Determining the Amount of Transpiration from a Schoolyard Tree

By Laurence Pomeroy

Investigation Procedures

Materials:

• Leaves, include petioles (mulberry or cottonwood work well)
• Small beakers (50 - 75 ml)
• Spring water
• Mineral oil
• Graph paper

Learning teams of students go outdoors and randomly collect five different sized leaves from a selected tree. They return to the lab and carefully trace the outline of each leaf onto a piece of graph paper. Count squares to find the area of each leaf (Determine area of square and multiply by number of squares). Add the areas of each of the five leaves together to determine a total area measurement in square centimeters.

The team now places its five leaves into a small beaker and fills it about half full with the spring water so that the petioles are entirely submersed. Pour a thin layer of oil on the surface of the water, so it is completely covered (carefully avoid getting oil on the leaves). The purpose of the oil is to prevent any water loss due to evaporation from the surface of the water. Mark the top edge of the oil with an erasable marker on the side of the beaker. Let the beaker sit out for 24 hours by a window or grow light.

After 24 hours, estimate how many milliliters the water has dropped overnight. Divide this number (the number of milliliters "transpired") by the total area of the five leaves to obtain the amount of water transpired in milliliters per square centimeter of leaf in 24 hours.

Student teams go back outdoors and estimate the number of leaves on their trees in the following manner: count the number of leaves on a branchlet, the number of branchlets on a branch, the number of branches on a big branch, etc. and multiply using calculators to arrive at an estimate. Pairs of team members should make their own estimates and then the team can take an average to arrive at the amount they will use.

To calculate the size of an average leaf, take the previously calculated total area of the five leaves and divide by five to reach an average. Multiply this average by the estimated number of leaves on the tree to arrive at the total leaf area of the tree. Now, multiply this by the milliliters of water transpired per square centimeter in 24 hours to obtain the number of milliliters transpired by the tree in one day.

Further Calculations:

Determine liters of water transpired per day (also pounds, etc. - there are 2.2 pounds per liter of water.)

Calculate yearly water loss of the tree through transpiration (if deciduous, use number of months the tree has leaves.)

We estimated in a temperate area that there would be about 36 trees per acre. Use this to figure out how much water is being transpired from trees per acre on a daily or yearly basis. The closing calculation is definitely a "guesstimate".

We assumed there to be about 300 - 400 trees per acre in a tropical rain forest and made an estimate of how much water is transpired by trees from an acre of tropical rain forest. Finally, we used the frequently stated statistic of one acre of tropical rain forest lost per minute (some figures range as high as one per second...) on Earth. We then determine how much water transpiring "potential" is lost after a year of cutting down rain forests. Also, 1.8 times as much oxygen is given off as water transpired--so you can multiply all your figures by 1.8. Don't forget that the missing trees cannot pull carbon dioxide out of the atmosphere (thus exacerbating the Green House effect).

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