Forecasting the Future
Plant Biology: Leafing Through the Past

Forecasting the Future - Background Explanatory


To demonstrate how scientists decipher clues about past climate from tree rings and leaf stomata.


Background Information

Have you ever noticed the patterns in wood grains? Examine nearby furniture or any other object made of wood. Some surfaces show straight lines. In others, the pattern is swirled. If you have ever seen a piece of wood sliced horizontally from the trunk of a tree, you have probably seen a pattern of concentric rings much like that in the surface of an onion cut horizontally. Each ring is really the cross section of a layer of wood the tree added to its trunk each year. If you look closely, you'll notice that the rings have different widths. Wider rings indicate that the tree grew fast; narrower rings signify a year of slower growth. Why do growth rates vary? Growth depends on changing factors, such as climate. Most trees grow faster during cooler, wetter years than they do when conditions are hot and dry, so scientists can use the relationship between ring width and annual precipitation to recreate climate history over the lifetime of the tree. Since many trees live for a long time - sometimes hundreds of years - the climate record preserved in very old trees extends back over many decades. (However, other factors besides climate cause trees to grow faster or more slowly. Injuries caused by fire or insect pests, or crowding by other trees, can slow growth and lead to narrower rings. Interpretation of the tree ring record must be conducted with care.)

Just as tree trunks preserve a record of past precipitation, tree and other plants' leaves offer clues to the composistion of past atmospheres. Why are scientists interested in the composition of ancient air? There is a link between the amount of carbon dioxide gas (CO2) in the atmosphere and the average global temperature. For reasons scientists don't fully understand, more CO2 was present in the air during times when the climate was warmer. During cooler periods, there was less CO2. How are samples of the atmosphere from a million years ago obtained? Tiny amounts of ancient air are available in bubbles trapped in glacial ice, but scientists have been able to locate and sample only relatively young bubbles dating back approximately 250,000 years. To learn how much CO2 was present before then, clues must be sought elsewhere. Fossils of leaves provide some such clues.

On the underside of leaves are tiny pores called stomata. The stomata are openings through which gases can enter and leave. In times of warm climate, when the atmosphere is rich in CO2, plants need relatively few pores to take in all the CO2 they need, so their leaves develop comparatively few stomata. In times of cold climate, more stomata are needed because less CO2 is readily available.

Given this, scientists can determine from the number of stomata on fossil leaves whether plants lived in warm or cool times. Fossils of leaves as old as 10 million years have been studied to learn about climatic conditions that existed when those leaves grew on living plants.

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Examining Tree Rings

Introduction to Activity

a. Create a model tree trunk using large flour tortilla and peanut butter. Spread peanut butter between two or three tortillas and roll them together jelly roll-fashion. Starting at one end and moving toward the other, cut the tortillas in several ways: once perpendicular to the roll, again at an oblique angle, and again parallel to the roll. What differences are observable? In this instance, how do layers' orientation vary? What differences in measurements and interpretations are suggested? Scientists try not to cut trees down when studying them. Likewise, at what angles would you make tortilla tree-trunk cores to secure the fullest display of information?

b. Collect samples of wood in which the grain is visible. Try to find pieces that were cut on various angles across and along the trunk of a tree. Discuss why you can see rings in some of the pieces, but only a straight grain in others. Again, what differences in interpretation are suggested by the wood grains' angle and orientation?

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a. Divide into 4 groups per trunk slice.

b. Using the ruler as a straight edge, draw a cross on the surface of the wood as though cutting a pie into four equal wedges. The marks should look like four spokes of a wheel extending from the center to the edge. Number the spokes 1, 2, 3, and 4 to correspond to the number of student work groups.

c. Count the rings along spoke numger 1, from the center to the edge. Record this number, which tells the age of the tree in years.

d. Use the ruler to measure the width of the rings starting at the center. Record the width of each ring in millimeters.

e. Graph the ring-width data on graph paper. (This can be done while fellow group members are still measuring.) Label the X-axis "Age in years." Number the X-axis in one-year increments, left to right, up to the number of years you've agreed to count. Label the Y-axis "Ring width in millimeters" and choose an appropriate scale for tick marks. Number will increase in value as you go up the Y-axis. Make sure the range of numbers on the Y-axis will include both the widest and narrowest ring. Once the aces are drawn, plot the data in a single solor. Connect the points with a smooth line.

f. Repeat c, d, and e for each group of students. Each group should measure their own spoke, and plot the data on a shared graph, each in a different color. Be sure the graph includes a key so that it is clear which color represents each group.

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Examining Plant Stomata

Introduction to activity

Producing leaves with more or fewer stomata is one way plants adapt to their changing environment. List other adaptations plants and animals have made to their environment.

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Collect some leaves. Use a magnifying lens to examine leaves' undersides for stomata.

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Do some of the leaves appear to have more closely spaced stomata than others? What other factors would contribute to the number of stomata observed?

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