How Does Bark Protect Trees From Fire?

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How does bark protect trees from fire? Mainly by insulation. Bark covers the living tissue of the trunk, and the sources focus on that living layer, the cambium. A 2025 study in the journal Fire describes the bark’s “protective (insulating) function” as one of its roles. Thickness is the variable the sources return to most often. This guide explains the mechanism, sets out what each source measured, and marks where the evidence stops.

Quick answer

• Bark works mainly as insulation. The Fire study names the bark’s “protective (insulating) function” as one of its roles.
• Thickness is the main variable. The same study says “the bark thickness is the dominant factor determining cambium heating and post-fire survival”.
• The living layer that heat damages is the cambium. A Forest Service report describes “cambium injury, which is dependent on bark thickness”.
• Bark also holds carbon reserves. The Fire study says the bark “also serves as a store of non-structural carbon reserves”.

What bark is and why its thickness varies

Bark is not one uniform layer. A 2021 paper in Frontiers in Forests and Global Change begins by saying “Bark is a complex multifunctional structure of woody plants that varies widely among species.” That variation matters for fire. The same paper notes that “Thick bark is a primary trait that can protect trees from heat generated in surface fires.”

Tree size is the simplest guide to thickness. The Frontiers authors write that “Tree diameter or wood diameter is often used as a predictor for bark thickness but little attention has been made on other factors that might affect bark development and allocation.” In other words, diameter is a shortcut, and the authors looked at what else changes bark allocation within a species.

Shape matters too. The same paper observes that “Outer bark on species that allocate resources to thick bark also tends to be rugose, with bark being thickest at the ridges and thinnest in the furrows.” So two trunks of the same diameter can carry different amounts of protective bark depending on how the surface is ridged. A single measurement at one spot on the trunk may not describe the whole tree.

How heat reaches the living layer

Fire damages a tree most when heat reaches the cambium, the living layer under the bark. The Forest Service report, a 2007 technical report on delayed conifer mortality in California, explains that “Duff smoldering around the base of trees or extensive flame exposure to the bole can cause some level of cambium injury, which is dependent on bark thickness.”

The report also explains why the cambium matters. It states that “In addition to crown injury, cambium death caused by lethal heating of the tree bole is another important predictive variable for mortality.” It adds that “The fact that fire-killed cambium can contribute to subsequent tree mortality is rarely disputed.” Heat that kills the cambium around the trunk therefore threatens the tree even when other parts look intact.

The report gives a way to tell live cambium from dead cambium. It says “Dead cambium is darker in color, often resin soaked and hard or gummy in texture.” By contrast, “Live cambium is lighter in color, moist and rather pliable.” These descriptions are the field test the authors used to judge the tissue.

What the 2025 black pine study measured

The 2025 study in Fire looked at Pinus nigra, a pine that grows in mountain and Mediterranean forests. Its authors measured bark thickness, roughness and moisture, then ran laboratory flammability tests. The study describes the flammability measures as “ignition, heat release, mass loss, as determined in laboratory flammability tests.” That is a controlled measure of how readily a bark sample catches and how much energy it gives off.

The results connected thickness with fire behaviour. The authors report that “The bark thickness correlated strongly with the ignition time, heat release, and mass loss.” Thicker bark therefore lasted longer before ignition and released its energy more slowly in the tests. The study also found that “the bark moisture content was significantly lower in previously burned trees”, which points to a second factor that changes with fire exposure.

The authors draw a clear split between the two causes. They write that “the age of the tree primarily affects the bark thickness and time to cambium death, while fire exposure primarily affects the bark moisture content, regardless of age.” In practice, age shapes how thick the protection is, and recent fire changes how wet the bark is. Both matter for survival, but they act through different parts of the bark.

Why age and thickness affect survival

Young trees are more exposed because they have thinner bark. The Fire study says that “Bark thickness increases with tree size, which explains the higher survival rate of mature trees after fire, while young trees with thinner bark remain more vulnerable.” That gap is why saplings are more at risk than old trees in the same stand.

The study also reports what happens when bark is thick enough. It says that “Under prescribed burning conditions, trees with sufficient bark thickness generally exhibit low cambium heating, minimal physiological impairment, and negligible growth reductions.” This is a finding about prescribed, low-intensity burns rather than wildfire.

The same study connects bark to the species’ long-term persistence. It states that “the bark thickness is the dominant factor determining cambium heating and post-fire survival” in Pinus nigra, and that thick bark is “central to P. nigra’s persistence under infrequent, low-intensity fire regimes.” For a tree, then, protection is not only a single-fire question. It depends on how often fires return.

Resprouting, fire intervals and carbon reserves

Bark thickness also shapes whether a tree can grow back after a fire. The Fire study notes that “There is often a negative correlation between bark thickness and resprouting ability: species that invest in thick bark are generally not resprouters, whereas species with thinner bark have a strong tendency to resprout after a fire.” Thick-barked species tend to survive by resisting heat, while thin-barked species tend to regrow.

The authors add that the pattern is not fixed. They write that “However, this trade-off is not universal; it varies by species, fire regime, and plant functional type.” A species can therefore be thin-barked and still persist if its fire regime is mild, which is a useful caution against applying one rule to every tree.

