# How trees defend themselves

> Trees cannot run, so they fight with resin, thorns, tannin, latex, thick bark and stump sprouts. A tour of the defenses, with the debated parts flagged.

Published 2026-09-15, updated 2026-10-24. Tags: How trees work, Protection & care, Forests & habitat. By Blue Whale Trees, a native tree nursery in the United States. Web version: https://bluewhaletrees.com/articles/how-trees-defend-themselves/

A tree cannot move away from a beaver, a deer, a caterpillar or a fire. Everything it does about them it does standing still, over a lifetime that can run to centuries. The result is a set of defenses that are easy to overlook until you start reading them: gum, spines, bitterness, armor, and a willingness to be cut to the ground and start over.

![Long straight thorns along a hawthorn twig, silhouetted against a blue sky](https://bluewhaletrees.com/img/articles/how-trees-defend-themselves-1-1000.jpg?v=3094a1a2 "Cockspur hawthorn, a native of eastern North America, arms its twigs with straight thorns. Photographed in May 2019 in the botanical garden at Wrocław, Poland.")
*Photo: Agnieszka Kwiecień, [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0) ([source](https://commons.wikimedia.org/wiki/File%3ACrataegus_crus-galli_G%C5%82%C3%B3g_ostrogowy_2019-05-24_06.jpg))*

## Sweetgum: gum, wings and a refusal to die

Sweetgum is the best single tree for this subject. Wound it and it bleeds a fragrant, sticky resin, the storax that once went into chewing gum and medicine. The resin seals the wound against drying and fungi, and presumably it is unpleasant to chew through, so a beaver or rabbit working on the bark gets a mouthful of it. Beneath the seal, the living edge of the wound rolls inward as woundwood and closes it over the following seasons. We do not grow sweetgum, honey locust or Osage orange.

The twigs and young branches carry corky ridges and wings. Nobody has proven what they are for. The plausible idea is that a mouthful of dry cork and resin is a poor meal and that the wings make twigs awkward to strip; it is also possible they do nothing much. What is certain is what happens after a beaver fells a sweetgum: the stump and roots throw up a dense thicket of sprouts, and one tree becomes a dozen. The beaver gets a tree and the sweetgum gets a colony, and the colony grows at a speed no seedling can match.

> "Although sweetgum seedlings reach a height of 4.5 feet (1.4 m) in 3 to 5 years, sprouts often reach this height in one growing season." (USDA Forest Service, Fire Effects Information System, *Liquidambar styraciflua*)

![Sweetgum twigs against a blue sky, each edged with thick, dark corky wings](https://bluewhaletrees.com/img/articles/how-trees-defend-themselves-3-1000.jpg?v=6714377d "Corky wings along sweetgum twigs. Nobody has proven what they are for; a mouthful of dry cork and resin is a poor meal, and the wings make a twig awkward to strip.")
*Photo: Gmihail, [CC BY-SA 3.0 RS](https://creativecommons.org/licenses/by-sa/3.0/rs/deed.en) ([source](https://commons.wikimedia.org/wiki/File%3ALiquidambar_styraciflua_twigs.jpg))*

## Spines and thorns

Honey locust grows branched thorns, several inches long and clustered on the trunk and lower branches, sometimes in impenetrable masses. [Hawthorns](https://bluewhaletrees.com/plants/washington-hawthorn/index.html.md) carry stiff, simple thorns along their twigs. Osage orange has stout spines and, like its relatives the mulberries, milky sap. All three are defenses against browsing mammals; the spines are where the mouths are.

Which mouths is the interesting question. Honey locust thorns are usually thickest low on the trunk and sparse in the upper crown, far above any deer. The pods, like [the fruit of Osage orange](https://bluewhaletrees.com/articles/what-tree-dropped-this/index.html.md), [persimmon](https://bluewhaletrees.com/plants/american-persimmon/index.html.md) and [Kentucky coffeetree](https://bluewhaletrees.com/plants/kentucky-coffeetree/index.html.md), are large and drop uneaten in quantity. One hypothesis, argued since the 1980s, is that these trees are still defending themselves against, and setting fruit for, the mammoths, mastodons and ground sloths that vanished from North America some thirteen thousand years ago. It is a persuasive story, and it is not proven. Either way, a thorn that discouraged a mastodon discourages a deer just as well.

![Long branched thorns bristling from the bark of a honey locust trunk](https://bluewhaletrees.com/img/articles/how-trees-defend-themselves-2-1000.jpg?v=4b93b7b0 "Honey locust thorns, some longer than a hand, arranged where a browsing animal's face would be.")
*Photo: MONGO, [Public domain](https://commons.wikimedia.org/wiki/File%3AHoney_locust_thorns.jpg) ([source](https://commons.wikimedia.org/wiki/File%3AHoney_locust_thorns.jpg))*

The paper behind the idea was about Central American forests, not North American ones.

