Why Do Trees Rot From the Inside Out?

Trees rot from the inside out because their innermost wood, called heartwood, is made of dead cells that have no active biological defenses against fungal invasion. The living outer layers of a tree trunk, by contrast, mount chemical and physical counterattacks against infection. Once decay fungi colonize the heartwood through wounds or broken branches, they work outward through tissue that cannot fight back, while the tree’s living sapwood and bark form barriers that slow or halt the rot’s outward march. The result is that signature pattern familiar to anyone who has seen a standing tree with a hollowed-out core surrounded by a perfectly healthy shell of wood.

Heartwood and Sapwood Are Fundamentally Different Tissues

A tree trunk is not a uniform cylinder of wood. It is built in annual layers, and the functional distinction that matters most is between the outer sapwood, where water and nutrients actively move, and the inner heartwood, which has been retired from plumbing duty. As a tree grows, it converts older sapwood into heartwood by filling the cells with resins, tannins, and other extractive chemicals, then sealing them off. These deposits give heartwood its typically darker color and a degree of natural resistance to decay, but the cells themselves are dead. They no longer receive water, sugar, or oxygen from the tree’s living tissues.

Research comparing the two zones in hardwood species has documented significant differences in water content, pH, and the concentration of extractive compounds between sapwood and heartwood. The heartwood in tested species had markedly different chemical profiles, including different levels of material soluble in water and alkali solutions, as well as different ash content and acidity compared to surrounding sapwood.¹ Those chemical extractives do slow fungal colonization in many species, sometimes for decades. But they are a fixed deposit, not an active defense. Once fungi begin to break those compounds down, the heartwood has no way to replenish them. Sapwood, on the other hand, is alive and can mount fresh responses.

How Fungi Get Inside

Decay fungi don’t bore through bark like insects. They need an opening. The most common entry points are branch stubs (either naturally shed or pruned), storm damage, wounds from machinery or animals, fire scars, and cracks caused by frost or lightning. Any break in the bark that exposes the wood underneath gives fungal spores a landing pad.

Once inside, colonization follows a predictable route. In functional sapwood, fungi face a hostile environment: high water content that limits available oxygen, and active cellular defenses that can produce antimicrobial compounds on demand. As one review described it, the high water content and low oxygen availability in healthy conductive sapwood restrict microbial colonization, while sapwood lesions caused by wood-decaying fungi tend to be bounded by multicellular barriers that both inhibit further pathogen spread and maintain xylem water-transport function.² The heartwood lacks all of these advantages. It is drier, better aerated, and biochemically inert. So while fungi may enter through a wound in the sapwood, they tend to proliferate most successfully once they reach the heartwood, where nothing is fighting them.

This is why heart-rot fungi can sometimes take extraordinarily long to produce visible damage. Decomposition is a slow process, and it can take heart-rot fungi hundreds of years to establish before hollowing even begins in some species.³ The fungi work through extractive-rich heartwood at a pace dictated by the chemistry of the particular species, the environmental conditions inside the trunk, and the species of fungus involved.

What Fungi Actually Do to Wood

Wood is built primarily from three structural polymers: cellulose, hemicellulose, and lignin. Different fungi attack these components in different ways, and the type of rot that results depends on which components the fungus targets first.

White-rot fungi are the most common agents of heart rot in living hardwoods. Some of them practice what researchers call simultaneous decay, breaking down cellulose, hemicellulose, and lignin all at once. Others are selective, preferentially degrading lignin and hemicellulose while leaving much of the cellulose behind, at least initially.⁴ The selective type leaves the wood looking bleached and fibrous, because the white cellulose fibers remain while the brownish lignin disappears. Brown-rot fungi, which are more common in conifers, do the opposite: they attack cellulose and hemicellulose while barely touching the lignin, leaving behind a crumbly, brown, cube-like residue.

Both strategies dissolve the structural integrity of the heartwood from within. The result, over years or decades, is a progressively larger zone of softened or absent wood at the tree’s core. Outside that zone, the living sapwood continues to function normally, and the tree keeps adding new growth rings on the exterior, completely oblivious in some cases to the hollowing in its center.

