Source: Laboratories of Margaret Workman and Kimberly Frye - Depaul University
A dichotomous key is a tool that identifies items in nature, such as leaves. This method is based on the idea of choosing between two characteristics. The word dichotomous comes from two Greek words that mean “to divide into two parts.” In a dichotomous key for leaf identification, each pair of phrases describes different features of the leaf. Only one of the phrases correctly applies to the leaf being keyed out. The correct phrase leads to the next pair of phrases, or states the name of the tree from which the leaf came. Using a field guide to trees and the iTree National Tree Benefits Calculator helps to identify trees in a field investigation, which shows the significance of trees in terms of their environmental benefits, such as storm water management, increasing property value, energy efficiency, air quality, and carbon sequestration.
Examining leaves is one of the most common ways to identify trees. Leaves are very characteristic of a particular tree species. There are many clues to look for on a leaf to help identify the tree from which it came. These include leaf shape, leaf arrangement, and leaf margins.
Broadleaf trees are very common in the United States (Figure 1). These trees have leaves with wide blades exposing a large surface area for photosynthesis (e.g. oaks and maples). Mostly, these trees are deciduous and drop their leaves in autumn.
The other type of tree is an evergreen tree. These have needlelike or scalelike leaves. Trees like pines and spruces have needlelike leaves, and trees like junipers and cedars have scalelike leaves. Generally, these leaves stay on the tree for more than a year.
Needlelike leaves have very little surface area; therefore, they are not able to capture much sunlight for photosynthesis. Needlelike leaves also have a thick coating to prevent excessive water loss. Trees with needlelike leaves are well suited to sites where water conservation is very important for survival. Because these needles last several years on a tree while broadleaves only live for one growing season, trees with needles have an advantage over broadleaf trees, in that the metabolic cost of leaf production can be recovered with photosynthesis over several growing seasons.
The shape of a tree’s leaves form over the course of a tree species’ evolutionary history. The shape gives the tree its best chance of survival based on the environmental factors in the ecosystem. A leaf’s task is to capture sunlight for photosynthesis, producing food for the tree. In this process, the leaf also receives heat. The shape of the leaf has therefore developed over time to balance these needs: maximizing sunlight but minimizing heat absorption and/or water loss.
Heart shaped leaves look exactly as the name implies – the leaf is in the shape of a heart (Figure 2). Obovate leaves are broadest above the middle and longer than they are wide. Elliptical leaves are broadest in the center and taper near the ends. Ovate leaves are broadest below the middle and longer than they are wide. Like the heart shaped leaves, the triangular leaves look as the name implies – the leaf is in the shape of a triangle. Lance leaves are much longer than they are wide (typically 4x longer), and although generally the same width throughout, they may be slightly wider in the middle.
There are other leaf shapes, depending on the source used. However, the ones mentioned are some very common, simple shapes.
Leaves can be arranged on a twig in one of three ways (Figure 3):
Opposite – leaves occurring in pairs at the nodes.
Alternate – leaves staggered or not directly across from each other.
Whorled – leaves occurring three or more on a single node.
The arrangement of the leaves minimizes the overlap between one leaf and another. This maximizes availability of sunlight and air. Opposite leaves usually have the adjacent tiers cross at right angles to minimize overlap. Alternate leaves are generally distributed in a spiral.
Most trees have alternate arrangement of leaves, making trees with the other two arrangements a limited group. In order to see the leaf arrangement, the leaves must be observed while still on the twig.
The margin of the leaf is the name for the shape of the edge of the leaf (Figure 4). A leaf that is smooth all the way around with no teeth or undulations has a smooth leaf margin. A leaf with a wavy or bumpy edge in the plane of the leaf is called rounded or sinuate. A margin with continuous, sharp teeth on the edge is finely serrated.
Leaf teeth serve as clues in the process of leaf identification of a tree. In environments with sufficient water and nutrients, the percentage of toothed leaves correlates negatively with temperature, i.e. the higher the temperature, the lower the percentage of trees with teethed leaves. Therefore, in cold climates, leaves have larger and more teeth. Paleobiologists often use this in paleoclimate reconstruction.
When looking at a leaf with a broadleaf shape (as opposed to needlelike or scalelike), the next thing to look for is whether it is simple or compound (Figure 5). A simple leaf has one leaflet, a petiole (stalk) and a bud at the base of the petiole. A compound leaf has two or more leaflets and a bud at the base of the petiole. A once pinnately compound leaf has one main petiole and leaflets arranged pinnately on each side of the petiole. A twice pinnately compound leaf has one main petiole and then secondary petioles arranged on each side of the main petiole. The difference between a leaf and a leaflet can be checked where the leaf attaches to the stem. If there is no bud, then it is a leaflet and not a leaf.
Figure 1. Examples of broadleaf, needlelike, and scalelike leaves.
Figure 2. Examples of heart shaped, obovate, elliptical, ovate, triangular, and lance leaves.
Figure 3. Examples of opposite, alternate, and whorled leaf arrangements.
