Did you know that the last week of September is National Fall Foliage week? As the season changes from late-summer to early autumn, we start to see the impacts of less light and cooler temperatures on the trees around us. Many are turning from green to red or yellow, and they are stunning!

As we start to observe and swoon over the fall foliage, let’s take a look at how to identify trees by their foliage so that we can start observing patterns in the coloration.

Ways to Identify Trees

There are so many ways to identify trees. You can examine the bark color and texture, overall shape and growth habit, or how the buds form along the branches. As the leaves fall, you can even examine the scar, or the place where the leaf originally attached to the branch! These scars will vary tree by tree.

However, one of the easiest ways to identify trees during the growing season and into autumn is to examine the leaves! Here, we’ll focus on Simple vs. Compound leaves, as well as leaf arrangement along the stem. Simple means that there is only one leaf that attaches to a petiole connecting the leaf to the branch. Think maples and oaks. Compound means that there are multiple leaflets that attach to the petiole that connects the entire leaf to the branch. Think Black Walnuts and Sumacs.

Simple Leaves

Within the category of simple leaves comes many subsets. As you examine a tree and answer the question of “Does it have simple or compound leaves,” you can start to see that each tree may have individual leaf shapes. Are the leaves long and skinny or are they egg-shaped?

Another observation to make as you examine the leaf is in regards to the edges, or margins. Do the leaves have teeth, like the birch, or entire (smooth) margins, like the Redbud? Are they finely serrated like the willow, or extremely lobed and dissected, like a White Oak or Japanese Maple?

By answering these questions, you will be able to start narrowing down the tree’s identity in your field guide.

Compound Leaves

Just as with the simple leaves, compound leaves also can have some subsets. In the diagram above, you can see that compound leaves can be Palmate or Pinnate.

Palmate leaves are ones whose individual leaflets radiate from one central point, like the fingers on our hands radiating out from our palm. The diagram above shows a blackberry leaf, however we are focusing on trees, not on berry canes (although blackberry leaves do turn a stunning deep red in the fall). A Horse Chestnut would be an example of a Palmately Compound tree.

Pinnate means that there are many leaflets that run along a single stem. The diagram above shows a Black Walnut leaf. In compound leaves, each of the individual blades is a leaflet. All of those leaflets that attach to the single stem is what comprises a Leaf. In the Black Walnut, sumac, ash, or even in a fern, that one long structure is a single leaf!

Another way to determine tree identity via the leaf is to examine the leaf arrangement along the stem or branch.

Leaf Arrangement

Answer some of these questions. Does your leaf (remember, one leaf can be made of many individual leaflets!) attach to the main stem or branch in an alternating or opposing fashion? There are only a few trees that have oppositely arranged leaves, which can be helpful in narrowing down your identification.

Remember this acronym to narrow down your id: MAD Horse. M (Maples), A (Ash), D (Dogwoods), Horse (Horse Chestnuts). If you spot a tree with opposite leaf arrangement, where the leaves are arranged directly across from one another, and not even a smidgen out of place, then you either have a Maple, and Ash, a Dogwood, or a Horse Chestnut. Some people find ash trees and other pinnately compound leaves difficult to differentiate between, but with this acronym, you can easily narrow it down! If the leaves are alternate, then it can’t be an ash, and therefore must be a sumac, walnut, or other tree.

From there, did you know that the fall foliage color can also help you identify leaves?

Fall Foliage Color & Tree ID

Trees don’t just willy-nilly decide to present as red, orange, yellow, or brown in the fall. While they all have various levels of the anthocyanin, xanthophyll, and carotenoid pigment levels, each trees will utilize those pigments in different ways and will express their colors uniquely.

For example, Sassafras trees turn a peachy-red or orange color, while Red Oaks turn a deep burgundy. Red maples may be red, yet they are usually a more vibrant, brighter red than a Red Oak. Beech trees and Aspen turn yellow, while Pin and Black oaks might just turn a dingy yellowish-brown.

Field guides may include images of the fall foliage coloration for this exact purpose! If you have an orangey-peach tree, yet your field guide is saying that the tree should be a bright sunshine yellow, you may want to reexamine the tree’s leaf structures, or use another identifying feature, such as bark or overall tree shape to get an accurate identity.

To complicate things a little, do keep in mind that environmental factors can impact fall foliage color. If there was a drought, or the late-summer storms were severe, the leaves may be more dingy and less vibrant. Plenty of rain (but not too much), a dip into cooler temperatures (without a freeze), and a happy summer make for vibrant fall colors.

What trees do you observe?

Upload your observations to iNaturalist or share them with me on social media using #theartofecology. There is such a wealth of biodiversity in the tree world and they all have their own stories to tell. Celebrate Fall Foliage Week in a whole new way this year!