Here is a fact that took me a while to believe: in the strict language of biology, there is no such thing as a tree. Not in the sense that trees are imaginary, obviously, but in the sense that “tree” is not a group of related organisms. It is a shape.
I am a writer, not a botanist, so I have leaned on the people who study this for a living. But the claim is not fringe, and once it clicks it changes how you look at a forest.
A tree is a strategy, not a family
Living things are usually grouped by ancestry. Cats form a family because they descend from a common cat-like ancestor. Trees do not work that way. A pine, an oak, a palm and a tree fern are all trees, and they are only very distantly related to one another.
What they share is a solution to a problem. If you are a plant competing for light, growing tall and building a stiff woody trunk to hold yourself up is a good way to win. That solution has been discovered independently, again and again, by lineages that were not trees to begin with. Biologists call this convergent evolution, the same answer arrived at from different starting points, the way eyes or wings evolved more than once.
The distances involved are enormous. Tree ferns, the ones that give a rainforest its prehistoric look, are not even seed plants. Their lineage was growing tall and trunked, in its own fibrous way rather than with true wood, hundreds of millions of years before the first oak or pine existed. A tree fern and an oak are both trees in the everyday sense, and about as distantly related as two plants can be.
So “tree” belongs in the same category as “swimmer” or “digger.” It describes what something does, not what it is descended from.
Why an oak is closer to a strawberry than to a pine
This is the example that makes the point land for me. An oak and a pine look like obvious cousins. Both are big, woody, long-lived trees.
But an oak is a flowering plant, an angiosperm, and so is a strawberry. A pine is a gymnosperm, from a lineage that split off before flowering plants existed, well over 300 million years ago. On the family tree of plants, the oak and the strawberry sit on the same major branch, while the pine sits on a far older one.
The oak, in other words, is more closely related to the little herb in your garden than to the conifer it resembles. Their shared treeness is a coincidence of strategy, not a sign of kinship.
The clearest evidence: trees keep getting reinvented
If the tree form really is just a repeatable trick, you would expect to catch plants in the act of evolving it. You can.
The most striking case is what botanists call insular woodiness, the tendency of ordinary herbaceous plants to evolve into woody shrubs and small trees after they colonize islands. A 2022 study in the journal PNAS, led by Frederic Lens and colleagues, counted more than a thousand island plant species that had made this shift, arising from at least 175 separate evolutionary transitions across 31 archipelagos.
The examples are vivid once you know to look. The giant Hawaiian silverswords, the tree-sized lettuces of the Canary Islands, and the woody viper’s-buglosses of the same region all descend from soft, herbaceous ancestors that became woody in place. Each is a small, recent, independent invention of something tree-like.
Part of what makes this possible is that woodiness appears to be an ancient capacity in flowering plants. Many herbaceous lineages, the soft green plants we think of as the opposite of trees, descend from woody ancestors and simply switched the trait off. Evolving back into a tree is often less a leap into something new than a reactivation of machinery their ancestors already had.
Woodiness turns out to be relatively easy for a plant to switch back on, which is exactly why the tree form is scattered so widely and so raggedly across the plant kingdom.
Why this is more than a technicality
It would be fair to ask whether any of this matters, since “tree” is a perfectly good everyday word and no one is proposing we stop using it.
What changes is the picture in your head. A forest is not a single family of organisms doing the same ancient thing. It is a gathering of very different lineages that independently hit on the same way of making a living, a convergence rather than a dynasty. The oaks and the pines in a mixed wood are less like relatives at a reunion and more like unrelated companies that happened to build similar towers.
The scale of that convergence is hard to picture. A 2015 study in Nature, led by Thomas Crowther, put the number of trees on Earth at roughly three trillion, many times higher than earlier guesses. The same work estimated that the total has fallen by nearly half since humans began farming. That is a staggering amount of independently evolved tree, and a staggering amount of it already gone.
For a fuller version of the argument, there is a widely shared essay, “There’s no such thing as a tree (phylogenetically),” that walks through the genetics in more detail than I can here.
None of this makes a tree less real when you stand under one. It just means the thing you are standing under is a shape that life keeps reaching for, not a branch of the family tree. The next time you see an oak beside a pine, it is worth remembering that they are far stranger neighbors than they look.
