Why deliberately insert fungus into living trees to grow decay? (Because it helps wildlife)

Hemlock dowel with fungus was inserted into a Douglas-fir tree to grow decay for future wildlife nesting. Photo: Dan Omdal/DNR.

There are many birds that nest in trees but also quite a few that use hollow cavities in trees for nests. Some excavate their nests from the soft rotten wood. Other birds and mammals depend on tree cavities that are already excavated by other animals or other natural processes.

Due to logging practices of the past, there are not many of the large, old, dead trees that offer this type of habitat. Forest protection rules now require timber harvesters to leave some large trees and snags as habitat. Still, that may not be enough in some areas because it can take decades for natural processes to turn trees into useful homes for birds and animals that nest and shelter in tree cavities. Dead trees and snags are effective for nesting sites, but they don’t last as long as living trees with slow-moving decay or rot.

Forest disease specialists have been working with wildlife biologists and foresters to figure out a solution. One they’ve come up with is to inoculate living trees with wood-rotting fungi. This can quickly and reliably create long-lasting internal decay in living trees without killing them.

Here’s how they figured it out

Several years ago, forest health specialists collected heart-rot fungi from forested sites around Washington and Oregon, and grew it on hemlock dowels. Then tree climbers drilled holes high in the trunks of selected trees, inserted one fungus-infected-dowel per hole and capped it with a short piece of plastic pipe to prevent the hole from closing over.

In 2010, they returned – now 7 to 14 years after the inoculation – to cut down some of the treated trees to see if there was any decay, measure its extent, and see if wildlife was using the sites.

You can view the results in the U.S. Forest Service report ‘Seven- to 14-Year Effects of Artificially Inoculating Living Conifers to Promote Stem Decay and Subsequent Wildlife Use in Oregon and Washington Forests.’

Although wildlife was not yet observed using the inoculation sites, these treatments did result in successful colonization by heart rot fungi and development of internal decay.

The group described many important observations, including:

  • The recommendation to drill three holes at each inoculation site in order to create a larger volume of decayed wood more quickly
  • Fomitopsis cajanderi (common term – rose colored conk) appeared to cause the most decay in Douglas-fir in Western Washington
  • Fomitopsis officinalis (common term – quinine fungus) was most effective for inoculating several tree species in Eastern Washington
  • Fast growing trees may not be good candidates for this treatment because they produce such thick layers of hard, sound wood outside the internally decayed area that it would likely be difficult for a bird or animal to excavate and reach the decayed area.

Learn more about this and other issues that DNR studies about forest health on DNR’s Forest Health Program web page.

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