by Lawanda Jungwirth
A dead tree is never entirely dead even though it may seem that way at first glance. Observe a dead tree for a while, and you will see that it is teeming with life. The healthiest, most biologically diverse forests have a good share of dead trees, broken tops, and downed logs along with living trees. They are not the tidiest looking forests, but they are the most ecologically healthy.
Dead trees in forests and home landscapes provide habitat for more than 1,200 wildlife species in the United States. According to the U.S. Forest Service, this includes about 85 species of birds, at least 50 mammals, and about a dozen reptiles and amphibians that rely on dead trees for survival. Then there are the dozens of invertebrates such as millipedes, beetles, spiders, worms, ants and more who call dead trees home or a snack bar. (Plus countless microscopic species essential to the natural community.) Some estimates indicate that removal of dead trees from wooded areas can cause loss of habitat for one out of five animals in a forest’s ecosystem.
At least 30 kinds of birds use snags as foraging perches, whether they are flycatchers scooping up flying insects – kingfishers, eagles and ospreys diving for fish – or owls and hawks searching for field mice. In addition, many bird species perch atop dead trees to sing their little hearts out. Dead trees are necessary at some point in the life cycles of up to 45 percent of all North American bird species.
The most well-known and observable use of dead trees is the cavities birds use for nests. Only 30 species of birds are able to make their own nest cavities in trees, but another 80 bird and animal species, called secondary nesters, depend on those premade cavities for their homes. Some of the secondary nesters are larger birds, squirrels, bats, raccoons, porcupines, and opossums. It is not uncommon for a series of ever-larger birds and animals to call the same cavity home.
Dead trees attract insects, and in the past, that was one of the reasons snags were removed from forests. However, those very insects draw predator insects and birds that actually help control the insect pests that can harm the overall forest.
Both standing dead trees and downed logs are home to hundreds of species of pollinating insects such as bees and wasps, the primary pollinators of a forest’s flowers and berry-producing shrubs.
How Many Dead Trees Are Enough?
For wooded acreage, three snags per acre is the minimum recommended to be left standing. A single dead tree in a small yard is also vital, sometimes even more so than on acreage. There is no upper limit for number of dead trees, just as there is no lower limit for size. Even a tree with a 6-inch diameter at breast height can host smaller birds. In general, though, the larger the snag the better, and the more snags the better.
When a Tree Should Go
When a dead tree is leaning against your house or may fall or drop branches on your house, car, or children – or your neighbor’s house, car, or children, it should go. Consider that it may be possible to remove part of the tree and let the remainder stand, or to move the dead tree or its branches to another part of your property.
After the Fall
But one day, after years of providing for birds, animals, and insects, a dead tree will finally topple. Even then it continues to perform useful ecological services, perhaps even more so than when it was standing. If it is propped up by its branches, animals such as bears, foxes, and porcupines may make a den in the protection of its branches. Generations of grouse may use it as a drumming stand.
Eventually, even those supportive branches will give way, and the trunk will lie flat on the ground where fungi and soil microbes will soften the wood, and insects and worms will arrive and thrive. Now the rotting log is performing ecological benefits such as soil fertilizer, erosion control, soil-moisture stabilization, and carbon control.
At this point, salamanders arrive and search the downed log for food. Salamanders don’t get much press, but did you know that Wisconsin has seven species of salamanders? And that in eastern forests salamanders make up more biomass than deer? Salamanders eat insects such as beetle larvae, ground beetles, spiders, sow bugs, and round worms that would like to consume all the leaf litter on the forest floor. When salamanders prey on these species, a deeper layer of leaf litter remains in the forest, holding moisture that is important for other forest plants and helping reduce erosion. Below that leaf litter is another layer of life dependent upon dead wood. A complex food web made up of bacteria, fungi, protozoa, springtails, mites, and more tiny organisms work to improve the soil for forest regeneration.
Back to salamanders for a minute. Do you know what eats them? Wild turkeys for one, so you can see how allowing downed logs to remain in the forest can directly affect the results of the spring turkey hunt. Other animals that prey on salamanders include snakes, birds, shrews, frogs, fish, skunks, raccoons, and other small mammals.
Deer also benefit from dead trees. They eat the lichens that often grow on the bark of dead or dying trees. They will also eat mushrooms that grow in the damp rotting wood and leaves, as do insects, turtles, birds, mice, and squirrels. Mushrooms can make all the difference in a deer’s survival of a difficult winter.
Over the years, as a log slowly decomposes, it releases its nutrients into the soil and becomes what is known as a nurse log. Bright green mosses and colorful mushrooms may grow on it, and tiny plants and even tree seedlings will find purchase on the rich decomposing wood.
When a tree ends up falling into a stream instead of the forest floor, it slows the movement of the water and traps sediment behind it. This provides habitat for fish and amphibians. Most trout fishermen know that streams edged with downed trees produce more fish. Scientists have not pinpointed a maximum density of logs in a stream that would be too much. Basically, the more wood, the more fish.
Note: Lawanda Jungwirth is a member of the Wild Ones Fox Valley Area Chapter and a Master Gardener. She has written a garden column in the Oshkosh Northwestern since 2000 and the “Plant Matters” column in Badger Sportsman magazine since 2010. A lifelong gardener, she has an intense interest in environmental issues. Lawanda received the 2010 Invader Crusader award from the Wisconsin Invasive Species Council.
E-mail photo by Donna VanBuecken.