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.

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)

Spines and thorns

Honey locust grows branched thorns, several inches long and clustered on the trunk and lower branches, sometimes in impenetrable masses. Hawthorns 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, persimmon and Kentucky coffeetree, 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.

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 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, 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 leaves release cyanide when they wilt, which is why farmers keep cattle away from fallen cherry limbs. Black walnut makes juglone. Black locust bark is toxic enough to poison a horse that chews it. Pawpaw 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 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 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, post oak and blackjack oak carry corky, deeply furrowed bark that insulates the living tissue and lets them hold the prairie edge for centuries. Black gum 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.

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, 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 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.

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. 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, 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, 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.