This time of year, it’s common to see neighborhoods, schools, and workplaces adorned with Halloween decorations and bowls of candy, an ode to the trick-or-treat tradition of our youth.

Large cavity at the base of an American sycamore (Platanus occidentalis) which has led to heart rot, hollowing out the center of the trunk (Photo taken by Zach Carnegie).

Many of these festive displays contain images of skeletons, ghosts, zombies and other undead creatures. The theme of this holiday is dominated by ghoulish figures reminding us of death. Of course, Halloween is also tied closely with the feeling of Fall. In fact, if we look around our forests right now we’ll find a display of death and decay on a scale that Halloween decorating enthusiasts could only dream of achieving! Dead leaves falling all around, littering the forest floor and leaving behind naked branches devoid of obvious signs of life. The usually abundant green carpet of herbaceous vegetation growing on the forest floor has wilted and browned, killed off by the frosts. This time of year, as the layers of our deciduous forests are no longer shrouded in green and the line of sight truly opens up, one of my favorite things to look for are cavities in trees, and these aren’t caused by eating too much Halloween candy!

Tree cavities come in all shapes and sizes. Each tells a story related to that tree and the wounds it has endured. When I say wounds, I’m referring to damage and decay that has entered and afflicted the outer layers of the tree; the outer bark, the phloem and cambium beneath, and the innermost layers which consist of xylem tissue.

Depiction of the cell layers that make up the woody tissue of a tree (Image courtesy of Virginia Tech, Forestry Outreach Site).

When a wound does occur it serves as an entry point for moisture and insects which may carry pathogens (bacterial and fungal). This speeds up the process of decay and can affect the long-term survivability of the tree. A healthy tree has a good chance of surviving wounds and slowing the spread of decay. This method is called compartmentalization and is a trait unique to trees. In the late 1970’s, Dr. Alex Shigo, a plant pathologist with the US Forest Service, modeled the process by which trees compartmentalize through the use of 4 barrier walls in and adjacent to the affected area. This model is referred to as the Compartmentalization of Decay in Trees (CODIT).

Wall 1 resists vertical spread of decay by plugging up xylem vessels, which normally transport water and minerals from the roots up through to the canopy (sapwood) or are dead but provide structural support as the innermost layers of the tree (heartwood). Wall 2 resists decay from spreading inward by chemicals previously deposited into the older cells of the heartwood. Wall 3 resists the lateral spread of decay by activating ray cells which radiate outward from the center of the tree, like spokes on a wheel. Rays normally function to store the carbohydrates over winter, which have been produced in the leaves and transported by the phloem. Wall 4 is the layer of new wood that is grown after the wound occurs.

Walls 1-3 are referred to as the reaction zone while Wall 4 is called the barrier zone. Dr. Shigo’s model determined that Wall 1 is the weakest or least effective at resisting the spread of decay. Wall 4 on the other hand is the strongest and most effective. The overall effectiveness and efficiency of this compartmentalization process varies by species and the overall health of the wounded tree. Sometimes only a small cavity is left behind or in subsequent growing seasons the wound is completely grown over. Other times, the wound is too large, the tree is facing other stressors and it fails to adequately compartmentalize, or pathogens are introduced quickly after the wound occurs. It’s not uncommon in these instances for fungi to feed upon and break down the interior woody tissue causing heart rot within the trunk, roots, or branches. As the internal layers of woody tissue are softened and broken down it becomes more susceptible to further breakdown by insects such as carpenter ants and termites. With several methods of decay and consumption, heart rot can completely hollow out portions of a tree, leaving only the living sapwood behind. This can go on for years, often undetected, as the rot resistant 4th Wall holds fast with an otherwise hollowing trunk.

Looking up into the hollowed out trunk of an American sycamore, which still had a living crown (Photo taken by Zach Carnegie).

