Opposite or Alternate? The Question That Halves the Answer

Most people try to identify a tree by looking at the shape of the leaf. That is the hard way. Experienced people look at where the leaves attach, because that one character eliminates most of the possibilities before shape enters into it.

How to read it

Look at a healthy twig, partway along rather than right at the tip.

Opposite: leaves grow in matched pairs, directly across from each other at the same point. Where one leaf joins, another joins on the exact opposite side.

Alternate: leaves are staggered, one at a time, spiralling along the twig. No two are directly across from each other.

Whorled: three or more leaves radiating from the same point. Rare in trees.

Two cautions:

• Do not judge from the tip of a shoot. Growth crowds there and alternate leaves can look almost paired.
• Check several twigs. Damage and browsing can distort a single one.

The mnemonic

In North America, the opposite-leaved trees are remembered as MAD Cap Horse:

• M — Maple
• A — Ash
• D — Dogwood
• Cap — Caprifoliaceae, the honeysuckle family, which includes viburnums and elderberries
• Horse — Horse chestnut and buckeyes

That is very nearly all of them. Add Paulownia and Catalpa and you have the practical list for most of the continent.

Everything else is alternate. Oaks, birches, beeches, hickories, walnuts, cherries, apples, poplars, willows, sycamores, elms — all alternate.

Why this is so powerful

There are hundreds of native tree species in North America. Fewer than a dozen groups are opposite.

So if you establish opposite arrangement, you are looking at maybe five possibilities before you have examined the leaf. Combine it with one more character and you are usually done:

• Opposite + simple + lobed → maple
• Opposite + compound → ash, or boxelder, which is a maple with compound leaves
• Opposite + simple + smooth edges + curving veins → dogwood
• Opposite + palmately compound → horse chestnut or buckeye

Four combinations, four answers. That is the whole reason a key starts here.

Simple or compound: find the bud

The second question trips people up more than the first, because a compound leaf looks like several leaves on a twig.

The test is the bud. A bud sits in the angle where a whole leaf joins the twig. There is never a bud at the base of an individual leaflet.

So: follow the stalk back. If you reach a bud, everything from there outward was one leaf. If you find no bud where the stalk meets what you took for a twig, you are still holding part of a leaf.

A second check: leaflets of a compound leaf all lie in one flat plane, like a feather. Separate leaves on a twig spiral around it in three dimensions.

This is why ash gets called a walnut and vice versa — both are compound, and people compare the leaflets instead of checking arrangement. Ash is opposite. Walnut is alternate. That settles it before you count anything.

In winter

Everything above still works, because buds follow the same arrangement as leaves.

Opposite buds mean an opposite tree. So does an opposite pattern of leaf scars — the marks left on the twig where last year's leaves detached.

You can therefore run the identification key in midwinter on nothing but a cut twig. That means the single most useful character in tree identification is available all year, on a bare twig, in February. Which is a large part of why bark-based winter identification is less mysterious than it appears: you are still narrowing to MAD Cap Horse first.