PURPOSE:  To investigate the ability of plants in winter to withstand desiccation.

MATERIALS:  Deciduous and evergreen twigs, pruning shears, laboratory balance sensitive to 0.001 g, petri dishes, aluminum foil, distilled water, refrigerator at 5° C (40° F), drying oven at 70° C (158° F).

INTRODUCTION:  Trees and shrubs in the northern part of the United States and southern Canada are in a state of dormancy during the winter months.  Although dormant, these woody plants have living tissues that require liquid water to stay alive.  Branches or the whole plant may be killed during a severe winter.  One cause of winter kill may be desiccation or drying. 

In this activity you will investigate changes in water content in deciduous and evergreen twigs during the winter.  Since you will be sampling twigs of different sizes from trees over an extended period of time, you will measure the relative water content (RWC) of the twigs rather than actual water content.  The RWC of a twig expresses the amount of water a twig actually contains as a percent of the maximum amount it could contain.  When the RWC of a twig is low, the tree is in danger of desiccation.  You will relate changes in RWC of twigs over the winter to temperature conditions, and consider the plant's ability to resupply water and withstand desiccation.

PREDICTION: Use what you know about plant structure and function to make a prediction.

1. Do deciduous trees or evergreen trees maintain higher relative water content during the winter?  Which type of plant is better adapted to withstand winter desiccation?  Explain your ideas.

PROCEDURE:

A. In the fall as soon as deciduous leaves have been shed, collect at least one evergreen and one deciduous twig with terminal buds. The number and species will be determined by your teacher.  The twig samples should be about 8 cm long. 

B. Determine the RWC of each twig using the following procedure:

C. Each week during a one month period in mid-winter, repeat step B to calculate the RWC for freshly picked twig samples from the same tree or shrubs.  In addition, record the maximum temperature on the day prior to sample collection from newspaper weather reports. 

D. The final samples should be taken in spring after woody plants have absorbed water to begin active growth.  Repeat step B and record your results on the data sheet.

E. Construct a graph with percent RWC on the left vertical axis and time on the horizontal axis.  Use a different color or symbol for each species. On the right vertical axis of the same graph, plot the maximum temperature on the day prior to sample collection.

CONCLUSIONS:

2. Based on the fall data, how does the RWC of deciduous twigs compare to that of evergreen twigs?

3. Judging from your graph, how does the RWC of woody twigs generally change from through the winter?

4. Is there a noticeable difference in the rate of change in RWC (desiccation rate) of deciduous twigs compared to evergreen twigs?

5. During the winter months, did the maximum temperature for the day before sample collection ever get above freezing?  What effect, if any, did this have on the RWC?

DISCUSSION:

6. What evidence, if any, suggests that either deciduous or evergreen plants are better able to resupply the water lost through desiccation?

7. What do you suppose is the source of the water used to resupply the twigs?  Is it water absorbed from the environment or water redistributed within the plant?

8. What evidence from the spring data suggests that the plants had broken dormancy?

9. Is there any evidence of winter kill in the plants you tested?  If so, describe it.

10. Can you offer another explanation, besides twig desiccation, to explain winter kill in woody plants?

Dates Species

Fall

Weekly Winter Data Collection 1 . . . . . . . . 2 . . . . . . . . . . 3 . . . . . . . . . 4

Spring

FW
TW
DW
RCW%
Temperature
FW
TW
DW
RCW%
Temperature
FW
TW
DW
RCW%
Temperature
FW
TW
DW
RCW%
Temperature