Tree Identification: How To Use a Dichotomous Key

Dichotomous keys are commonly used in science to identify items in nature, including trees. Users progress through sets of two-choice questions, leading to the identity of the sample.

In a dichotomous key, questions are posed as paired phrases or questions, in which only one can be correct. The correct phrase then leads to the next question or phrase, until finally, after a number of steps, it leads to identification of the item being keyed out.

For tree identification by dichotomous key, users study the features of leaves and leaf arrangement, and move though the paired phrases until reaching identification of the tree the leaf came from.

This video will illustrate the layout of a dichotomous key, how to use it, and some of the leaf features used in dichotomous keys for tree identification.

Leaves are very characteristic of individual tree species, and are commonly used to identify trees. Leaf shape, arrangement, margins, and multiple other characteristics can be taken into account when identifying a tree sample.

Broadleaf trees are common in the United States, and are characterized by leaves with wide blades that expose a large area for photosynthesis. Most broadleaves are deciduous, dropping their leaves in autumn.

The second major tree type in the United States is the evergreen. These have needle or scale-like leaves, which generally stay on the trees year round. Needle-like leaves have little surface area for photosynthesis, and a thick waxy coating to prevent water loss, making needled evergreens well suited to areas where water conservation is important for survival.

Leaf shape is tied to the evolutionary history of a tree species, and depending on the ecosystem demands, the leaf's needs to maximize sunlight capture while minimizing heat absorption and water loss. Overall leaf shape is a trait often used to categorize broadleaves in dichotomous keys.

Leaves may be categorized as heart shaped, triangular, lance, ovate, or obovate. Other shaped leaves occur, but these are most common. Leaf arrangement on the twig is another characteristic used in key identification of trees. Opposite leaves are those occurring in pairs at the nodes, usually displaying adjacent tiers at right angles to minimize overlap. Alternate leaves, the most commonly seen arrangement, are staggered, not directly across from one another, and often arranged in a spiral along the twig. Whorled arrangements have three or more leaves occurring at a single node.

The edge of the leaf, or leaf margin, may also provide features to aid identification. They may be smooth, have projections, teeth, or undulations. A wavy or bumpy edge is called rounded or sinuate. Again, this may be related to environmental conditions. In colder climates, native trees tend to have larger and more teeth.

With broadleaf trees, leaves may be categorized as simple or compound. Simple leaves have one leaflet, a petiole or "stalk", and a bud at the base of the petiole. Compound leaves have two or more leaflets and a bud at the base of the petiole. Further, once-pinnately compound leaves have one main petiole and leaflets arranged on each side of the petiole. Twice-pinnately compound leaves have one main petiole, and secondary petioles arranged on either side of the main petiole. This can be used to check the difference between a leaf and a leaflet; if there is no bud where it attaches to the stem, it is a leaflet, not a leaf.

Now that we are familiar with some of the traits of common trees and leaves, and some categories used for tree identification, let us walk through the use of a key.

Before attempting to identify leaf samples, it is necessary to first become familiar with the traits and the categories examined within the key itself.

The first question in the key asks whether the leaf specimen is needlelike or scalelike, or if it is broadleaf. The first two are categorized as evergreen, and the second as deciduous. If the answer is "needlelike or scalelike" proceed to row 2 of the key. If it is "broadleaf", proceed to row 3.

Row 2 relates to needlelike or scalelike leaves, and asks which of these best describes the sample. Needlelike leaves have very little surface area, and have a thick, waxy coating to prevent excessive water loss. Scalelike leaves also have narrow surface area, but are comprised of small, individual scales. If the leaves are scalelike, the key states that the specimen is a Red Cedar. If the leaves are needlelike, the key states that the specimen is a Scotch Pine.

Question 3 asks if the leaf is simple or compound. Simple leaves are those with one leaflet per petiole or stem, and a bud at the base of the petiole. Compound leaves are those with two or more leaflets, a petiole, and bud at the base. If the leaf is simple, move to row 4, and if compound, to row 5.

