"To be able to walk under the branches of a tree that you have planted is really to feel you have arrived with your garden."
-- Mirabel Osler
They are one of the world's most efficient pumps, drawing hundreds of gallons of water a day from roots to leaves at speeds of more than 100 miles per hour. Their shade and the water they transpire cools us, especially in asphalt jungles where streets and buildings absorb and release heat. Noise levels ease as trees absorb and deflect sound waves. They clean the air by filtering and trapping fine particles and prevent erosion with topsoil-anchoring roots and leaves that break rainfall.
Through their food-making process, trees make our lives, well, livable. For every ton of new wood a tree grows, it produces 1.07 tons of oxygen and removes 1.47 tons of carbon dioxide, which many scientists believe is the primary culprit in global warming. From coffee to kumquats, trees provide sustenance and medicines for the world's population, along with a host of products that keep us warm, dry, and otherwise in business. Then there are the less tangible emotional responses that trees evoke as they create sanctuaries in schoolyards and neighborhoods, or as we plant them as gifts for future generations.
But enough about us. Many species of wildlife -- from beetles to blue jays -- are utterly dependent on trees for shelter, nesting, food, and food storage.
With a resume like that, it's a wonder we so often take trees for granted. Tropical rainforest studies capture much attention, but it also makes sense for students first to get to know their own arboreal neighbors.
Revealing Hidden Hues
Most leaves look green because they contain the green pigment, chlorophyll, which is a vital component in photosynthesis. Other colors also exist, but are masked by the strong green pigment. In temperate areas, shorter days and longer nights trigger deciduous trees to release a hormone that restricts the flow of nutrients to leaves in preparation for winter dormancy, causing chlorophyll to break down. Other formerly masked colors are then revealed. The yellow and orange pigments are carotenoids. The red pigment, anthocyanin, is formed when nights cool and sugars are trapped in the leaves. (Scientists still don't fully understand the role of these substances.)
Your students can uncover "hidden" colors in leaves through a process called chromatography. Start by inviting them to collect and keep track of leaves from different types of trees and locations (e.g., shade and sun). Next, have them crush the leaves and put each type in a test tube or small, lidded glass jar. Carefully add enough of the solvent acetone (you can use nail polish remover) to create a milkshake consistency (for a test tube, this will be about 2 cm). Cover and let the mixtures sit for 24 hours. (Pigment molecules dissolve in acetone.) Make narrow strips from the middle of a coffee filter, then place one end in each solution and let it sit overnight.
Remove the strips and let them dry, making sure they don't get mixed up. (The solvent carries pigments up the strip, but some will "drop out" of the solution sooner than others. Because each pigment has a characteristic rate of movement, they will separate from each other on the strip, typically as green, yellow, and light orange bands.)
Ask, What conclusions, inferences, and new questions do you have? Did some leaves have the same mix of pigments? Were they similar in other ways? Did sunlight seem to make a difference? Would you expect to get the same results all year round? How might the colors in tree leaves compare with those in garden plant leaves, such as spinach? (Find out!)
Consider finding another school through our Garden in Every School registry: www.kidsgardening.com/School/searchform.html that is willing to exchange data on a leaf pigment project.
If appropriate to your region, also challenge students to explore questions related to seasonal color changes: Do all trees of the same species turn the same colors? How does color appear over time on individual leaves? Which weather factors, if any, seem to affect how and when leaves change color?
Sorting Out Leaves
Have students collect a variety of leaf types, then describe different characteristics: color, shape, size, margins, texture, vein pattern, and so on, or create a scavenger hunt for leaves with particular characteristics (a "palmate" shape, for instance). Consider inviting students to divide a variety of leaves into two piles based on a comparable observable characteristic (e.g., hairy/smooth). You might have young students practice this a number of times using different qualities. This is the first step in creating a dichotomous key in which only two descriptive alternatives are available at each step.
Once students have divided leaves by one characteristic, they should take each pile and choose two new alternatives. (For instance, the "hairy leaf" pile might then be divided by type of leaf margin (leaf margin serrated/leaf margin smooth). If students select subjective qualities, such as small/large, encourage them to quantify their choices (leaves<10cm and leaves>10cm). Students should continue in this fashion until there is only one leaf left in each category. Once your young taxonomists have created their unique keys, consider using some published guides to identify trees.