Figure 4. Examples of various leaf margin, including smooth, rounded, finely serrate, and double serrate.
Figure 5. Examples of leaf type, including simple, once compound, and twice compound leaves.
1. Identification of a Set of 10 Unknown Samples
Use the dichotomous key (Table 1) to identify the 10 unknown leaf samples (Figures 6-15).
- Pick a leaf, and starting at number 1 on the key, answer each of the questions.
- Choose the statement that best describes the leaf in question.
- The column on the right lists either the tree species or a number that lists the next set of statements to consider.
- Continue until the key lists the name of the tree from which the leaf sample came, and fill in the blank table supplied (Table 2).
2. Field Investigation
Collect leaf samples from 5 trees, properly identify the trees using a field guide to trees, and record on a data sheet (Table 3).
- Select a tree to be identified.
- Collect one representative leaf sample from the tree.
- Glue it to a herbarium sheet with regular glue.
- Note whether the leaves have an alternate or opposite arrangement on the stems.
- Record this on the herbarium sheet and the data sheet.
- Measure the diameter at breast height (dbh) of the tree in inches.
- This is done by measuring the circumference of the tree at 4½ feet above the existing grade. The diameter of the tree is calculated from the circumference using the formula d = C/Π. Record the circumference and the diameter on the data sheet.
- Note on the data sheet what type of land use is nearest to the tree: single family residential, multi-family residential, small commercial business, industrial/large commercial business, or park/other vacant land.
- Repeat steps 2.1 – 2.7 for 4 additional trees. 5 total leaf samples should be collected.
- Using a field guide to trees of choice, identify the leaf samples. Record the species on the data sheet.
3. National Tree Benefits Calculator
Using this software, the benefits of street-side trees can be calculated. This includes a tree’s annual benefits for storm water management, property value, energy efficiency, and carbon sequestration.
- Open up the iTree for education software tool found at http://www.treebenefits.com/calculator/ created by the USDA Forest Service, which means iTree tools are in the public domain.
- Using the National Tree Benefits Calculator (Figure 16) and the data collected on the trees, calculate the environmental benefit of each tree.
- Record the results.
Figures 6-15. Unknown leaf samples.
Figure 16. National Tree Benefit Calculator.
|Is the leaf needlelike or scalelike?
Is the leaf a broadleaf?
|Is the leaf scalelike?
Is the leaf needlelike?
|Is the leaf simple?
Is the leaf compound?
|Is the leaf lobed?
Is the leaf unlobed?
|Is the leaf once compound?
Is the leaf twice compound?
|Is the leaf pinnately lobed?
Is the leaf palmately lobed?
|Does the leaf have teeth on the margin?
Does the leaf NOT have teeth on the margin?
|Does the leaf have 3 – 5 deep lobes with opposite leaf arrangement?
Does the leaf have 3 – 5 shallow lobes with alternate leaf arrangement?
|Does the leaf margin have double teeth, elliptical shape and asymmetrical at the base?
Does the leaf have a single teeth margin?
Table 1. Tree Identification Dichotomous Key.
Table 2. A blank table to fill out tree species for each unknown leaf sample.
|Tree Sample Number
|Leaf Arrangement (opposite, alternate, or whorled)
|Circumference at 4½ feet above grade (inches)
|Diameter at 4½ feet above grade (inches) *calculated
Table 3. A blank data sheet to fill out leaf arrangement, circumference, dbh, land use, species, and notes.
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!
Table 4 contains the correctly identified leaves for the identification of a set of 10 unknown species.
Results for the field investigation will vary depending on the samples collected. Representative results for trees found in the Chicagoland area (zip code 60031) can be found in Table 5.
The results for using the Tree Benefit Calculator can be found in Table 6. This calculator provides an estimation of the benefits individual street-side trees provide. When the data from the Field Investigation is input, including zip code, species, diameter, and land-use, the environmental and economic benefit provided by each tree can be seen.
Table 4. The unknown leaf samples and their correctly identified tree species.
|Tree Sample Number
|Diameter at 4½ feet above grade (inches)
Table 5. Representative results for trees found in the Chicagoland area.
|Tree Sample Number
|Storm Water Management
Table 6. Tree Benefit Calculator results.
Applications and Summary
Understanding the benefits trees provide for a community is important. Converting this benefit to a monetary value or ecosystem services value allows for a concrete understanding of exactly the role trees play in an ecosystem. Trees are important for health, the economy, and the environment, and once this is realized, a discussion about ways to protect the trees and increase their benefits can begin. As trees age and grow, their benefits increase. This provides a reason to protect mature trees (Figure 17).
This information can be used to determine which trees would be more beneficial to plant in a community. It also can be used by city officials to inform decisions about building infrastructure (e.g. policy about number/types of trees required to be planted on new building construction). Stakeholders can also decide how many/type of trees to plant on their property to help decrease energy bills (e.g. schools, businesses, government offices).
Figure 17. An example of an old, mature tree.