come to eat decaying material or also find shelter. You’ve probably turned over a log before and seen all the things that skitter away and noticed veins of fungal mycorrhizae in particularly decaying material. These things may not be immediately seen as valuable to us in the city, but they’re part of the biodiversity that keep our ecosystems up and running.
Appealing to people to care about invertebrate detritivores, hiding in the dark, eating decaying wood, is a hard sell. But if you want to attract wildlife, even a piece of wood that isn’t large enough for a bird cavity, may attract insects, which will in turn become food for birds. By getting rid of deadwood unnecessarily, we are picking out a cross-section of biological processes and trying to take nature into our own hands. On the Hill we can find a happy medium between tidy, well landscaped yards, and wildlife habitat, and we’ll be all the wealthier for it.