Douglas-fir Stumps Healed By Helpful Neighbours

A capped Douglas-fir stump in an old growth forest

One of the curiosities of Vancouver Island's Douglas-fir forests are stumps that have healed over completely such as the one shown above. These capped stumps make one wonder how a dead tree can continue to grow in this way. If you could peer into the darkness underground you would find answers,  because it is all about the roots.

Several species of trees produce root grafts including Douglas-fir and the true firs. Grafting happens when the root systems of two trees come into contact and grow together. Trees joined in such a way become in effect one organism.

This scenario is common in Douglas-fir forests, and stumps adjacent to live trees may grow over and heal completely in a strange mushroom-like bark cap.

Douglas-fir roots graft together where they cross each other

In the Douglas-fir forest, a single tree could be connected with most of its neighbours. When a tree is cut or broken off, adjacent trees treat the stump as if it was a part of themselves and will send help through their connected roots.

This starts with resin soaking into the exposed wood to prevent pathogens and decay from entering, much as a tree responds to any insult to its bark or branches. Once soaked with resin, the cambium (active growth layer just under the bark) starts to form a callus in order to heal the wound. Sometimes this goes on until the stump is fully capped.

But it is not all selfless action that is occurring in the tree community. Grafted roots can also be responsible for spreading vascular diseases from one tree to another, so a cut or damaged stump can be a way for disease to enter and spread throughout the connected trees.

A damaged or cut tree can introduce disease to surrounding trees
through grafted roots

Fully capped stumps remain connected to their neighbours, and while Douglas-fir can not sprout a new tree from the stump like some deciduous trees, the capped stump can continue to increase in girth over the years.