The most poorly understood part of the
water cycle is the process of evapotranspiration by which plants give off
water from their leaves to the atmosphere. The process of evapotranspiration
has been greatly ignored in most introductory science courses and only
lately is entering science curricula as rain forests, a major source of
atmospheric water, continue to disappear at an alarming rate.
In this investigation, tree parts will
be bagged and a water evapotranspiration to biomass ratio will be calculated
from the collected data.
Objectives: 1) Bag trees (technique will be demonstrated
in class).
2) Determine mass of water given off.
3) Determine mass of tree part.
4) Calculate water transpiration mass to tree biomass ratio (divide
grams of water by grams of tree and label water mass/tree mass ratio).
Procedure: Make a ‘Data /Calculations’ section on your
paper and show all data recorded. In that section make a chart or table
as shown below to place your data in. Make sure you use units on all numbers.
Show your calculations.
1) Bag the trees after class demo. Make sure the tape is a snug
fit and touches both the tree and bag all the way around. Allow the trees
two or more days to photosynthesize if possible. Read the steps below and
set up an appropriate data table.
2) Carefully collect the bagged tree parts so as not to lose any water.
Keep the opening upward.
3) Mass the bag, tape, tree parts, and water together.
4) Separate the tree parts from the bag and tape. The water may be
poured out now.
5) Mass the dried bag and tape after waving in the air to dry.
6) Mass the tree parts after waving in air to dry. Pick up any leaves
or needles that fall to include them.
7) By subtraction, determine the mass of water.
***8) Determine the ratio by dividing the water mass by the mass of
the dried tree parts. ( #4 / #3)
Water to Tree Mass Ratio = ____________
Analysis: 1) If the area you bagged had a volume
that was 1 ft3, how much water would you expect to see transpired for a
tree that was 1000 ft3?
2) Would a 1000 ft3 tree be a relatively large or small tree?
3) In six months, how much water would the 40,000 such trees put into
the atmosphere?
4) How and why would each of the following factors probably affect
transpiration?
Temperature
Length of Day
Time of Year
Drought
5) Look at your water to biomass ratio. Notice that it shows how much
water is transpired for each unit of tree biomass. How long would it take
the tree to put water equivalent to its entire mass into the atmosphere?
Conclusion : Write a short paragraph commenting on what you learned from this activity.
Supplement for AP Environmental Science
copyright Ken Baxter 2001