The fourth question asks if the leaf is lobed or unlobed. Lobed leaves are those with projections that give the leaf shape. Unlobed leaves have a consistent leaf edge. If the leaf is lobed, the key instructs to move to row 6. For unlobed leaves, row 7 should be consulted next.

Row 5 follows on from question 3 and asks if the leaf is once or twice compound. Once-compound leaves have two or more leaflets arranged pinnately on each side of the petiole. A twice-compound leaf has one main petiole, and then secondary petioles arranged on each side of the main petiole. This is a terminal question, with once-compound leaves listed as belonging to Green Ash, and twice-compound as Honeylocust.

Question 6 deals with lobed broadleaves. Is the leaf pinnately or palmately lobed? Pinnate lobes are those where the lobes all attach to a central axis or vein. Conversely, in palmate leaves, the lobes all radiate from a single point. For pinnately lobed leaves, the leaves belong to Bur Oak. For palmately lobed leaves, the key moves on to Row 8.

In row 7, the key asks if the leaf samples have teeth on the margin. Teeth are classed as continuous and serrate, versus a smooth leaf, which has no serrate or pointed projections on the margin. Toothed samples lead on to Question 9, and untoothed are classified as Redbud leaves.

Question 8 asks if the leaf samples have 3-5 deep lobes with opposite leaf arrangement, or 3-5 shallow lobes with alternate leaf arrangement. Deep lobes are those that extend far into the leaf surface, and opposite leaf arrangement is seen when leaves occur in pairs at the nodes. Shallow lobes are those that extend less into the leaf surface, and alternate leaf arrangements are those in which leaves are staggered, or not directly across from one another. Deep lobes and opposite leaf arrangement leads to Silver Maple, whereas shallow lobes and alternate arrangement leads to Sycamore.

Finally, question 9 asks if the leaf margin has double teeth, elliptical shape, and is asymmetrical at the base, or if instead it has a single toothed margin. If the former is true, the sample is identified as American Elm, and if the leaf has a single toothed margin, it is from a Cottonwood tree.

Now, use the leaves pictured along with the dichotomous key to practice identification.

After examination of the key and the characteristics described, field identification of trees can be carried out. First, select a tree to be identified. Collect one representative leaf sample from the tree, and affix it to herbarium paper using glue.

Next, note whether the leaves have an alternate or opposite arrangement on the stems, and record this on the herbarium sheet. Measure the diameter at breast height of the tree in inches by taking the circumference of the tree at 4.5 ft above the existing grade, and calculate the diameter. Record the circumference and diameter.

Note what type of land use is nearest to the tree: residential, small commercial, industrial, park or vacant land. Using the dichotomous key, identify the leaf samples and record the tree species on the herbarium sheet.

Tree identification has many practical applications, and dichotomous keys are useful and practical tools for quick identification.

Tree identification is an important first step in understanding the benefits specific trees or tree species provide in a community environment. Using tree identification data, and the National Tree Benefits Calculator, scientists and urban planners can use tree data to inform decisions about building, infrastructure, or planting strategies to maximize benefits to health and the environment, and decrease energy consumption.

Dichotomous keys are commonly used to identify many different types of organisms. For example, they can be used to identify anything from species of venomous snake, to insect pests of citrus trees, or types of aquatic plant. This technique can allow users unfamiliar with a specimen to key out an identify subjects simply in the field or laboratory setting.

The ability to identify trees or their pests by key can be extremely useful in pest or disease control. For example, the Asian Longhorned Beetle is becoming an increasingly common pest in US woodlands. An insect identification key can be used to identify and distinguish these from other native longhorn beetles, and also identify at-risk tree species in woodlands where this pest has been found. In turn, this can help to curb the spread of this highly invasive pest.

You've just watched JoVE's introduction to identifying trees using a dichotomous key. You should now understand how dichotomous keys work, and how to apply a dichotomous key to tree identification. Thanks for watching!