Academic journal article Science and Children

CSI for Trees: Dendrochronology Gets Upper Elementary Students Thinking like Scientists

Academic journal article Science and Children

CSI for Trees: Dendrochronology Gets Upper Elementary Students Thinking like Scientists

Article excerpt

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The circles and patterns in a tree's stem tell a story, but that story can be a mystery. Interpreting the story of tree rings provides a way to heighten the natural curiosity of students and help them gain insight into the interaction of elements in the environment. It also represents a wonderful opportunity to incorporate nature of science.

As a botanist with a special interest in dendrochronology (tree-ring science) and a professor helping preservice teachers understand and teach nature of science, we found we could combine our interests and use tree rings as a vehicle to effectively present both concepts. In this activity, appropriate for grades 3-6, students make connections with the work of a scientist as they solve a minimystery using tree-ring evidence.

Making "Cookies"

Having a real wood sample for students to observe is preferable, but it is not necessary for this activity. We have made scanned images of tree slices, known as cookies, available online (see NSTA Connection). Cookies can also be obtained from science suppliers (see Internet Resources). We have found that photocopying or scanning cookies so that each student has a cookie worksheet works best, and black-and-white photocopies also work well. For those with a do-it-yourself streak, we've posted instructions for making your own cookies available online (see NSTA Connection).

Providing cookies from different types of trees and from the same type of trees growing in different environments can greatly add to the exercise. For example, students can compare the growth rate and wood anatomy between angiosperms and gymnosperms. Comparing the same species of tree growing in a yard versus a mature forest will also allow students to see how factors such as resource competition greatly influence growth rate and overall size of a tree. Open-grown trees from yards often grow faster and have larger annual rings than forest-grown trees.

Engaging Students

Initially, students are asked to make observations about their tree cookie using their senses (with the exception of taste). Magnifying lenses are optional. Students should then be given the opportunity to share their observations with the class and generate a list of questions they have about their tree cookie. Students often ask why the tree rings vary in size and why the earliest-formed rings are often much larger than the later-formed rings. Students also like to share previous experiences, such as how they counted rings on a stump or memorable trees they have encountered (usually large trees). Simple questions about the age of the tree (how many rings), the date of the largest and smallest rings, and the size of the ring in which each student was born and went to school can be asked and discussed. By looking at a cookie, a diary of the tree begins to unfold, and the mystery begins.

Explaining Tree Growth

Next, share information about tree growth and tree rings. Each tree ring represents a year in a tree's life. An individual tree ring is made up of a light and dark band (in many trees). The light and dark regions in an individual tree ring represent the yearly earlywood and latewood growth, respectively (Figure 1). Earlywood, formed in the spring, has visibly larger cells with thin cell walls. The dark section of the ring is the latewood formed in the late spring and summer. These cells are much smaller and harder to see. The size of the cells correlates with the availability of resources. For example, abundant moisture in the spring allows for rapid cell growth, development, and consequently, larger cells. More stressful summer conditions translate to smaller, thicker cells produced by a slower-growing stem. The cycle of yearly cambial growth forms a new layer or ring each year. The cambium of the tree consists of specialized cells that produce xylem, or wood, to the inside of the stem and phloem to the outside of the stem. …

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