How Trees Contain the Damage

Trees cannot heal in the way animals do. They can’t replace destroyed heartwood. Instead, they wall off the damaged area and keep growing around it. This containment strategy has been formalized in what researchers call the CODIT model, which stands for Compartmentalization of Decay in Trees. The model describes four conceptual “walls” that limit the spread of decay: barriers that exist in the wood at the time of wounding, and new barriers built after wounding to prevent decay from moving into freshly formed wood.⁵

The strongest of these walls is the barrier zone laid down by the cambium, the thin layer of dividing cells that produces new wood each year. After a wound occurs, the cambium produces a chemically distinct layer that is particularly resistant to fungal penetration. New wood formed outside this barrier zone is essentially walled off from the decay happening in the older wood behind it. This is why you often see a tree with a completely hollow core but a thick, structurally sound shell of living wood that was all formed after the infection began.

Trees also defend their sapwood through vessel occlusion. When infection or injury threatens the water-conducting vessels in the sapwood, living cells adjacent to those vessels can grow balloon-like plugs called tyloses, or secrete gums, that block the vessel lumen. This slows pathogen spread, limits water loss in damaged areas, and reduces the drying and aeration that would make the wood more hospitable to fungi.⁶ The heartwood, being composed entirely of dead cells, cannot produce tyloses or gums in response to new threats. Whatever vessel-blocking structures exist in heartwood were formed long before, during the original sapwood-to-heartwood transition.

Why Trees Don’t Bother Defending Their Core

An obvious question is why trees haven’t evolved better chemical defenses for their heartwood. Some species actually have, to a degree: black locust, white oak, and many tropical hardwoods produce heartwood so saturated with toxic extractives that fungi take decades or centuries to break through. But many other species invest far less in heartwood chemistry, and some researchers argue this isn’t a failure of evolution but a rational allocation of resources.

The evolutionary argument, as proposed in a widely discussed 2014 paper, is straightforward: losing heartwood costs the tree almost nothing. A hollow tube is nearly as strong in bending as a solid cylinder of the same diameter, because mechanical stresses during wind loading are concentrated in the outer layers of the trunk, not the core. Since heartwood contributes relatively little to structural support, and contributes nothing to water transport or nutrient storage, investing heavily in defending it would be a poor use of energy that could instead go toward growth, reproduction, or sapwood defense.³ In other words, many trees are hollow not because something went wrong, but because defending the heartwood was never worth the cost.

This doesn’t mean hollowing is always harmless. But it does help explain why so many mature trees in forests worldwide are at least partially hollow without showing obvious signs of distress.

When Does Internal Decay Actually Threaten a Tree?

A tree with a modest hollow core can stand for centuries. The trouble starts when the ratio of remaining solid wood to the trunk’s overall radius gets too thin. Engineering analyses of hollow tree trunks have identified distinct failure modes depending on how much wood is left. For one well-studied cypress species, when the wall thickness dropped below about 6% of the trunk’s radius, the dominant failure mode switched to a catastrophic flattening of the cross-section under wind load, a type of buckling that solid or thick-walled trunks never experience.⁷ At more moderate levels of hollowing, trees are more likely to split lengthwise, with cracks running along the trunk rather than the trunk snapping cleanly.

The type of wood matters too. A study across eleven tree species found that hardwoods are more likely to fail by conventional bending (snapping), while softwoods and tropical woods tend to fail by longitudinal cracking when their trunks are hollow.⁸ This means the risk profile of a hollow tree depends on both how much wood remains and what kind of tree it is. A hollow oak and a hollow pine with identical wall thickness ratios don’t have the same failure risk.

For the arborists assessing trees in parks and along streets, these distinctions are critical. A tree with internal decay is not automatically dangerous, but it requires judgment about how much wood remains and how it is distributed around the trunk, because the decay is rarely perfectly centered or symmetrical.

