HBT’s

A Hollow-Bearing Tree (HBT) is a dominant or co-dominant living tree, where the trunk or limbs contain hollows, holes or cavities. Such hollows may not always be visible from the ground but may be apparent from the presence of deformities such as protuberances of broken limbs, or where it is apparent the head of the tree has broken off.

Hollows may occur in tree branches as well as the trunk. Hollows also include fire scars in the butt of trees and fissures or cracks in the branches or the main trunk.

Department of Environment and Climate Change
NSW, Australia

The HBT above was photographed minutes before the tree was destroyed. The cavity, 40-50 feet above the ground, is large enough to home a nesting owl. The tree was one of approximately 25 Cottonwoods in the grove, all of which were destroyed by the property owner.

why are hbt’s important?

The study of Hollow-Bearing Trees (HBT) is widespread in the United Kingdom, especially in Australia, where at least 303 native wildlife species rely on hollows to nest, breed, shelter and feed. This includes 31% of native mammals and 15% of native birds.¹

In much of North America, on the other hand, trees with cavities are often thought to present a risk, and are therefore subject to removal. In urban settings the removal of HBT’s frequently happens because their value is not recognized.

Appendix: HBT’s as human habitations

Trees as homes may seem like a child’s fantasy, or the stuff of ancient history. But as Dr Luke Bauserman demonstrates, the use of tree cavities as habitations is extensively documented. Cavities have effectively served as barns, taverns, hideouts, meeting rooms, storage sheds, and even homes.

The tree that has most often been used is the American Sycamore (Platanus occidentalis), native to eastern North America. In western North America the American Sycamore is sometimes confused with the London Planetree, a close relative. London Planetrees are not known to produce large cavities, perhaps because few have yet achieved the requisite age.²

notes

1. Why Are Hollow-Bearing Trees So Important? The Wilderness Society (AU)
2. It is estimated that approximately 75% of the plane trees in Walla Walla, WA are London Planetrees, and the remaining 25% are American Sycamores.

Other Sources

In North America the scholarly research on tree cavities is primarily focused on the economic impact of cavities on forest management. One exception is the following:

Tree cavity availability in urban cemeteries and city parks; by Ryan A Bovyn. Journal of Urban Ecology, Volume 5, Issue 1, 2019

The following lines are taken from the paper’s abstract:

Tree cavities provide shelter, nesting sites and food storage for many species of birds, mammals and insects. While tree cavities are present in a variety of habitats, most prior research focuses on forests, with fewer studies completed in urban areas. However, city parks provide some habitat, and cemeteries may also provide adequate, or even better, habitat for cavity-nesters, especially in urban areas where cavities are a limiting resource.