PURPOSE:  To investigate how tree trunk size changes with air temperature during the winter.

MATERIALS (per group):  pencil, ball point pen or nail, measuring tape, metric ruler, graph paper. For the class -- 3 or more dendrometers, max-min or other thermometer.

INTRODUCTION: Matter usually expands when heated and contracts when cooled.  Pure water is unusual because its maximum density is at 4^oC.  Below 4^oC, it expands to form the crystalline structure of ice.  Substances dissolved in water decrease the temperature at which water freezes.  The sap in the vascular tissue of trees contains water and other substances such as sugars and minerals. Changes in tree trunk diameter can indicate when the sap freezes, and when spring growth resumes.

In this investigation you will use dendrometers to measure how the trunk size of living trees changes as winter temperatures vary.  A dendrometer is a steel band which is placed around the trunk of a tree and attached with a taut spring.  The difference between the marks on the inner and outer bands are used to measure expansion or contraction of the tree trunk.  A wooden telephone pole will serve as the control for this experiment.

PREDICTION: Use what you know about the freezing point of pure water and the composition sap to make a prediction.

1.  At what temperature do you think sap will freeze?  Do winter temperatures in your area get cold enough to freeze the sap in trees? 

PROCEDURE:

A. Locate an area that has both deciduous and evergreen trees and a wooden telephone pole within walking distance.  Record the date, and the air temperature in ^oC.  If a max-min thermometer is available, set it up in an area sheltered from the sun.

B. Attach a dendrometer around the wooden telephone pole about 1.5 m from the ground.  Make sure that the band is tight and the spring is taut.  Scratch a mark with a ball point pen or nail on the inner band exactly under the notched reference mark on the outer band.  Use a measuring tape to determine the circumference of the telephone pole in mm just below the dendrometer.  Record the data.

C. Select an evergreen tree and a deciduous tree with approximately the same size trunk and attach dendrometers as above.  Record the species and circumference of each tree on the data sheet.  If more dendrometers are available, increase the number and species of test trees.

DENDROMETER

D. After 24 hours return to the study area.  Record the date and the air temperature.  (If you are using a max-min thermometer, also record the lowest temperature and then reset it.)  If the marks on the inner and outer band of each dendrometer are still aligned, no change has occurred in the circumference.  Expansion or contraction is indicated by the distance between the two markers measured in millimeters.  To determine the new circumference, add or subtract that number from the original circumference measurement.  (Do NOT remeasure with the measuring tape.  The dendrometer is far more accurate for measuring any changes in circumference).  Complete the information required on the data sheet.

E. Make four more observations over the next several weeks.  Choose days that have marked temperature differences -- mild days and very cold days.  Be sure to take measurements on days you think it might be cold enough to freeze the sap in trees.

F. After you have completed the data collection, reorder the data according to increasing temperatures.  List the measurements for the coldest day first and the warmest day last.

G. Graph the data.  Plot air temperature (^oC) on the horizontal axis and circumference (mm) on the vertical axis.  Use a solid line, a dotted line, and dashed line (or three different colors) to represent the data for the telephone pole, the evergreen tree and the deciduous tree.

CONCLUSIONS: Answer the following questions based on the graph.

2. Did the wooden telephone pole expand and contract with changing air temperatures?  How did it compare to the trees?

3.How did expansion and contraction in the evergreen tree compare to that of the deciduous tree?

4.Did the sap freeze at 0^oC?  What is your evidence?

5.What was the coldest temperature you recorded?  (If you used a max-min thermometer, use that data).  Did the sap freeze at that temperature?  What is your evidence? 

DISCUSSION:

6.What was the reason for using the wooden telephone pole?  How does the composition of the telephone pole differ from that of the living trees?

7.What evidence is there that northern trees are able to resist freezing?  What accounts for freezing resistance in trees?

8.What evidence is there that trees are able to resist damage when freezing occurs?

9.Is there any evidence of winter kill to trees in your area after a severe winter?

GOING FURTHER:Sustained increase in trunk size is evidence that spring growth has begun.  New xylem cells are produced by cell division in the lateral meristem (cambium) resulting in a new annual ring.  To determine when the tree breaks dormancy and resumes growth, leave the dendrometers in place or reattach in late winter.  Make weekly observations and record the circumference for each tree.

Name_ ______________________________________________________

  TREE TRUNKS AND TELEPHONE POLES DATA SHEET

 Evergreen Species:_ _ ______________________________________

 Deciduous Species:_ ________________________________________