TEACH!: Unit 6, Lesson 1.

Forests, Fields and Meadows: Upland Communities:

Choice A or Choice B:
Using a Dichotomous Key to Identify Trees


The participants will understand how to use a dichotomous key to identify common trees.


  • A copy of Common Trees of Pennsylvania for each participant (see resources)
  • Sample leaf types
  • Trees or botanical tree specimens to identify
  • Snack foods for introductory activity (gumballs, triangular tortilla chips, potato chips, cheese puffs, and chocolate chip cookies)
  • Overhead, poster or chalkboard drawing of the snack food key


Have you ever found a leaf and wondered about the name of the tree to which it belongs? You could ask someone (if they’re around and know the answer), you could randomly flip through a tree field guide (frustrating and time consuming), or you could try your hand at using a dichotomous key to identify the specimen yourself.

A dichotomous key is a tool that allows the user to determine the identity of items in the natural world, such as trees, wildflowers, reptiles, rocks and fish. “Dichotomous” means “divided into two parts,” and a dichotomous key consists of a series of couplets (two choices) describing mutually exclusive characteristics of biological organisms. The user compares the characteristics of the specimen at hand against the distinction in the couplet and decides which statement applies most to the specimen. The key then directs the user to another couplet with additional distinctions, and so on until the series of choices leads the user to the correct name of the specimen. Dichotomous keys usually begin with general characteristics and lead to progressively specific characteristics. Some keys employ numbering or coding systems.

Using a dichotomous key requires familiarity with vocabulary used by scientists to describe a species. When using a tree key, understanding a few botanical terms will greatly aid in making correct choices. For example, do you know the difference between compound and simple leaves? Can you distinguish between opposite and alternate branching patterns? Do you know where to find the leafstalk? Use the illustrations in the

Common Trees of Pennsylvania guide to help you recognize a few key visual characteristics of trees and leaves.

Tips for using a key:

  • Always consider all of the choices. Although the first on e may seem to fit your sample, the second one may be even better.
  • Be sure you understand the meaning of the terms involved. Do not guess. You may need to use additional reference material.
  • When measurements are given, use a scale or ruler. Do not “eyeball” it.
  • Work with multiple samples of your specimen whenever possible. Living things are always somewhat variable, and keys reflect typical presentations of a species’ characteristics. Therefore, avoid basing your conclusion on a single observation.
  • If the choice is not clear, for whatever reason, try pursuing more than one sequence of couplets. After working through a few more descriptors, it may become apparent that one line of choices does not fit your sample at all. If you end up with multiple possible answers, read the narrative descriptions of the choices to help you decide.
  • Arriving at an answer in a key is not the end of the identification process. Be sure to check the narrative description of the organism and a picture or an illustration to see if these agree with the unknown specimen. If not, you made an error at some point in the process or the key isn’t intended to identify the species you are observing.


To set up this lesson, the instructor should have available botanical samples as well as the snack foods where everyone can see them. This lesson assumes there is a suitable outdoor location with a variety of trees to identify. If such a location is not available, the instructor may bring indoors a variety of botanical tree specimens (leaves, branches, etc.) for the class to identify.

1. Begin by asking participants how many trees they can correctly identify. How can you distinguish one type of tree from another? If there’s a tree you don’t recognize, how can you learn what type it is? Introduce the concept of dichotomous keys and explain their uses in multiple settings.

2. Walk the participants through the Snack Food Key. After identifying all the snack foods, discuss the following:

  • What terminology is necessary to use this key properly?
  • Which snack food characteristics were difficult to classify?
  • How can you use this sample to prepare to use more complex keys (more types of snacks, snacks very similar to each other)?


1a Snack items are flat go to 2

1b Snack items are not flat go to 4

2a Snack items are round go to 3

2b Snack items are triangular tortilla chips

3a Snack item is thinner than ¼ inch potato chip

3b Snack item is ¼ inch or thicker cookie

4a Snack item is tubular cheese puff

4b Snack item is spherical gumball

  • Once all participants understand how the key works, be sure to eat the snack foods!

3. Instruct participants to look at pages iv and v of Common Trees of Pennsylvania and study the illustration of various leaf types. Discuss the terminology used in the booklet: simple, compound, doubly compound, bundled, clustered, opposite, alternate, smooth, toothed, doubly toothed, lobed, leaflet, leafstalk, leaf base, vein, and bud. The instructor should show students real life examples of as many leaf types as possible to illustrate these terms.

4. Go outside and locate a sample tree to identify. Walk the students through the tree key found in Common Trees of Pennsylvania. Review the tips listed above for successfully using tree keys.

5. Instruct participants to practice identifying trees using the dichotomous key. This may be done alone or in small groups.

6. After participants have used the dichotomous key, wrap up the lesson with the following discussion points.

  • What strategies helped you to be successful using the dichotomous key?
  • What skills, knowledge and qualities are helpful when using a dichotomous key?
  • In what ways is using a dichotomous key easier and harder than other methods of tree identification (asking someone, flipping through a field guide, etc.)?
  • Which tree species are easiest to recognize? Which are hardest to tell apart?

7. Advanced option: To help participants really understand how dichotomous keys work, encourage them to construct their own keys. They might make a key of breakfast cereals, shoes, beans, cartoon characters or even their classmates.


There are many guides to the trees of Pennsylvania. An easy one to use in learning about dichotomous keys is Trees of Pennsylvania: A Complete Reference Guide by Timothy A. Block and Anna Anisko.

Another good guide that covers species in the larger northeast region is the Peterson Field Guide. A Field Guide to Trees and Shrubs: Northeastern and north-central United States and southeastern and south-central Canada by George A Petrides and Roger Tory Peterson.

The Pennsylvania Department of Conservation and Natural Resources has a good guide to Pennsylvania’s trees, too — Common Trees of Pennsylvania. It is available online at

Finally, you can learn about tree anatomy from an article in the Minnesota Conservation Volunteer — “Tremendously Marvelous Tree” by Dawn Flinn. It is available online at https://files.dnr.state.mn.us/mcvmagazine/young_naturalists/young-naturalists-studyguides/marveloustrees_studyguide.pdf

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Unit 6, Lesson 2.:  Forest management

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Unit 5, Lesson 2: The Delaware River game

Updated: May 2018