We’ve all seen deadwood… a decaying log or stump, a fallen tree, hollows in trees, the branches of a dead tree stark against the sky, the dead or dying limb of an old tree trying to cling onto life. Humans often feel the need to tidy up this ‘lifeless’ wood. After all, having dead things lying around is generally considered to be a bad thing. Leaving an unsafe decaying tree in a place where it could fall and hurt someone or damage property would not be a good idea but is this wood as lifeless as we think it is?
Dead and decaying wood is actually an important part of the woodland ecosystem and a healthy forest should contain about 30% deadwood. Woodland in the UK generally contains much less. Saproxylic invertebrates are those that are dependent on dead or decaying wood (or dependent on other organisms that are themselves dependent on dead wood) and there are over 2000 saproxylic species in the UK alone. Species found on deadwood include beetles, hoverflies, flies, parasitic wasps, moths, bugs, sawflies and spiders. The National Trust has a list of invertebrates that they have observed on dead and decaying wood on their Trelissick estate; Attingham Park is known for its saproxylic species. Fungi such as candle snuff, which grows on the rotting stumps of broadleaved trees, also live on deadwood.
Saproxylic species are often specialists with specific habitat requirements so removing deadwood has a significant impact on biodiversity. For example, 650 beetles depend on deadwood in the UK. A fallen tree can provide habitat and shelter for 20-30 years where fungi and lichen can thrive and insects such as centipedes and woodlice in turn provide food for birds and bats. Birds use holes or cavities in dead trees or dead limbs for nesting and trees with hollows provide ideal roosting sites for the great spotted woodpecker as well as many owl species. Deadwood also acts as a carbon store, slowly releases nutrients and nitrogen back into the soil and slows the flow of water therefore helping to prevent soil erosion.
How does deadwood form? Beneath a tree’s protective bark are xylem vessels which transport water and nutrients from the roots to the canopy. As the tree grows it creates new vessels and the old ones die, forming dead heartwood. If the heartwood becomes exposed, for example through woodpeckers boring in or a storm damaging the tree, fungi and insects are able to move in and start the process of decay and decomposition. Specialist invertebrates follow and the longer the process is allowed to continue, the greater the number of species that can be supported. Unfortunately, we are losing deadwood habitats through loss of traditional woodland management techniques and our tendency to “tidy up”. Pesticide use also impacts on the insect population.
What can we do to help preserve deadwood habitat and the species that depend on it? Firstly, leave dead trees and shrubs if it is safe to do so. It is also possible to take the limb off a tree or remove a ring of bark to expose the wood underneath and engineer the process of decay. If a tree or shrub must be removed, consider leaving the stump to decay naturally. Another option is to have a pile of deadwood in your garden. Dig a shallow hole so the logs are properly in contact with the ground and stack them tightly together. You can pile leaf litter or soil around and amongst the logs and then leave them to rot.
We also need to think long-term. Buglife has an interesting project called Back from the Brink: Ancients of the Future which aims to ensure continuity between trees that are already old, and those that are much younger.
You really don’t have to take on a massive project. If all of us just do one small project, it will all add up to one big whole with huge benefits to both us and nature.
The Woodland Trust’s Wood Wise has an article about deadwood.