TOPICS: Plant anatomy and physiology, ecology

LEVEL: High school and advanced placement

TIME: 10-15 min on 3 successive days in fall and spring and 10-15 min on 3 successive days each week for four weeks in midwinter; additional time for pre- and post-activity discussion.

ADVANCE PREPARATION: Bring in newspaper weather reports for the days prior to twigs collection (step C).

INTRODUCTION: You may wish to point out the conceptual similarity between RWC and relative humidity (RH) in meteorology .

PREDICTION: In class discussion, encourage creative, logical, and critical thinking. We recommend that you introduce the following ideas if they are not suggested by students: Students may think that low temperatures cause winter kill in woody plants, but have not considered how such temperatures result in tissue death.  Both deciduous and evergreen twigs tend to lose water by evaporation, or freezing and sublimation.  Water is lost through lenticels and by cuticular transpiration.  Excess water loss can result in desiccation and thus winter kill. It has been suggested that evergreens have a greater ability to resupply water to tissues than deciduous species, whether or not the air temperature goes above freezing.  The potential mechanisms involved in this rehydration process are not known.  Determining water content over a one month period in winter should hit some of the low points and high points of desiccation and rehydration .

PROCEDURE: The number of species (step A-B) and trials (step C-D) is to be determined by the teacher .

STEP A:  Select deciduous and evergreen species that are readily accessible since the twig samples must be collected fresh several times over the winter.  Any time lag between collection and measurement of fresh weight will result in experimental error .

STEP B:  If needles are left on conifer twigs in step B3, they fall off in the drying oven.  Loosely wrap the conifer twigs in foil to retain the needles before placing them in the drying oven or strip the needles off the conifer twigs in step B1 before fresh weight is determined .

STEP C: January to March is appropriate in many parts of the country to do step C .

STEP D: Water absorption for resumed growth begins in late winter or early spring.  Your county extension agency may be able to offer guidance on timing in your region.

QUESTIONS 6-10 require higher order thinking skills.  We recommend that you discuss these questions in class before students write out their answers .

2. The percent water content measured in the fall may be somewhat higher in evergreen twigs than in deciduous twigs .

3. The water content of both deciduous and evergreen twigs will decrease over the winter months, although evergreen twigs may lose less water .

4. The decrease in RWC may be steadier and, less variable in deciduous twigs than in evergreens .

5. Air temperatures will differ according to local weather conditions of the study area.  However, rehydration may occur even though the air temperatures do not go above freezing .

6. The graph may show that evergreen trees are better able to resupply water to tissues .

7. Rehydration probably involves redistributed water as opposed to that absorbed from the environment because rehydration can occur even when air temperatures are below freezing .

8. Steady rehydration of the twigs in spring or earlier suggests that dormancy is coming to an end and the plant is resuming active metabolism .

9. Failure to rehydrate in the spring samples is evidence of winter kill.

10. In woody plants, winter kill can result when heavy snow or ice bends the branches and/or needles resulting in physical damage to the twig.  Intracellular ice can cause physical damage by rupturing the organelles; it can also cause coagulation of proteins.