> "Frugivory by extinct horses, gomphotheres, ground sloths, and other Pleistocene megafauna offers a key to understanding certain plant reproductive traits in Central American lowland forests." (Janzen and Martin, *Neotropical anachronisms: the fruits the gomphotheres ate*, 1982)

## Chemistry: tannin, latex and poison

- **[Tannins](https://bluewhaletrees.com/articles/glossary/index.html.md#tannin)** are the bitter, astringent compounds in oak leaves, bark and acorns. They bind proteins, making leaves harder to digest for insects and mammals alike. Acorns of the red oak group carry more tannin than those of the white oak group, which is part of why squirrels eat white oak acorns on the spot and bury red oak acorns for later.
- **Latex** is the milky sap of [red mulberry](https://bluewhaletrees.com/plants/red-mulberry/index.html.md), Osage orange and their kin. It is sticky, gums up the mouthparts of chewing insects and carries compounds most caterpillars cannot handle. The silkworm, which eats nothing else, shows how specialized an insect must be to get past it.
- **Poisons.** [Black cherry](https://bluewhaletrees.com/plants/black-cherry/index.html.md) leaves release cyanide when they wilt, which is why farmers [keep cattle away from fallen cherry limbs](https://bluewhaletrees.com/articles/native-nuts-and-fruit-poisonous-to-dogs-horses-and-children/index.html.md). [Black walnut](https://bluewhaletrees.com/plants/black-walnut/index.html.md) makes [juglone](https://bluewhaletrees.com/articles/juglone-what-to-plant-under-a-black-walnut/index.html.md). [Black locust](https://bluewhaletrees.com/plants/black-locust/index.html.md) bark is toxic enough to poison a horse that chews it. [Pawpaw](https://bluewhaletrees.com/plants/pawpaw/index.html.md) leaves and twigs are so well defended that deer almost never touch them. Each of these trees has made itself expensive to eat.

Dead leaves may count as chemistry too. Young beeches and oaks hold their dry, tannin-loaded leaves all winter, and one explanation for [marcescence](https://bluewhaletrees.com/articles/marcescence/index.html.md) is that a mouthful of dead leaves makes the buds behind them less worth the trouble to a deer.

## Armor against fire

Fire was a regular visitor across much of the eastern forest before the twentieth century, and the trees that live with it wear thick bark. [Longleaf pine](https://bluewhaletrees.com/plants/longleaf-pine/index.html.md) spends its first years as a tuft of needles at ground level, its bud buried where a grass fire passes over it, then grows plated bark that a surface fire cannot burn through. [Bur oak](https://bluewhaletrees.com/plants/bur-oak/index.html.md), [post oak](https://bluewhaletrees.com/plants/post-oak/index.html.md) and [blackjack oak](https://bluewhaletrees.com/plants/blackjack-oak/index.html.md) carry corky, deeply furrowed bark that insulates the living tissue and lets them hold [the prairie edge](https://bluewhaletrees.com/articles/native-trees-for-the-great-lakes-and-midwest/index.html.md) for centuries. [Black gum](https://bluewhaletrees.com/plants/black-gum/index.html.md) is more modest: its bark thickens with age enough to take a light ground fire, and when a hotter fire kills the top, the roots send up new stems.

Two eastern pines can do what pines generally cannot. Shortleaf pine seedlings and saplings sprout from a crook at the base of the stem after their top is burned off. Pitch pine sprouts from buds along the trunk and branches, so a burned pitch pine barren greens up within weeks.

![Two pitch pine cones in an open hand on a woodland path, one opened wide and one still closed, dry pine needles across them](https://bluewhaletrees.com/img/articles/how-trees-defend-themselves-5-1000.jpg?v=5ef60a19 "Two pitch pine cones in May 2024, one open and one closed. On some pitch pines the cones stay sealed with resin for years and open only in the heat of a fire.")
*Photo: Blue Whale Trees*

## The last resort: grow back

Almost every eastern hardwood carries dormant buds around the base of the trunk and on the roots, waiting for the top to be removed. Cut an oak, a hickory, a maple, a black gum or a sweetgum and it sprouts; cut a black locust, a [sassafras](https://bluewhaletrees.com/plants/sassafras/index.html.md), a persimmon or a pawpaw and the roots send up shoots yards away. This is the defense that makes the others survivable. A tree can afford to lose a fight with a beaver, a fire, [an ice storm](https://bluewhaletrees.com/articles/ice-and-snow-damage-on-trees/index.html.md) or a logging crew because the fight is not the end.

Blue Whale Trees uses this defense in the seed beds. A crooked seedling cut back to a stub in winter sends up a straight new leader the following summer, on a root system the seedling has already built.

![Round shallow cavity in a trunk, dark old wood ringed by a thick rolled rim of new wood, the bark around it deeply ridged](https://bluewhaletrees.com/img/articles/how-trees-defend-themselves-4-1000.jpg?v=75e2ef53 "A limb lost years ago, May 2024: the tree has rolled new wood around the hole from every side.")
*Photo: Blue Whale Trees*

It is also the defense you are most likely to work against by accident. A stump you cut and forget becomes a clump, [one tree that was cut rather than three that were planted](https://bluewhaletrees.com/articles/multistem-or-single-stem/index.html.md). A tree cut back hard answers with more shoots, not fewer. Trees are built for a world that keeps trying to eat and burn them, and they are better at surviving it than their reputation suggests. Choose [a species that fits the site and its hazards](https://bluewhaletrees.com/tools/tree-finder/index.html.md), and most of the defending takes care of itself.

