Some leaves are unique and easy to identify. For other species, it takes careful observation to detect the differences; luckily, studying trees is an enjoyable challenge for most nature-lovers.
When looking up a tree in a field guide, you will need to note several immediate criteria:
- Does the tree have needles or broad leaves? In this article, we will focus on the broad leaves of deciduous trees. See Pines, Conifers, Evergreens for information on needle-bearing trees.
- Are the leaves simple (a single leaflet per stem) or compound (having multiple leaflets on each leaf stem)? The leaflets may have their own stems. The leaf stem is the part that connects to the twig and falls off when the tree sheds its leaves.
- Are the leaves entire (smooth-edged), toothed (with jagged edges), or lobed (with bulges)?
- Do the leaves grow in an opposite pattern (growing in pairs on opposite sides of the twig) or an alternate pattern (occurring first on one side of the twig, the next one down on the other side of the twig, and so on, alternating sides)? More rare are whorled leaves, with three or more leaves arranged around the twig at the same point.
Most field guides to trees are organized around these criteria, which will lead the student to a particular section of the book. From there, it is necessary to look further at the leaves for more subtle features, for example:
- For toothed leaves: size and number of teeth; whether teeth are blunt or sharp
- For lobed leaves: size and number of lobes; whether lobes are rounded or pointed
- For compound leaves: number and relative size of leaflets; whether leaflet arrangement is pinnate (leaflets situated in rows on opposite sides of the leaf stem), as in hickory, sumac, ash, or palmate (leaflets all arising from the end of the leaf stem, like fingers on a hand), as in horse chestnut
- For simple leaves: whether the sides of the leaf base meet at the stem or one side hits the stem lower than the other, as in elm and witch hazel
- Texture of leaf: rough, smooth, hairy
- Odor of leaf: having a distinct odor when crushed, as in sassafras, black walnut, ailanthus
To securely identify a tree, it may be necessary to look at other features besides the leaves. Some trees have distinctive bark (flaky, rough, smooth, striped, mottled, powdery), fragrant inner bark, twigs with chambered pith, tendency to shrubby growth (having multiple slender trunks), thorns, limited height, or a host of other characteristics. Buds and leaf scars may be helpful for identification, especially when there are no leaves on the trees.
Careful observation yields not only an ability to distinguish different species but also an intimacy with trees that can be deeply satisfying.
See the article Trees with Oval Leaves for more information on identifying simple leaves.
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