Transpiring Trees

PART 1. Investigation Set-Up

Tell the students that it is possible to do the same thing with trees and tree branches. As a part of the PINE MAP study, researchers are looking at drought and its effect on the growth of Loblolly Pine forests commonly grown for the lumber industry. In this study one thing that is measured is the transpiration rate (amount of water usage) for an average pine tree. One method for measuring this is by cutting down an entire tree and placing it in a bucket of water. The researchers can then measure exactly how much water the tree uses over a period of time (see image below). This is not much different than the setup for keeping Christmas trees alive after being cut down.

Tell the students that they will be investigating the factors that might be influencing water usage for individual tree limbs using a similar method. Explain to students that each group will receive a live tree branch a large #10 coffee can and the necessary measuring tools.

Show the students the sample set-up below for the investigation. Tell the students, “Before we can begin we need to define the parameters of our investigation. What characteristics of the tree branch could we measure and compare with transpiration rate (water usage) to find possible correlations?”

Once the students have identified the three characteristics to use as the independent variables the students will need to create a hypothesis and a null hypothesis for the investigation.

Students will also need to specify how each variable will be measured. This is important because during the data analysis, the class will combine and use all the data collected by the class(es). Consistency in the measurements will lead to a more accurate statistical analysis.

Show the students the suggested measurements methods (See figure below):

• Tree limb length – Is measured vertically from floor to the top of the tallest point of the limb.

• Tree limb diameter – Is measured directly at the top of the can.

• Total leaf surface area – Will be described.

To measure Total leaf surface area students will need to construct a mathematical model to calculate total leaf surface area. The simplest and most common method for measuring leaf surface area is to use 1cm grid paper.

1. Place leaf on 1cm2 grid paper and trace.

2. Count the number of whole squares covered.

3. Count the number of partial squares covered. Divide this number by 2.

4. Add the number of whole squares to your answer from step 3. This is the surface area of the leaf.

Even though this method is fairly straightforward is takes time, especially for measuring leaf surface area for an entire tree or tree limb. A second method is to create a mathematical model (an equation) in which you can measure the length of the leaf and input this number into the model and get the leaf surface area for each leaf, then add the surface area together for the entire limb or tree.

To do this you should have 2-3 tree limbs available for your classes. Have each student use the grid method to find the surface area of at least 5 leaves. Students will also measure the length of each leaf. Have the students record their data. Have each of your classes do this. This will provide a large enough data set to enter into the excel spreadsheet (see attached excel spreadsheet instructions document) used to create an accurate mathematical model to utilize during the transpiration investigation. The mathematical model will be unique to the species of tree that you use.

PART 3. Transpiration Investigation & Data Collection

Place students into groups of 3-4. Give each group the materials for the investigation.

• Live tree branch

• #10 tin can or similar size container.

• Plastic sheeting

Have students set up investigation as described in the sample set-up figure below. Have students write their group name on the can and place a mark about 3/4 of the way up on the inside of the can. Students will then fill the can with water up to this mark.Next have students use the pruning shears to cut a couple of inches off of the already cut stem while it is underwater inside the tin can (see photometer video).

• When cutting the branches off the trees place them cut end into a bucket of water as soon as possible, however cutting the branches off the trees and getting them into water within 30 minutes shouldn't cause any problems.. You can store them like this for a few days before the investigation in needed.

• A few inches of each transported branch will need to be cut off below the water surface in the trashcan.

• When students begin to set up the system for their investigation, a few more inches will need to be cut off the branch under water in the in the can.

This allows the vascular system, the xylem and phloem, to function properly in regard to water uptake. Have students use a ring stand, table or other available object to attach the tree branch in order to keep the branch vertical.

Have students measure and record initial data. (See Transpiration investigation set up figure above)

Tree limb length (TLL) – Is measured vertically from floor to the top of the tallest point of the limb.

Tree limb diameter at can (TLDC) – Is measured directly at the top of the can.

Total leaf surface area (TLSA) – Have students measure the length of each leaf on the branch and record these measurements in a table. (Students can use a permanent marker to mark each leaf to indicate which leaves have been measured.) Have students figure the surface area for each leaf using the equation created in part 2. Have students add all of the individual leaf surface areas to obtain the Total leaf surface area. (This can be done using a calculator and paper or can easily be setup in an excel spreadsheet to figure the TLSA. See attached excel spreadsheet instructions document.)

Water height/volume - Water height/volume can be measured using two methods.

1. Use a ruler to measure the height from the bottom of the can. Repeat everyday for the duration of the investigation (5-10 days).

2. Place a mark on the inside of the can. Each day of the investigation add water to bring the water level up to the line. Record the volume of water required to “refill” the can to initial level.

Have students record the water used by the tree branch once everyday for the duration of the investigation. The investigation should last at least 5 days but could be longer as more data will result in more accurate analysis. Students can collect data in a journal or a provided data table similar to the table below.