A hollowed tree in a park setting, or adjacent to buildings and critical infrastructure would be particularly concerning. The heartwood of a tree provides significant structural support and without it, many property managers would recommend removal to prevent unnecessary damage or hazard to life/property should the compromised tree break and fall over. In a forested setting free from those concerns it’s common to find trees of varying sizes, both in height and diameter, with portions of the trunk completely hollow inside. In fact, that’s the case of one of my favorite trees from my youth. It was a massive American sycamore, several feet in diameter, that had a relatively small wound about 2’ up the base of its trunk. For who knows how many years, rot and decay ensued. This provided food for thousands, if not millions of insects that lived and fed within that rotting wood. Birds and mammals alike undoubtedly fed upon those insects and lived within the ever expanding cavity. Yet the tree stood fast as it put layer after layer of new wood on. By the time I came to know the tree, it had become what I referred to as nature’s greatest fort. Climbing through the opening at the base was a bit of a challenge, but once inside, the tree opened up and 8 or more people could comfortably sit inside. It was hollowed out vertically, up to (at least) 30’ high.

While it may not have been the safest place for my friends and I to congregate, with only a few inches of sapwood bearing the weight of what was left of the canopy, it was an experience that has stuck with me for life and ignited an incessant search for a similar specimen. It also showed me that even in declining health, trees provide excellent habitat for wildlife, and wild youth.

Sometimes I’ll spot cavities that are nearly perfectly round in shape. These may occur at a place along the trunk or a large branch where a limb once grew and either broke off or shed naturally after being shaded out by competing trees through a process called cladoptosis. It may even be a sign of past improper pruning activity done by humans. On occasion you can find large cavities that stretch vertically along the trunk of a tree. These may be the result of a frost crack or a lightning strike. I’ll also find cavities that start at or near the base of the tree indicating damage caused many years ago by severe weather events and wildlife or human activity. For example, logging activities or other instances of equipment operations which lead to strikes against the bark of a tree. Livestock grazing amongst trees can lead to similar wounds as they rub or chew the bark. I’ve even seen horse pastures where many of the trees had substantial wounds around the base of their trunks due to rubbing and removal of the bark with their teeth.

The image here shows an American sycamore tree growing along Morgan Run in Carroll County, MD. This tree has a large cavity opening that starts at the base of the trunk and faces directly upstream. I believe that during a flooding event years ago, a large piece of debris was carried with the flow, likely another tree trunk, and slammed against the base of this one, causing significant damage to the bark. This may have occurred multiple times throughout the years.

Tree cavities of all shapes and sizes are utilized by wildlife for shelter, food storage, and reproduction. Many bird species rely on such habitat for roosting and nesting. One such bird, the pileated woodpecker (Dryocopus pileatus) not only inhabits, but creates such cavities, particularly in dead or dying trees, referred to as snags. Snakes may use an accessible tree cavity as a safe space to shed their skin. Mammal species, such as racoons, squirrels, opossums, and black bears, also utilize large cavities and hollowed out trunks as safe spaces to shelter in.

Two smaller cavities in the trunk of the same American sycamore tree. A pileated woodpecker (Dryocopus pileatus) hsa been spotted popping in and out of one of these cavities in recent years (Photo taken by Zach Carnegie).

Below is a black cherry tree (Prunus serotina) which has provided such habitat. It appears that at some point this tree experienced damage from frost cracking. This occurs during the cold season when sunlight warms the bark to a temperature higher than that of the interior wood. As the sunlight fades, the temperature of the outer bark and phloem drop rapidly causing them to constrict at a quicker rate than the interior layers of the tree, forming a vertical wound as the outer layers tear under the stress. Frost cracks may be relatively narrow and the tree may close over the wound with new layers of wood. If the frost crack occurs repeatedly, it can create a larger, deeper fissure that allows pathogens and insects to enter and decay occurs faster than compartmentalization can occur. The black cherry below is a species that is notoriously poor at compartmentalization of wounds, so this frost crack has become a much larger cavity that has not only provided food for wildlife but also shelter for a cozy squirrel’s nest.

A black cherry tree with what appears to be an old frost crack wound. The lower wound houses a squirrel’s nest (called a drey), insulated on 4 sides by the wood of the tree (Photos taken by Zach Carnegie).

When you get out for a walk in the woods this winter, take a look both high and low for cavities. You may be surprised at how many you find in trees that appear otherwise healthy. If you’re lucky, you may just see wildlife entering or exiting their nests or stumble upon a great hollow trunk like my bygone sycamore fort!