Tim-ber? … Not So Fast: The Important Role of Dead and Dying Trees | EarthCare Northwest

Western Screech-owl nestling | Ken Shults | Audubon Photography Awards

by Stuart Niven

Birds need trees. This is a fact that probably all the members of Seattle Audubon know, as well as anyone else who cares about birds and the environment. Trees provide places for birds to build nests and rear their young, shelter from adverse weather, gather to communicate with one another, perch in while feeding and hide from predators. However, what is less obvious is that birds need trees in a variety of conditions. Even trees in a state of decline, or that have already died, provide value to a backyard habitat.

Solo dead tree in healthy stand, Aimstock/Canva

Dead trees provide excellent habitat for birds

As Seattle and surrounding cities become increasingly dense, with decreasing tree canopy cover and forested ‘green spaces’, the number of standing dead trees is diminishing. This happens when dead trees removed along with healthy ones when land is cleared to build new homes, and because there is a common misperception that all dead trees are dangerous, especially if they are near homes. However, depending on the cause of the decline of a tree’s health, a standing dead tree can be relatively stable for many years. Leaving a dead tree standing can provide excellent habitat for birds and other wildlife.

Depending on the species, a dead tree will generally begin to shed its bark and the wood will soften, especially the innermost portion of the tree’s core, commonly known as the heartwood. Cavity-dwelling species, such as woodpeckers, take advantage of dead trees where the wood is softer to ecavate perfect nest sides to rear a family. Birds survey trees to discover these soft spots, and use their beak and skill to identify areas of increased decay. This is similar to how consulting arborists use what is known as sonic tomography to identify and map the density of wood as it relates to areas of decay and possible structural weakness.

Common cavity-dwelling birds

Wood Duck in a tree | RC Keller | Canva

Gauging safety during structural decline

While it is important to preserve standing dead trees in urban areas, it is not always safe or practical to leave the whole tree standing. Over time, there is an increased risk of parts of the tree breaking and falling to the ground. Typically, as trees lose structural strength, they shed parts of their branches, commonly losing the extremities first. As the area of decay increases, larger sections of branches will fall. Sometimes, larger branches or stems may break, especially during extreme weather events, such as the occasional snowstorm experienced throughout the Puget Sound region. However, it is relatively unusual for a whole tree to fail at the base once the tree has died, unless the cause of the decline was related to root decay or an issue with the base of the tree. In that case, an experienced consulting arborist would recommend the removal of the whole tree when necessary.

Many of you may have noticed the dramatic decline of the non-native European White Birch (Betula pendula) trees around Seattle, due to a combination of cumulative drought stress and the  effects of an insect known as the Bronze Birch Borer. Typically, these trees initially tend to have dead tops and branch tips, which decline at differing rates until the whole tree eventually succumbs. During this process, the trees usually lose the smaller tips of branches first, then as the larger diameter wood softens as part of the decaying process, the larger portions of branches and stems may begin to fall, especially when they are in a more horizontal position. The result is a standing stem or ‘snag’ with short stubby branches that is likely to be able to stand upright for many years, crumbling down in sections over time.

Birch trees losing ground by The Oregonian / Oregon Live.

Stages of birch tree decline from infestation. From left to right: Healthy tree. Early stages. Advanced infestation.

‘Snag’ some wildlife for your urban landscape

Since branches will eventually fail and fall from dead trees, an experienced climbing arborist will remove the upper section of the canopy and lower the height of the dead tree to a height that is sensible, relative to the species and the location of the tree on a property. A good example of this is the remaining trunk of a large Douglas-fir (Pseudotsuga menziesii) tree that has been standing in front of the Seattle Audubon Nature Shop in Wedgwood for many years.

Just like the natural decline of white birches described above, it is important to preserve branches lower in the canopy, along the remaining ‘snag’, for perches for birds, as well as areas where insect activity can be present. If branches are too long to safely leave intact, they can be cut shorter, using a technique known as ‘natural fracture pruning’. This technique mimics the natural failure of a branch leaving torn bark and more ridges in the wood, a natural look to the tree. Birds love to feed on the insects that feast on decaying wood and will benefit from as many branches and structures within the tree that already exist. For a similar reason, the top of the snag can be cut in such a way, typically with a chainsaw, to create more fissures and areas where water can collect, encouraging insect activity and fungal growth, again mimicking the natural look of a tree stem that has suffered a failure. (If branches are removed and the top is not cut in anyway, the result may look more like a utility pole than a tree!)

Cavity-dwelling birds, as well as some species of bats and even ducks, will look for the ideal area to excavate* to site their nests. This will typically be a certain height above the ground, to help protect themselves from predators. If the height of the snag is not tall enough to attract the right species, the resulting nesting activity will be reduced.

* Not all cavity-dwellers do the excavation. Some seek out existing cavities for their nest site.

Seattle Audubon snag with Pileated Woodpecker | Wendy Walker

Branches, stumps, and woody debris

In most cases dead and decaying wood is a benefit to the environment. Therefore, it is not only dead trees that are essential for our local and wider ecosystems. Dead branches and stems in trees and on the ground also play an important role in the ecology of a landscape. It is unfortunately a common recommendation by tree-cutting companies that all dead branches, or deadwood, should be removed from trees. This is sometimes referred to as ‘deadwooding’ or ‘crown cleaning’ and the explanation given is usually a claim that, “if dead branches remain in trees that they will cause disease and decay to the rest of the tree, so it is essential for the health of the tree to remove all dead branches and wood.” This is a false claim: the way trees function, a dead branch in essence has already been ‘walled off’ from the rest of the tree. Even if it is decaying and has fungal and insect activity in it, there is no risk that its existence will impact the rest of the tree.

Trees have a complex way of protecting themselves from disease and external elements that may be harmful to them, so there is no need to remove dead branches or stems, unless they may cause harm to pets or people, or physical damage to other trees, plants, or structures on a property.

Dead branches, depending on their size, can be excellent nesting material for birds. They also provide a great source of food for birds once they fall onto the ground. Decomposing branches create fungal and insect activity, attracting birds. The birds’ foraging can help evolve the soil to become healthier and nutrient rich, so that artificial soil amendments are not required.

This woody debris, regardless of size, can be both a hidden or visible feature in a garden. It will not be long before the local wildlife comes to explore the natural bounty contained within, under, and around it.

Another benefit to leaving dead trees and woody debris in our backyard, is the resulting variety of surfaces on which other plant life, such as mosses, lichens, ferns, and even other trees and shrubs, may grow. Moss and lichen, known as epiphytes, are incredibly important to our local ecology, but that is a story for another time…

Wood debris placed in a planting strip (the area between a street and sidewalk) can provide good bird habitat and is often overlooked and underutilized. 

Wood debris along a sidewalk | Stuart Niven

Other articles in this issue of Earthcare Northwest

Patio Pals: The Birds and the Bees

While full-scale gardening allows for a large, varied planting palette that birds love, container gardening on a balcony or patio can also be accomplished. Jose Gonzales, a local garden professional and plant expert, walks us through how to get the most out of a small space container garden that encourages birds and other natural pollinators to thrive.

Take the Backyard Habitat Quiz

How does your outdoor space measure up? Have you minimized threats to birds? Are you offering food, water, and shelter options? Discover some obvious and subtle ways you can create a wildlife haven in your own backyard.

Connecting Urban Nature

How can we improve access and connectivity for people and wildlife? Seattle Audubon, the Capitol Hill EcoDistrict, and the Seattle Bird Conservation Partnership are finalizing a guide to improving habitat values and human access to green space in urban Seattle.