Wind-hardened tree form

Why does a sheltered sapling snap in its first storm while its exposed neighbor bends?

Two saplings of the same species, the same age, the same seed lot. One grew in the lee of a wall, tall and slim and, frankly, the better-looking tree. The other grew on an exposed edge, shorter, thicker, a bit lopsided. The first storm takes the handsome one at the base and leaves the ugly one waving.

The tempting explanation is that the exposed tree was "toughened" by adversity, which sounds like a moral rather than a mechanism. So let me press on the word: what could wind actually do to a growing stem, other than break it, that would leave it better able to survive the next wind?

Begin with what a tree cannot do. It cannot know the local wind climate in advance. A seed has no way of encoding whether it will land on a ridge or in a hollow, and building every stem to ridge specification would waste enormous quantities of carbon on trees that never need it. So if a tree is going to match its structure to its loads, it must measure those loads while it grows. What would it have to sense, and what would it usefully change?

Sensing first. When a stem bends, its outer tissue on one side is stretched and the other compressed. Plant cells detect that deformation — mechanoperception — and the earliest signal is a rapid influx of calcium into the cytoplasm, within seconds of the touch. That calcium transient is picked up by calmodulin and related proteins and propagates into changes in gene expression. Braam and Davis showed in 1990 that simply touching Arabidopsis, or spraying it with water, switched on a set of genes within minutes; they named them TCH, for touch. Ethylene and jasmonate signalling are also implicated downstream, though the full pathway from bend to wood is not completely mapped, and I would rather say that plainly than pretend the chain is closed.

Now the response. Jaffe coined the term thigmomorphogenesis in 1973 after rubbing bean stems daily and finding they grew shorter and stouter than untouched controls. That is the general pattern: repeated mechanical stimulation suppresses elongation and redirects growth into radial thickening, altered wood anatomy — shorter fibres, more flexible tissue, sometimes called flexure wood — and heavier investment in the root plate and the base of the stem.

Ask yourself why shorter and thicker is the right answer rather than merely a stunted one. The bending stress a wind imposes rises with the height of the crown above the point of attachment, because the wind's force acts through a longer lever. The stiffness that resists it rises very steeply with stem diameter — roughly as the fourth power for a solid cylinder. So a tree that loses a little height and gains a little girth is trading a small amount of light-capture for a very large gain in resistance to the load it has actually been feeling.

And now the sheltered tree becomes explicable without any appeal to character. It was never stimulated, so it never received the signal to shorten and thicken. It spent that carbon on height instead, which in a sheltered site is the correct investment — it wins the competition for light. Its slenderness is not a defect. It is a solution to a different problem, and it fails catastrophically when the problem changes.

Does that not put the phrase "adaptation" under some strain? Nothing here evolved. The two trees carry the same genes. What differs is which developmental programme the environment switched on — adaptation within a single lifetime rather than across generations. Is that the same word doing two jobs, or two different ideas?

Two clarifications matter.

First, this is acclimation, not toughening in the folk sense. There is no reservoir of hardiness being filled. The wind is carrying information, and the tree is using that information to allocate carbon. Remove the information — as a nursery stake does when it holds a young trunk rigid — and the tree builds as though it lived in shelter. Arborists have long observed that firmly staked nursery stock develops thinner, more slender trunks and is prone to failure when the stake comes off; the standard advice, to stake loosely and briefly, follows directly from this mechanism.

Second, there is a cost, and it is not small. A wind-hardened tree is shorter, and in a closed forest shorter often means shaded and eventually dead. So the response is not simply "better"; it is a bet placed on the conditions the tree has already sampled. When conditions change abruptly — a shelterbelt felled, a neighbouring stand clear-cut — the trees left standing are the ones that grew to the old regime, and the wave of windthrow along a fresh forest edge is exactly that bet being lost.

• How foresters use thinning intensity and edge design to avoid creating stands of unhardened trees.
• Whether the same signalling explains the compact, ground-hugging form of alpine and coastal krummholz.
• How mechanical stimulation interacts with drought and light signals, which sometimes pull growth allocation the other way.

Every term the collection defines is gathered in the glossary.