Capturing Tree Sweat
As leaves manufacture food (through photosynthesis), they take in carbon dioxide and give off water and oxygen (through pores called stomata in leaves and lenticels in bark). A mature tree, in fact, can release (or transpire) more than 200 gallons per day when it's actively growing.
Challenge your students to design a setup to investigate this phenomenon. For instance, put a plastic bag over a group of leaves and seal it with twist-ties. Check the bag after 24 hours. Consider pursuing the following questions: Do leaves in the sun or shade transpire at a greater rate? Does leaf size or type, or tree species, affect how much water transpires? How can we compare the amount of water in bags on different trees? Do leaves of evergreens or trees that thrive in hot and dry climates also transpire? (Yes, but they have adaptations to reduce water loss, such as waxy coatings, surface hairs, and small surface areas.)
Producing enough food for a massive plant to thrive is no small thing. Tree leaves are arranged to absorb as much sunlight as possible for making food in a particular environment. (A mature maple, in fact, can have half an acre of surface area!) Challenge students to estimate the surface areas of different leaves, then to trace leaf outlines on graph paper and count fully covered and partially covered squares. With that data in hand, they could estimate the sun-collecting surface for an entire tree by counting or estimating the number of leaves on a small twig, twigs on a branch, and so on. How does the total leaf surface area relate to tree size? Which individual tree or species has most surface area for absorbing light?
Tree Cues for Planting Clues
Share with students that people living close to the earth often used cues from nature to determin to plant their crops. For example, early settlers were told by native people in New England to plant corn when the white oak leaves were the size of a mouse's ear. Because light and/or soil temperatures trigger many such changes, the advice was often well-founded. Students in certain regions may want to test this concept by matching local corn-growing recommendations with the appearance of this leaf size, then noting related climatic factors (temperature and day length, for instance). Or they might create their own planting tips based on cues from other local trees.
In the natural cycle in a forest, tree leaves and other organic matter fall and decompose to create a rich natural compost called humus (the fallen debris also harbors insects and other animals). The nutrients in this humus are released and returned to the soil where they can, in turn, help plants thrive. Dead trees (snags) provide shelter for birds and other animals before also decomposing. But in urban and suburban areas, humans intervene in this natural process.
Fallen leaves and clipped limbs are often considered waste and are removed. Artificial fertilizers replace the natural fertilizers released through decomposition. Lawn grasses compete with trees for nitrogen, and lawn chemicals can damage delicate feeder roots. Invite your students to investigate, then draw their own comparisons of forest and urban ecosystems. Challenge them to brainstorm how they might mimic aspects of natural forest cycles (by composting tree leaves, then using that to side-dress trees, for instance).
Trees Feed the World
Challenge students to explore their homes and supermarkets for edible tree products. (You may want to first have them brainstorm or conduct research.) Some examples follow.
Fruits: apple, cherry, tangerine
Nuts (technically fruits): almond, cocoa, cola, coffee, walnut
Other parts: bark (cinnamon), flowers (cloves), leaves (tea), sap (chicle for gum, maple syrup)
In early spring through summer, have students search for tree flowers. What can they infer about how different types of trees are pollinated by examining the flowers? (For instance, wind- pollinated tree flowers, such as the catkins of birch and willow, tend to be less conspicuous than those of insect-pollinated trees, such as apple.)
About five billion acres on Earth were once covered with tropical rainforests. Less than half of that original acreage exists today. Some researchers estimate that we're losing roughly 50 million acres of tropical rainforest every year an area equivalent to the size of Great Britain! If the trend continues at this rate, how old will your students be when the last piece of rainforest is gone?
Challenge your stewards to dig up information on rainforest ecosystems; the factors that contribute to their deforestation; and the impact of this loss on plants, humans, other animals, topsoil, and weather. Then learn about efforts that are taking place to slow destruction and/or ameliorate these impacts, and consider taking action.
Two good educational Web sites are the Rainforest Information Center http://forests.org/ and the Rainforest Alliance www.rainforest-alliance.org.