Identifying Trees With Pinnately Compound Leaves

The term "pinnate" comes from the Latin word pinnātus, meaning feathered or winged. Combine that with the word "compound," made up of two or more parts, and you get pinnately compound leaves, essentially leaves divided into smaller leaflets above the stem.

Pinnately compound leaves vary in lengths and attach to either side of twig-connection petioles. The axis they form is called a rachis. The leaves are often are joined by smaller leaflets on the petioles. Some pinnately compound leaves can branch again and will develop a second set of pinnately compound leaflets. The botanical term for leaves with these secondary leaf branches is called a bipinnately compound leaf.

There are many trees and shrubs with pinnately compound leaves in North America, including hickory, walnut, pecan, ash, boxelder, black locust, and honey locust (which is bipinnate). The most common shrubs and smaller trees are mountain ash, Kentucky yellowwood, sumac, and the invasive exotic mimosa, alanthus, and chinaberry.

Degrees of Compound Leaves

There are many degrees of "compoundness" in more complicated leaves—you can even have tripinnately compound. Leaf compoundness may cause some tree leaves to grow extra shoot systems, easily confusing a novice leaf identifier.

It is always possible to distinguish a compound leaf attachment to the stem from a leaflet attachment to the petiole and rachis. A leaf attachment to the stem features axillary buds found in the angle between a true branch stem and the leaf petiole. This angle between the stem and petiole is called an axil. You will not find axillary buds present in the axils of leaflet attachments to the leaf rachis.

It's important to note the axils of a tree's leaves because they define which level of compound the leaves are experiencing, from simple pinnately compound leaves to the multitiered tripinnately compound structure.

Compound leaves also come in other varieties, including paripinnate, imparipinnate, palmate, biternate, and pedate, each of which is defined by how the leaves and leaflets attach to the petiole and rachis (and/or secondary rachis).

Simple Pinnation

Trees having a leaf that is pinnately compound will have leaflets growing from several places along the stalk or rachis. There can be as many as 21 leaflets and as few as three.

In most cases, there will be an oddly pinnate leaf in which there is a single terminal leaflet followed by a series of opposing leaflets. This can also be referred to as imparipinnate as the number of pinnate leaflets on each petiole is uneven and therefore not paired. Leaflets at the top of these are typically larger than those closer to the base of the petiole

Hickory, ash, walnut, pecan, and black locust are all pinnate-leafed trees that can be found in North America.

Bipinnation and Tripinnation

Trees having a leaf where at least some of the leaves are doubly compound and the leaflets have mostly smooth margins are known as bipinnate. The leaflets on these petioles appear on the rachis then are further subdivided along secondary rachises.

Another botanical word for bipinnate is pinnule, which is the word used to describe leaflets that are further pinnately divided. This term is used to describe any leaflet that grows in such a way, but it is most commonly associated with ferns.

The most common North American tree species with bipinnate leaves is a honey locust, though Bailey Acacias, silk trees, flamegolds, chinaberries, and Jerusalem thorns are also examples.

Bipinnate leaflets can be easily confused with tripinnate leaflets, so it's important for those identifying trees by their leaf configuration to note whether the leaflet attaches to the first rachis or a secondary rachis—if it's secondary, the leaf is tripinnate.