Understanding the Spread of Decay in Trees

Throughout the course of their lives, trees receive lots of wounds, even when branches die, where decay-causing fungi can invade the wood and begin to spread. Trees have survived for millions of years because they have evolved into compartmented organisms that wall off damaged and infected wood internally.

When an animal is wounded, it can heal by creating new cells at the site of the wound. When a tree is wounded, it cannot heal and can only defend itself from the spread of infection (internal decay) by walling off the damaged area and giving up those cells. This walling process is called Compartmentalization of Decay in Trees, or CODIT, as a short acronym. The walls created when the tree is wounded or injured are mostly chemical and internal. There is also a physical wall creating a barrier between the new wood growth created after the wound and the older wood that was present at the time of wounding. That physical wall is created by the cambium cells found just under the bark that form callus tissue at the time of wounding, followed by woundwood that grows over the wounded area, working to close the wound over time.

Brown Rot Wood Decay Consuming the Base of a Street Tree photo credit: V. Cotrone

In the 1970s, USDA Forest Service Forest Pathologist Dr. Alex Shigo developed a model describing the compartmentalization of decay in trees (CODIT) to teach practitioners such as foresters and arborists. The CODIT model explains how trees react to wounds by developing or forming four walls or barriers designed to slow the spread of decay internally:

• Wall 1 is created when the tree plugs vascular channels (water-conducting xylem) with tyloses or gums adjacent to the wound site to limit the spread of fungal hyphae up and down the length of the wounded branch or trunk.
• Wall 2 develops in latewood (xylem) toresist the spread of decay inward toward thecenter of the wounded branch or trunk by producing chemical compounds that resist the spread of decay.
• Wall 3 forms in living ray cells using active defense chemicals to prevent the circular spread of decay.
• Wall 4 is the Barrier Zone, where cambium cells produce chemical compounds to prevent the spread of decay outward into new sapwood. The cambium cells initially develop new specialized wood callus tissue immediately after the wound forms. This callus tissue has the strongest resistance to decay expansion.

Wall 1 tends to be the weakest because it is difficult to plug up vascular channels (xylem) tight enough to prevent decay fungi from spreading along the length of a branch or the trunk. Wall 4 is the strongest wall because it is both a chemical and physical barrier zone. In addition to chemical compounds, new callus wood formed by cambium cells creates a physical barrier to protect new xylem, cambium, and phloem.

Wound Closure

Following a wound, callus tissue consisting of thin-walled, undifferentiated cells (Shigo 1989) is formed at the site of the wound. Within months, the callus tissue forms woundwood, which is highly organized vascular wood with lignin (Shigo 1989). Lignin is an important compound in binding wood tissue together and contributing to the structural strength of wood. Woundwood is what we see growing over to close a wound on a tree trunk or where a branch was removed.

Years ago, people thought decay could be prevented from entering a wound by sealing it with tar or oil-based paints. That does not work to prevent or stop the spread of decay. Oil-based sealants will inhibit wound closure by damaging callus tissue and woundwood.

As decay progresses internally, the tree might lose the center sapwood (xylem tissue) and become hollow. This may become a part of aging for some trees, while others form heartwood that resists decay. This is usually driven by tree species and genetics. Species such as oak, hickory, sugar maple, and redwood are strong compartmentalizers and resist the spread of decay. Other species, such as silver maple, cottonwood, and willow are weak compartmentalizers and try to outgrow the column of decay, putting on new wood while their centers become hollow.

Old arboricultural practices included filling decay cavities in trees with concrete thinking strength was added. More recently, some have used expanding foam to fill cavities to keep water and wildlife out of the tree. Concrete, expanding foam, or other materials provide no benefit to thestructure of a tree with a cavity. This practice of filling cavities may actually cause more harm by preventing water from draining out and blocking air circulation, both favorable for decal fungi growth. Additionally, when the tree eventually needs to be removed, materials like concrete hiding inside a tree trunk can be a nasty surprise for a tree crew, breaking chainsaws and potentially causing injuries.

Types of Wood Decay

White Rot Fungi produces enzymes thatrapidly break down cell walls and lignin. The remaining wood is the light-colored cellulose that becomes soft and spongy (or stringy). Common white rot fungi are Armillaria root rot and Ganoderma applanatum, which produce the artist conk on hardwood trees.

Brown Rot Fungi consume the cellulose with degrading enzymes, rapidly weakening wood without damaging the lignin. The remaining wood is brown from the lignin, dry, fragile, and crumbling into cubes. Examples of brown rot fungi are Ganoderma lucidum and Hypoxylon canker.

Soft Rot is caused by both bacteria and fungi that produce enzymes that rapidly break down cell walls and materials between cell walls. Fungi grow slower than brown or white rots.

Signs of Decay in Trees

Several positive indicators that decay is expanding within a tree are:

• the presence of conks and mushrooms on the trunk, branches, or roots
• cavities and nesting holes in a tree trunk
• carpenter ants create colonies in the decayed wood but do not consume wood
• visual presence of wood decay seen on trunks and branches.

How do we prevent or slow the spread of decay?

• Avoid wounding trees with mowers, string trimmers, tree climbing spurs/spikes, heavy equipment, etc. Avoid damaging or severing large roots.
• Prune properly, leaving branch collars intact so that wound closure happens and decay is compartmentalized to the branch wood only. Avoid making flush cuts that cause rapid internal decay in the trunk. Avoid making heading cuts or topping trees because these wounds do not close well. Heading cuts and topping result in decay spreading readily through sapwood because Wall 1 in the CODIT Model is not very strong.
• Prune trees to train for good form while young. This allows you to prune branches while they are smaller so that wounds close quickly. By establishing a good form while young, you will be less likely to have large diameter branches that need to be pruned in the future.
• Plant strong compartmentalizing trees such as oak, hickory, walnut, elm, sugar maple, hornbeam, honey locust, or London planetree.
• Monitor wound closure over time, especially on tree species that are weaker compartmentalizers, such as silver maple, birch, magnolia, cherry, apple, or linden.
• Don't use wound sealants or pruning paints. It might work well for wooden decks and houses, but they don’t help living trees.

Keep the tree healthy by mulching properly, watering, and providing nutrients that are lacking. A healthy tree, with lots of foliage and stored carbohydrates, will have more energy and resources to allocate to wound closure and compartmentalization.

If decay is present in trees and there are concerns about safety and potential tree failure, it is best to hire an ISA Certified Arborist with the Tree Risk Assessment Qualification (TRAQ). Decay is typically present in many older trees that are still structurally strong. An evaluation by a trained and qualified arborist can help assess the condition of a tree, including the impact of decay on the health or structure of the tree and the extent to which the tree remains safe or poses a risk of failure.

References:

Wood Decay Fungi of Living Trees Website

Wood Decay in Living and Dead Trees: A Pictorial Overview - USDA Forest Service Publication

Tree Decay: An Expanded Concept - USDA Forest Service Publication

Compartmentalization of Decay in Trees - USDA Forest Service Publication