Building biodiversity in your forest: Cavity trees, snags, and deadwood
Forests are home to many species of trees, plants, and wildlife. A healthy mix of species helps forests withstand stress, which is essential as they face the impacts of climate change.
As woodland stewards, we can help improve the biodiversity of our forests. One of the easiest ways is by leaving behind cavity trees, dead standing trees, and downed dead trees. These trees create habitat for many species of mammals, birds, insects, amphibians, and reptiles. Maintaining these trees has a positive impact on the rest of the ecosystem.
Cavity trees and wolf trees
Cavity trees are live trees with holes in their trunks. These holes may form due to animal use (such as woodpecker damage), when branches fall off, or from genetic factors affecting the tree’s shape.
Wolf trees are large trees with crooked branches, making them less valuable for timber. But these characteristics make wolf trees a perfect habitat for cavity-nesting wildlife and other species.
If you plan to harvest timber, leave behind as many cavity and wolf trees as possible.
Big trees, such as those with a diameter at breast height of more than 12 inches, provide excellent habitat for larger animals, including ermines, flying squirrels, bats, fishers, porcupines, and black bears. But even a smaller cavity tree with a minimum of 6 inches in diameter can help small birds like the downy woodpecker and black-capped chickadee.
These special trees are usually rare in a forest; identify and mark them before timber harvesting so they don’t get cut. Also, leave some larger, healthy trees behind, as they will eventually become cavity trees and snags.
If you are not planning a timber harvest, leaving these cavity and wolf trees behind is easy. You might think that these trees are unhealthy and should be removed, but unless they are infested with an insect or disease of major concern, they are great for wildlife.
Snags, deadwood, and slash
Snags are dead standing trees, such as those that died due to wind damage, insects or decay.
These trees are full of insects, providing food for many birds and mammals. Small mammals also eat the lichens, mosses and fungi that are found on these dead standing trees.
Minnesota’s forest management guidelines recommend leaving behind all snags when possible during harvesting. As long as dead standing trees do not pose a threat to homes or other buildings, they provide critical habitat for wildlife and support ecosystem biodiversity.
Downed dead trees, also called deadwood, are trees that have fallen over. These logs are used by insects (including pollinators), amphibians, reptiles, and mammals.
Over time, deadwood decays and adds nutrients back to the soil, which is essential for long-term ecosystem health. Leave behind these downed dead trees, especially those with larger diameters, to support habitat for many species.
You can also support biodiversity by leaving the tops and limbs of harvested trees, known as slash.
After implementing a timber harvest, leave at least a third of all slash from harvested trees. Scatter slash across a timber harvest site or create slash piles. Scattered slash will break down faster than slash piles, supporting soil organisms.
Slash piles will take longer to decompose, but will support a wider range of species that use them as habitat.
When not to leave dead trees
In some cases, it’s best to remove a dead or dying tree.
If a dead standing tree or a live tree with broken branches threatens your home or other structure, have it promptly removed by a professional.
Retaining too many dead trees increases the risk of crown fires. While fire is a natural disturbance in fire-dependent forests, dead standing trees can act as a ladder for a ground fire to climb to the forest canopy, quickly making a fire worse.
Dead balsam fir trees are especially problematic “ladder fuels.” If you have a lot of dead balsam fir, consider removing some to reduce the risk of crown fires.