Cavities in Chaos – Bridging the Gap of a Microhabitat in Decline

One of the most important microhabitats of ancient and veteran trees are the rot holes and cavities used by everything from barn owls and bats roosting in the hollows to rare beetles burrowing through the fungi-infested dead wood.

Rot holes form when heart-rot fungi begin to decompose the heartwood at the centre of the tree. The initial decay softens the wood and allows insects like beetles and other species such as woodpeckers to excavate. Over time a succession of invertebrates and fungi break down the wood to form a hollow. The bottom of the cavity fills with wood-mould, a rich mulch similar to soil, that provides a stable environment for generations. In long-living trees like beech, oak and lime, cavities and their wood mould build up over hundreds of years.

The way we’ve managed trees and forests over the last 200 years has meant veteran and ancient trees are now scarce in the landscape. The move away from traditional forest management practices of pollarding and coppicing towards plantation forestry and whole tree extraction, the intensification of the farmland with its removal of hedgerows and boundary trees and a risk averse culture to tree ageing have all played their part in the loss of our old trees and the species they support.

This is a problem for the tree-cavity community. Heart-rot fungi are rarely able to move in before the trees are mature, as much as 120 years in beech and 180 years in oak and you can add another 50 years before for a cavity to form large enough for honeybee nest. Trees planted today won’t have a thriving community of beetles burrowing through the wood mould for another 200 years!

As our veteran trees gradually die of old age, disease or human intervention, there isn’t a cohort ready to replace them. There just aren’t enough trees growing to a ripe old age where cavities begin to form.

We now recognise the value of old trees and the issue associated with their loss. Good tree management will keep some to become veterans of the future. This is amazing progress, but we still need to bridge the gap between in cavity development between our hollow-bearing old trees the ones set to replace them.

One method we are trialling is to artificially inoculate trees with heart-rot fungi to initiate decay much earlier than it would usually occur. The process is straightforward – colonise wood blocks with the fungi > cut a hole in the tree  > leave the tissues to dry out a little > insert the colonised block > leave for a couple of years and see if we have been successful by re-isolating the fungi from the tree.

Colonise wood blocks with fungi

Leave the tissues to dry out a little

Insert the colonised block

Matt Wainhouse

The method is not new and has a long pedigree in North America where artificial inoculation has been used successfully for creating the heart-rot conditions that woodpeckers need to excavate their nest cavities. It’s too early to tell whether our heart-rot inoculations will be successful here in the UK. Different trees, fungi and climate will all have an impact and we lack very basic knowledge about the ecology of the fungi. If it does work, this remarkably cheap and simple conservation action could be rolled to help to bridge the temporal gap in cavity habitat.

Additional Information - Inoculating wood-decay fungi into living trees for habitat creation and species reintroduction: Developing a conservation tool

Matt Wainhouse

PHD student at Cardiff University

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