Dead, but alive.

October 29, 2020

Snags and Coarse Woody Debris (CWD) By: Robin Eng, Restoration Gardener

As our climate changes and new stressors add to the old, many people might be noticing that big trees in their yards are beginning to decline and die. Often when people see the downward trajectory of a tree they hire someone to come in and remove it, under the guise that the tree may be hazardous or an eyesore. This conception is one that I would like to challenge all people to re-evaluate. Dead and dying trees are immensely valuable to a multitude of wildlife species and for a variety of ecosystem services.

The death of a large tree is more like a milestone in its existence – no longer a living organism, the tree becomes a thriving ecosystem. For years to decades after they have stopped producing leaves snags (the term for standing dead trees) continue to provide a unique habitat and abundant resources to an incredible array of species. Particularly in highly managed spaces, such as lawns and parks, snags can support an ecological community of species with very high beta diversity (ie. they support a different suite of organisms than the surrounding landscape). Furthermore, in our fragmented urban and suburban landscapes, large snags continue to provide resting spaces and valuable food sources for birds and other species traveling between patches of higher quality habitat.

Both as a living tree and as a majestic snag, a tree's branches are host to an array of epiphytic (tree-top dwelling) species. Truly stunning mosses and lichen splatter the bark with organic abstract artistry. Beautiful and often delicious fungi will colonize and spread beneath the bark of a declining tree, beginning the process of decomposition while the tree still stands. Insect larvae - which will later become pollinators of wildflowers and food crops - also rely on the softened wood and loosened bark of these old trees as a safe and plentiful location to live out their first stage of life. Of course, these insect larvae are more than just pre-pollinators, they are also an important food source for woodpeckers, nuthatches, and a great number of other species.

Once you get in the habit of looking, you might be surprised to notice how many large trees have wonderful sections of a hollow trunk. The cavities are often found at the base of the tree or high in the treetops. They provide many functions for a variety of species depending on the size and location of the hollow space. Many of our native and charismatic birds, mammals, and reptiles use tree cavities to take refuge from predators and inclement weather. For several species, such as owls, woodpeckers, squirrels, and fishers, these tree cavities are vital spaces for rearing their young.

Even as a dead or declining tree starts to lean, its heaving roots create new spaces for organisms like salamanders, chipmunks, weasels, and friendly snakes to take refuge and search for food. As a tree trunk approaches the ground those spaces around its roots grow, as do their functions; larger animals like foxes and skunks often excavate burrows in the root mounds of these fallen giants, utilizing the space left behind by the rotting roots and lifted soil.

Once on the ground, a dead tree takes on a new role as Coarse Woody Debris (CWD). In this phase of its existence, the tree begins its slow transition back into the soil. Over the years, hundreds of organisms living on and in the fallen tree will convert their accumulated nutrients back into healthy living soil. Tiny new forests of mosses, lichen, and saprophytic fungi form an insulating layer along the top of these logs, capturing carbon, catalyzing nutrient cycles, and trapping moisture in this rich new ecosystem. Fungi, insects, reptiles, amphibians, and small mammals form corridors throughout the length of these decaying tree trunks, mixing nutrients and leaving seeds that await germination. With time, the log will lose its shape and structure. Hungry mammals may dig into the decaying wood in search of tasty insect prey, while plant roots blur the lines between decayed wood and the soil beneath. Eventually, all you can see of the old fallen tree is a strip of vibrant new growth; perhaps you might even notice some ambitious tree seedlings eager to begin the cycle all over again.

Management Suggestions for Snags and CWD: 

Declining trees, Snags, and Coarse Woody Debris are all elements of a healthy ecosystem that we have the power and practical knowledge to maintain. Maintaining safety and removing hazards from populated spaces is of undeniable importance, however, there are steps we can take to maintain habitat elements while also reducing the risks of human-tree conflict. As with any tree work, it’s important to call in a professional to evaluate and manage large or hazardous trees and branches.  

  • Evaluate the structural stability of an existing snag. If there are no signs of leaning or rot, it is likely that it poses little risk and will remain stable for many years. Remember to have snags and declining trees periodically re-evaluated by a professional, as the structural stability of a dead tree will change with time. 
  • If a tree leaning or unstable, evaluate if it is likely to fall onto a road, structure, or heavily trafficked space. If its falling poses no threat, consider allowing the trunk to fall (and roots to heave) naturally so it can fulfill its many services as an ecosystem and natural resource. To further reduce risk, consider using flags, barricades, or signage to dissuade people from lounging beneath the tree. 
  • When a tree has broken or fallen, consider leaving it in place, you can still clean up the small branches and debris, but you’ll be amazed by the many species that might visit your fallen giant  
  • If a tree breaks during a storm event, leave the remaining trunk and any surviving shoots! Often these trees are still alive and will continue to grow for many years. 
  • If there are branches that pose a risk to people or property, consider removing only the hazardous branches, or cut the tree to a shorter height so that it no longer poses a threat to roads, trails, or structures. Whenever possible leave at least six feet of branches above major forks in the tree, and at least 18 inches of branches greater than six inches in diameter. 
  • Check the tree for cavities! If there are cavities, consider timing your tree work to avoid nesting season (typically during the winter). If you must remove (that part of) the tree, try to keep that section of branch with the cavity intact and find an edge or place on your property to leave that cavity for species to continue using. 
  • In general, ask to keep sections of the trunk and branches as long as possible. This allows for better habitat connectivity for the many small organisms that inhabit CWD, and also increases moisture retention  
  • Consider leaving the large woody material in place (or nearby) to restore soil health and provide habitat as CWD. Keeping some smaller branches help provide protection and accumulate leaf litter can be great for invertebrate, amphibian, and bird diversity!  
  • Consider reducing your mowing regime in the 5-10 feet around a fallen tree or CWD. This can be a good opportunity to restore habitat in an isolated part of your yard with an interesting focal point at its center.  

As we carry on in an era of climate change and ecosystem collapse, we must continue to challenge our perceptions about how humans can facilitate co-existence with our urban and suburban ecosystems. In many cases, our land-care practices are selected out of habit and to accommodate antiquated norms, despite clear knowledge about the destructive nature of these actions. While change is often initially uncomfortable, it can lead to growth and beauty in unexpected ways.  

In addition to benefitting out our biodiverse community, ecological stewardship practices can often save you the time, effort, and money of beating back the living world. In addition to embracing the grand old trees in your yard, leave the leaves and perennial flower stalks in the fall; celebrate the clover, violets, and dandelions in your lawn; let dead trees stand and let fallen trees lie. All that’s left to do is to appreciate the flashes of color and myriad of the diversity of life that wants to share this world with us. 

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Posted in Pittsburgh Parks Conservancy Blog