Science Standards by State
in the next few days or weeks you will walk under a mature deciduous tree.
A deciduous tree is one that sheds its leaves in winter. In summer,
you may enjoy and appreciate the shade the tree provides. It may
be a maple, oak, a cottonwood, or any of several kinds of tree.
On a hot day in summer, its shade may be most welcome. But that shade
isn't there in winter. In wintertime, the leaves of a deciduous tree
are only buds that will swell in the warmth and moisture of spring, and
will open and enlarge in the warmth of summer. How large an area
of welcome shade will a mature, deciduous tree provide? Can you find
out? Sure! And besides, even though it's a bit of work,
it's fun. You'll learn what most people simply do not know or really
A mature sugar maple tree or oak tree is a good one to work with. By "mature", find one that has a "dbh", or "diameter breast high", of a foot or more. Examine the tree and try to estimate the total area of the leaves. Do you think it's more than the circle of shade case on the ground under the tree? How much more? Here's a way to find out. (You can print a helpful worksheet by clicking on the hand at left).
Measure the total area of the tree's leaves by means of a representative sample. This is most easily done on a day when the leaves have fallen and are ready to be raked. First, collect 10 leaves that you think are of average size. Lay them on white or light-colored paper, trace around them, and cut out the paper shapes. The reason for using 10 is to get a large-enough sample to be representative. Put all ten cutouts on a scale that weigh small amounts. Your school should have such a scale. When you find the weight of 10 cutouts, one-tenth of that is the approxiamte weight of one leaf cutout. Compare this with the weight of one square foot of the same paper on which you traced the leaves. What fraction of a square foot is the area of one average leaf? This is an important number, as you will see.
By weighing ten leaves, you made up for little irregularities such as a bit of leaf that had been eaten by insects, or that had been damaged by the wind. By weighing ten, any irregularity in one is made up by nine others that are undamaged. When you found the weight of the paper cut-out of one average leaf, you found its area because you found the weight of one square foot of the same paper. Next, you need to know how many leaves there are, or were, on the tree.
You can do this by (1) weighing an empty leaf bag, then (2) raking all the leaves and putting them in one or more large, plastic leaf bags just like the one you weighed. Add together the weight of all full leaf bags, and subtract the weight of the empty leaf bags. Then you will know the weight of the leaves in the bags. You have already weight a sample of ten leaves gathered right at the beginning. If you found that those ten weighed, say, 12 grams, then 100 similar leaves would weigh 10 times that, or 120 grams. And 10,000 similar leaves would weigh 120 x 100, or 12,000 grams, or 12 kilograms. One kilogram is about 2.2 pounds, and 12 kilograms would be about 26.4 pounds.
When you find the total weight of the raked leaves, how would you find their area? If the total weight of leaves in the bags was 16 kilograms, there must have been 20,000 leaves in the bags. You found the area of a 10-leaf sample. When you know that, and you know how much all the leaves weighed, then you can find the total area.
Wow! One mature sugar maple or oak tree has a huge leaf area! And all those leaves are making food through photosynthesis They also provide shade from the sun in their own way. But the shade isn't simple like that of an umbrella. If an umbrella lost a square inch of cover, the sun could shine through that hole. But if a deciduous (one that loses its leaves in autumn) tree lost a leaf, it probably wouldn't show as a lighted spot on the ground. There are lots of leaves above it. The spot would probably be shaded by other leaves. There should be many other leaves to shade the opening formed by the loss of one. The redundancy (more than enough) helps to close any gaps left by leaves damaged or eaten by insects, or blown off by strong winds.
From the area of a single leaf,
and the total number of leaves that you estimated for a single tree, about
how much area is there in all the leaves you raked under one tree?
How does this compare with the area out- lined by the shadow of the tree
on the ground below it? What a huge shadow could be cast by all the
leaves on the tree if they didn't overlap! But how fortunate that
the leaves are redundant. Then, in summer, there is almost certain
to be enough leaves on a deciduous tree to make the food needed by the
tree, and to guarantee you shade. Redundancy in leaves of a shade
tree is a little like redundancy in the hair on your head. If you
lose one hair, no matter; there are lots more where that one came from!
In what other objects in your life does redundancy guarantee success?