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:
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:
SNACK FOOD KEY
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
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.
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
Unit 6, Lesson 2.: Forest management
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Unit 5, Lesson 2: The Delaware River game