Detecting What You Can’t See

The fundamental challenge with interior decay is that a tree can look perfectly healthy on the outside while being extensively hollowed inside. Traditional methods involved drilling into the trunk with an increment borer or tapping the trunk and listening for a hollow sound, both of which are crude at best. Modern arboriculture relies increasingly on sonic tomography, which measures how fast sound waves travel through the trunk cross-section. Decayed or hollow zones transmit sound more slowly, allowing the device to create a color-coded map of the trunk’s interior.

The PiCUS sonic tomograph, one of the most widely used devices, works by measuring variation in the speed of sound across the trunk to determine patterns of wood integrity. It has been shown to be effective at detecting and visualizing patterns of decay, generally within about 5% of visual estimates from actual cross sections, though it has limitations with incipient (early-stage) decay.⁹ Electrical resistance tomography offers a complementary approach, detecting anomalies that sonic methods sometimes miss. In one comparison, sonic tomography detected internal decay in about 28% of tested trees, while electrical resistance tomography flagged about 30%, because the two technologies are sensitive to different types of anomaly.¹⁰

These tools have changed how cities manage their urban tree canopy. Instead of preemptively removing any tree suspected of internal decay, arborists can now map the extent and distribution of the hollow zone and make risk assessments based on actual wall-thickness measurements. Many trees flagged as suspect turn out to have more than enough solid wood to remain standing safely for decades.

Does Tree Age Predict Internal Decay?

It would seem intuitive that older, larger trees would have more internal decay. After all, they have had more time to accumulate wounds and more years for fungi to work. But the relationship is surprisingly weak. A tomography-based study of living trees found that decay incidence was unrelated to trunk diameter, age, species, or any tested interaction, with all statistical tests coming back non-significant. Even the volume of decay bore no relationship to how old or large the tree was.¹¹

That said, other research focusing on large old trees in urban settings in Beijing found that a cavity index was linked to trunk diameter, tree age, and trunk health, though the relationship may be stronger in urban environments where trees accumulate wounds from construction, pruning, and vandalism over time.¹² The divergence between these findings probably reflects the difference between managed urban trees, which endure repeated anthropogenic wounds, and forest trees, where wound frequency is more random and species-specific chemical defenses play a bigger role. The takeaway for the average person is that a big old tree is not necessarily a rotten tree, and a relatively young tree is not necessarily a safe one.

How Much Carbon Goes Missing Inside Living Trees?

Forest ecologists have recently started asking how much the carbon sequestration estimates for forests are thrown off by uncounted internal decay. If standard methods calculate a tree’s biomass from its external dimensions using allometric equations, they assume a solid trunk, and any internal void means the real biomass is lower than estimated. A study of oak stands in Europe with a high proportion of large old trees, measured by sonic tomography, found that internal stem decay and tree cavities resulted in only about a 1% overestimation of stand biomass.¹³ At the stand level, that is negligible. But scaled to continent-wide or global analyses, even small percentages translate into large absolute errors in carbon accounting, which is why the issue has drawn attention from climate modelers.

Hollow Trees as Habitat

Interior decay, far from being purely destructive, creates one of the most ecologically valuable microhabitats in any forest. Tree hollows are home to a specialized and species-rich community of invertebrates that depend on dead wood, along with cavity-nesting birds, bats, and small mammals.¹⁴ Owls, woodpeckers, parrots, possums, and tree frogs are among the more charismatic tenants. In many ecosystems, the availability of tree hollows is a limiting factor for populations of these species, and the loss of hollow-bearing trees through logging or urban development is one of the less conspicuous but more damaging forms of habitat destruction.

Conservation programs in some regions have even begun deliberately inoculating living trees with heart-rot fungi to speed up the development of hollows, since natural formation can take centuries. Whether or not the tree itself benefits from being hollow has been debated. One hypothesis held that trees might gain a nutritional advantage, reabsorbing minerals released as fungi decompose the heartwood. But research on termite-hollowed eucalyptus trees in Australian savannas found the opposite: the net effect of hollowing on individual tree growth and survival was negative.¹⁵ The tree tolerates the hollow, but it doesn’t profit from it. The beneficiaries are the hundreds of other species that move into the space the fungi created.