Oak trees

Quercus is a genus of roughly 500 species across the northern hemisphere, and almost every generalisation about oaks fails somewhere in it. The defining feature is the acorn: a nut sitting in a cup of fused scales, produced by no other genus. Everything else — leaf shape, whether the tree drops its leaves, how long it lives, how fast it grows — varies enormously.

Deciduousness is the clearest example. Northern European oaks are deciduous and the assumption travels with the word, but Mediterranean holm oak and cork oak are evergreen, and the live oaks of the southern United States hold their leaves through winter. "Are oak trees deciduous?" has no answer that is not first a question about which oak.

The winter leaves in that last row are worth naming. Many oaks hold dead brown leaves on the lower branches until spring, when new growth finally pushes them off — a habit called marcescence, which is why an oak sheds leaves in April rather than October. Why it happens is not settled: protecting buds from browsing deer, trapping snow, or timing leaf litter to arrive when the tree can use the nutrients are all proposed.

Oaks are not fast. A young oak in good conditions puts on perhaps 30–60 cm of height a year and slows steadily; a poplar of the same age is three times taller. What oaks do instead is persist. Five hundred years is unremarkable for an English oak, a thousand is possible, and the oldest known individuals substantially exceed that.

The usual framing has these trees living a very long time and then dying. What actually happens is stranger: an ancient oak retrenches. It sheds its upper crown, reduces the volume it has to supply, and regrows a lower, denser canopy — becoming shorter and wider as it ages. A "dying" veteran oak with a stag-headed crown of bare upper branches is frequently doing this rather than failing.

Hollowing is part of the same story and is also not decline. Heartwood at the centre of the trunk is dead tissue serving as structural support, and fungi digest it out over centuries. The living tissue is a cylinder near the outside, so a hollow oak has lost the least useful part of itself — and a tube is a more efficient structure than a solid rod for resisting bending. Hollow ancient oaks routinely outlive solid younger ones.

• Roots: mostly in the top metre of soil and spreading well beyond the canopy, with a taproot mainly in the first years.
• Bark shedding: normal in many species as the trunk expands, rather than a sign of disease.
• Sap: oaks bleed from wounds in spring when pressure is high, which is why they are pruned in summer.
• Summer branch drop: large limbs shed from apparently healthy trees in still hot weather, still not well explained.

Oaks support an unusually large associated community, and the reason is a combination of three things rather than anything mysterious: they are long-lived, so a single tree offers a stable habitat for centuries; they are structurally complex, especially once hollowing and deadwood arrive; and they are abundant enough across their range for specialists to have evolved on them.

• Gall wasps: dozens of species, each inducing a structurally distinct gall — the tree builds the structure, and the wasp supplies the instructions.
• Caterpillars: several hundred moth species feed on oak foliage in Britain alone, and the spring flush is timed against them.
• Deadwood invertebrates: beetles whose larvae develop only in decaying heartwood, which requires trees old enough to have some.
• Mycorrhizal fungi: hundreds of species associating with oak roots, exchanging minerals for carbon.
• Acorn eaters: jays, squirrels, mice, deer and pigs, some of which plant more acorns than they eat.

The figures quoted for "species supported by oak" — often over two thousand — come from specific national surveys with specific inclusion criteria, and they count anything recorded on the tree rather than anything dependent on it. The number is real and it is not the same as two thousand species that would disappear without oaks. NatureHQ cites the pattern rather than the headline.

An oak in a mast year can drop tens of thousands of acorns; two years later it may produce almost none. The swing is far larger than year-to-year weather explains on its own, and trees across a wide area do it together, which is the part that needs an explanation rather than the quantity.

Spring temperature is the cue that appears to synchronise them, in interaction with the resources a tree has stored. That is the proximate mechanism. Why the behaviour is favoured at all is a separate question, and it has two candidate answers.

The explanation everyone has heard is predator satiation: produce so many acorns in one year that the squirrels and mice cannot eat them all, and starve them in the years between so their numbers stay low. It is a real hypothesis with real support, and it is routinely stated as settled fact with an intention attached that no tree has.

The result is conditional rather than confirmatory, which is more useful. Satiation works where the seed eaters are resident and cannot simply arrive — a population of wood mice can be swamped. It weakens or fails where they are mobile generalists, and jays will travel to a mast crop rather than be overwhelmed by it.

There is also a second explanation with independent support, and it has nothing to do with predators. Oaks are wind-pollinated, and wind pollination works far better when everything flowers at once — a tree flowering alone is broadcasting pollen into a landscape with nothing to receive it. Synchrony pays for itself in fertilisation before any acorn is eaten. Both mechanisms fit much of the same data and NatureHQ carries both.

An acorn is heavy and falls straight down, which is useless — under the parent is the worst place for it. What moves acorns is animals, and jays above all: a single jay can cache thousands of acorns in an autumn, carrying several at a time, often hundreds of metres and preferentially into open ground where a seedling stands a chance.

It is not cooperation. The jay is storing winter food and retrieving most of it; the oak’s return is the fraction forgotten. But the effect is directional in a way wind is not — a jay chooses open sites and buries acorns at a useful depth — which is why oak woodland can advance across grassland faster than a heavy-seeded tree has any right to.

Oak tissue is heavily loaded with tannins — astringent compounds that bind proteins and make leaves and acorns hard to digest. That is a defence against herbivores, and it is also why oak resists rot, why oak bark was used for tanning leather, and why acorns need leaching before people can eat them.

Oak galls are something stranger. A gall wasp lays an egg in oak tissue and chemically hijacks the tree’s own growth, which then builds a custom structure — a marble gall, an oak apple, a knopper on an acorn — that feeds and shelters the larva. The tree is doing the building; the wasp is supplying the instructions. Oak supports more gall-forming species than almost any other plant.

• How do gall wasps actually redirect oak development?

  Why it matters: The insect builds nothing — the tree does — and the chemical signals that hijack plant growth this precisely are still not fully identified.

• How much does jay caching shape where oak woodland can establish?

  Why it matters: If oak expansion depends on one bird’s caching preferences, that has direct consequences for rewilding and woodland restoration.