Three characters, in order
Leaf first, because it is the largest and the one you can reach. Bark second, because it is the one that stays when the leaves are gone. Fruit last, because it is the most reliable of the three — a tree grown in deep shade, on poor ground, or clipped into a hedge will change its leaf and its habit, but it will still make the fruit its species makes.
Three-step key
Three-step key · Take leaf, then bark, then fruit
Results appear once the third character is chosen.
How the narrowing works
Every option stays live only while something in the library still carries it. When you pick a character, the other rungs grey out the choices that have just become impossible — so the list under the key can never send you to a combination that no published species satisfies. The count above the list is the honest size of the answer set, and it updates with each choice. It counts what is in print today, so it grows as the library does.
What the key will not do
It covers the species in this library, which is a working collection and not a national flora. A tree that is not in it will not match, however carefully you read it. It also assumes you are looking at a mature tree in the open: coppice regrowth, hedging and nursery stock all produce juvenile foliage that breaks the rules — a clipped beech hedge keeps its leaves through winter, and a young English oak can hold leaves that look nothing like the ones on the plate.
Reading a leaf properly
Take the leaf from the outside of the crown, in good light, and take a whole one — a leaf torn off at the base loses the petiole, and the petiole is often the whole answer. For a compound leaf, take the whole leaf, rachis and all, rather than a single leaflet: an ash leaflet alone looks like a small simple leaf. Then put it flat against something white. Half of all disagreements about a tree are settled by looking at the margin against a plain background instead of against the sky.