## Common questions

### Why does honey locust have such long thorns?

Nobody knows for certain, but the thorns are thickest low on the trunk and sparse high in the crown. That pattern has been argued since the 1980s to be a defense against browsing mammals that vanished about thirteen thousand years ago.

### Why are acorns bitter?

Tannins bind proteins and [make leaves and nuts harder to digest](https://bluewhaletrees.com/articles/native-nuts-and-fruit-poisonous-to-dogs-horses-and-children/index.html.md), and acorns of the red oak group carry more of them than acorns of the white oak group.

### Will a hardwood grow back if it is cut down?

Almost always, from dormant buds around the base of the trunk or from the roots, which is why a cut stump becomes a clump rather than a gap.

### How do trees protect themselves from insects?

With chemistry in their leaves and sap. Tannins make oak leaves hard for insects to digest, and the sticky latex of mulberries and Osage orange gums up the mouthparts of chewing insects.

## Sources

- [Neotropical anachronisms: the fruits the gomphotheres ate](https://doi.org/10.1126/science.215.4528.19): Janzen and Martin, Science 215 (1982): the case that thorns and big fruits were built for animals that are gone.
- [Gleditsia triacanthos, Fire Effects Information System](https://www.fs.usda.gov/database/feis/plants/tree/gletri/all.html): USDA Forest Service: honey locust, its thorns and its sprouting.
- [Liquidambar styraciflua, Fire Effects Information System](https://www.fs.usda.gov/database/feis/plants/tree/liqsty/all.html): USDA Forest Service: sweetgum's biology and its vigorous sprouting.
- [Pinus palustris, Fire Effects Information System](https://www.fs.usda.gov/database/feis/plants/tree/pinpal/all.html): USDA Forest Service: longleaf pine's stemless grass stage, the bud protected in a dense tuft of needles, and bark that thickens and insulates.
- [Pinus rigida, Fire Effects Information System](https://www.fs.usda.gov/database/feis/plants/tree/pinrig/all.html): USDA Forest Service: pitch pine sprouting along the trunk and branches within days of a fire.
- [Pinus echinata, Fire Effects Information System](https://www.fs.usda.gov/database/feis/plants/tree/pinech/all.html): USDA Forest Service: shortleaf pine seedlings and saplings top-killed by fire sprouting from dormant buds at the root collar, the buds guarded by a basal crook.
- [Quercus macrocarpa, Fire Effects Information System](https://www.fs.usda.gov/database/feis/plants/tree/quemac/all.html): USDA Forest Service: bur oak's thick, deeply grooved bark insulating the cambium from fire, and bur oak in frequently burned prairies and savannas.
- [Nyssa sylvatica, N. biflora, Fire Effects Information System](https://www.fs.usda.gov/database/feis/plants/tree/nysspp/all.html): USDA Forest Service: black gum's bark growing thick on older trunks, and young trees top-killed by fire sprouting from the root crown.
- [Robinia pseudoacacia, Fire Effects Information System](https://www.fs.usda.gov/database/feis/plants/tree/robpse/all.html): USDA Forest Service: black locust bark among the parts poisonous to livestock, horses included.
- [Cyanide Poisoning of Livestock from Cherry Tree Leaves](https://extension.psu.edu/cyanide-poisoning-of-livestock-from-cherry-tree-leaves): Penn State Extension (2023): wilted leaves on a downed cherry tree or limb releasing cyanide that poisons livestock.
- [Black Walnut Toxicity](https://hort.extension.wisc.edu/articles/black-walnut-toxicity/): University of Wisconsin-Madison Extension (2024): juglone in every part of a black walnut, most of all the buds, nut hulls and roots.
- [Growing Your Own Oak Seedlings](https://extension.msstate.edu/publications/growing-your-own-oak-seedlings): Mississippi State University Extension (2024): red oak acorns higher in tannins than white oak acorns.
- [Pawpaw Planting Guide](https://www.kysu.edu/academics/college-ahnr/school-of-anr/pawpaw/pawpaw-planting-guide.php): Kentucky State University: deer that will not eat pawpaw leaves or twigs.

## Related

- Next step: [Open the tree finder](https://bluewhaletrees.com/tools/tree-finder/index.html.md)
- [Marcescence: why young beeches and oaks keep their dead leaves all winter](https://bluewhaletrees.com/articles/marcescence/index.html.md)
- [Why new leaves come out red](https://bluewhaletrees.com/articles/why-new-leaves-come-out-red/index.html.md)
- [Which native nuts and fruit poison dogs, horses and children, and which are food](https://bluewhaletrees.com/articles/native-nuts-and-fruit-poisonous-to-dogs-horses-and-children/index.html.md)
- [Juglone: which plants a black walnut poisons, how far its reach goes, and what to plant beneath it](https://bluewhaletrees.com/articles/juglone-what-to-plant-under-a-black-walnut/index.html.md)