Hollow trees make such ideal habitats for so many types of birds, squirrels and other animals that it almost seems as if nature created them primarily to fulfill such purpose. Hollow trees across the world are home to many types of wildlife, and have also played important roles in folklore throughout the years. According to National Geographic News, male tree-hole frogs in the rain forests of Borneo have been found to utilize the unique acoustics of hollow trees to tune the pitch of their mating calls to be louder and more appealing to females. Rotting, hollow tree branches serve as the mating and breeding habitat for these tree-hole frogs as they take advantage of the sheltered, moist environment to lay eggs. Hollow trees fall naturally into the life cycle, but have you ever wondered why some trees are hollow in the first place?

Understanding the reason some trees are hollow involves first taking a look at how trees sustain life. Tree trunks and branches are comprised of a series of layers, with each serving a vital purpose. The visible outer bark of the tree protects the interior of the tree from the elements as well as disease, fungi, animals, insects and dehydration. The next three layers just inside the outer bark transport food and sugars from the leaves to the rest of the tree, create new growth and transport water and nutrients from roots to other parts of the tree. The innermost layer of the tree, called heartwood, serves as a supportive core and is comprised of strong woody tissue that is no longer alive. Tree growth occurs when new layers of tissue form and push outward, while dead cells become part of the supportive heartwood.

In order for a tree to defend itself, exterior layers must compartmentalize decay before it spreads to the supportive heartwood of the tree. If outer layers fail to do so, a hole known as a rot cavity will form, which is likely the result of fungi causing decay in the heartwood of the tree. This often happens after the tree has acquired a wound, such as from pruning or damage from a storm. Over time, this rot cavity can become larger, eventually resulting in a complete hollowing of the tree's heartwood. As long as the outer, living layers of the tree remain intact and are of sufficient thickness as compared to the hollowed area, a tree can often survive after developing a hollow.

While historically hollow trees have been filled with cement and a number of other substances, most experts now agree that filling a cavity or hollow trunk is more detrimental than effective at supporting the tree. It is important to note that while some hollow trees are structurally sound, others represent a hazard and should be supported or removed. Hollow trees that are weak can become hazardous and pose threats to people, buildings and other property in the vicinity. If you have a hollow tree on your property, it is advised to have it inspected by a certified arborist, who can determine whether it is living and assess its strength. If the tree is structurally weak, an arborist can determine whether it can be salvaged through effective cabling and bracing techniques, or if removal is necessary.