The fire interval matters as well. The study says that “The survival of individual trees is largely determined by the length of the fire interval, which must be sufficient to allow the development of bark thickness adequate to resist subsequent fires.” A tree burned again before its bark rebuilds is exposed to the same heat with less protection.

Bark also stores energy. Beyond insulation, the study says the bark “also serves as a store of non-structural carbon reserves, which are essential for recovery processes after a fire, such as resprouting, wound healing, and activation of the defences.” It warns that “these reserves can be severely depleted and/or not replenished under drought conditions.” Reserves are therefore part of recovery, and drought can cut them down.

How to use this information

The practical lesson is that bark protects a tree by slowing heat, and the amount it protects depends on thickness, age and fire history. Trees that are young, thin-barked or recently burned are the most exposed. Anyone who wants to judge a tree’s risk can start with its size, its bark condition and whether the trunk shows char.

Char is a visible clue, but it is not a full diagnosis. The Forest Service report compared bark char with cambium condition, and its results vary by char level. It states that “Light and deep bark char codes are relatively accurate in predicting live and dead cambium, respectively.” It then adds that “the moderate bark char rating is not a good predictor of cambium status.” A mid-level char mark therefore cannot replace looking at the cambium itself.

For newly planted trees, the bark question is about the first years of growth. Guides on how to water new trees and how to mulch trees cover the care side of that work.

How we chose these sources

The sources were chosen for three reasons. First, each one had to be readable in full and return HTTP 200 when checked. Second, the set had to include one government technical report, one peer-reviewed study on bark flammability, and one peer-reviewed study on bark structure. Third, each source had to address how bark affects the living tree, not just the bark’s general appearance.

The Forest Service report is a United States Department of Agriculture technical report written for land managers. The Fire study and the Frontiers paper are peer-reviewed journal articles. Each source was opened and quoted directly, and no claim in this guide relies on a summary of a source that was not opened.

What we could not verify

The sources answer the mechanism question well, but they do not settle every question a homeowner might ask. The limits below are stated in the sources themselves or follow directly from their scope.

What the sources do not say

The black pine study is limited in place. The authors note that “Only a few studies have investigated the relationships between the bark traits, bark flammability, and fire resistance of P. nigra” and that “there are no published studies addressing these relationships in black pine populations in the Balkan Peninsula.” Its findings therefore come from one species in one setting.

The Forest Service report also flags an open question about cambium damage. It says that “how much dead cambium causes tree mortality, either alone or in combination with crown kill” has not been settled, and that this “has not been determined for many species.” A reader should not take a single cambium rating as a precise survival forecast.

The Frontiers paper is about how bark is allocated across seven co-occurring southeastern United States tree species. Its fire statement is a single sentence, quoted above, and the paper does not supply a bark thickness that guarantees survival.

Frequently asked questions

Is thicker bark always better protection?

The sources treat thickness as the main factor, not the only one. The Fire study says “the fire resistance of tree species is often viewed through the lens of the bark thickness”, but it also shows that bark moisture changes after fire. Thicker bark generally slows heating, and that is the strongest finding in the sources.

Does bark moisture affect fire protection?

The Fire study reports that “the bark moisture content was significantly lower in previously burned trees.” The authors list moisture alongside thickness as a measured trait, and they say that “fire exposure primarily affects the bark moisture content, regardless of age.” A tree that has already burned may therefore carry different bark moisture than an unburned one.

What is cambium, and why does it matter?

Cambium is the living layer that sits beneath the bark. The Forest Service report says that “cambium death caused by lethal heating of the tree bole is another important predictive variable for mortality.” Protecting the cambium is therefore the main job of the bark.

Does bark thickness change as a tree gets older?

Yes, according to the Fire study, which states that “Bark thickness increases with tree size,” and links that growth to better survival of mature trees. Young trees remain more exposed for longer.

Can a tree recover after its bark is damaged?

Recovery depends on stored reserves. The Fire study says the stored reserves are “essential for recovery processes after a fire,” but adds that “these reserves can be severely depleted and/or not replenished under drought conditions.” A damaged tree in a drought year has a harder time recovering.

Sources

• Popovic, Z., Misic, N., Protic, M., and Vidakovic, V. Morpho-Physiological Traits and Flammability of Bark in a Post-Fire Black Pine Population. Fire 2025, 8, 342. https://mdpi-res.com/d_attachment/fire/fire-08-00342/article_deploy/fire-08-00342.pdf
• Hood, S. M., Smith, S. L., and Cluck, D. R. Delayed Conifer Tree Mortality Following Fire in California. USDA Forest Service Gen. Tech. Rep. PSW-GTR-203, 2007. https://d10.nrfirescience.org/sites/default/files/2023-09/psw_gtr203_019hood.pdf
• Shearman, T. M., and Varner, J. M. Variation in Bark Allocation and Rugosity Across Seven Co-occurring Southeastern US Tree Species. Frontiers in Forests and Global Change, 2021, doi:10.3389/ffgc.2021.731020. https://www.frontiersin.org/journals/forests-and-global-change/articles/10.3389/ffgc.2021.731020/epub