PURPOSE: To identify common weeds in your area and determine which ones are dispersed by winter winds.

MATERIALS (per group): Field -- suitable winter clothing, Key to Winter Weeds, index cards, transparent tape, one numbered seed collector, Tanglefoot^TM with applicator, compass, hammer (optional).  Lab -- string or yarn, newspaper, hand lens.

INTRODUCTION:  Weeds are plants growing where they are not wanted or where they were not planted.  They can be found in city lots, along roadsides, in fields or forest openings.  Typically such plants can tolerate a wide range of soil and climatic conditions.  Many of these plants, now widespread in North America, were introduced from Europe either deliberately or accidentally.

Plants with dead stems that remain standing throughout the winter may be called winter weeds.  The seeds of most of these plants are dispersed throughout the autumn and winter.  Some seeds are eaten by birds and small mammals.  However, many of them are dispersed by winter winds, or by attachment to animal fur.

In this field and laboratory activity you will identify some common winter weeds, describe their seeds, and speculate about their dispersal mechanisms.  You will then set up seed collectors to test your ideas.

FIELD PROCEDURE:

A. Go to an area where there are remnants of herbaceous plants, such as an abandoned lot, open field, or roadside.  Using the Key to Winter Weeds, identify as many species as you can by their common name.  Make an index card for each species as follows:

(3)  Look closely for seeds on each species you identify.  If present, estimate and record the quantity of seeds on each plant (few or many) and describe or sketch how the seeds are attached to the plant. 

(4) Tape a few of the seeds to the card for later reference.

B.  Smear Tanglefoot^TM, a non-drying sticky substance, on the plastic covering of your seed collector.  WARNING:  TANGLEFOOT^TM IS VERY DIFFICULT TO REMOVE FROM CLOTHING AND HANDS!

C. Your teacher will help you determine appropriate sampling sites for your study area.  Set the pole of your seed collector in the snow or soil so that the collecting surface is a small distance above surface level.  Orient the marks on the collector in the proper direction using the compass.

D. Retrieve your seed collector after a period of several days.

PREDICTION: While you are waiting to collect seeds, use your field notes on plant identification and seed structure to make a prediction.

1.  Which seeds are you most likely to find on your collector?  Explain your ideas.

LAB PROCEDURE:

E.  Hold the collector by the pole to avoid getting Tanglefoot^TM on your hands, clothes, or lab surfaces.  Turn the collector upside down and rest it on a newspaper.

F. Examine the collecting surface with a hand lens to distinguish seeds from any debris that might have been transported by the wind and caught on the collector.  Record the compass direction with the largest number of seeds in the appropriate column on the data sheet.  Using your field notes for identification, count and record the number of seeds of each species.

G.  Share your data with your class and complete the information on the data sheet.

CONCLUSIONS:

2. Among the winter weeds you identified, which were the most common in the field?  Which were rare?

3. On the basis of size and quantity of seed, which weed species may provide winter food for birds and small mammals?

4. Which weed species may provide nesting material for animals?

5. Which seeds were you most likely to pick up on your clothing?  How are these seeds usually dispersed?

6. Based on the class data, what was the direction of the prevailing winds during the time of seed collection?

7. Which seeds were found most commonly on the collectors?  How did the class results compare to your predictions?

8. What are some of the distinctive characteristics of seeds that are dispersed by wind?

DISCUSSION:  Plant species that can readily colonize distant areas disturbed by agriculture, forest fires, and so on are called successional species.  Plants that eventually become the permanent residents in a particular region are called climax species.  Studies have shown that the seeds of these two kinds of plants differ in size and abundance.

9. Would you expect successional plants to produce large or small seeds?  Would you expect successional plants to produce large or small quantities of seeds?  What is the selective advantage of this strategy?

10. What size and quantity of seeds do you suspect are produced by climax plants?  What is the selective advantage of this strategy?

11. Which category (climax or successional) describes the winter weeds you collected?  How do your observations on the size and quantity of weed seeds support your answers to questions 9 and 10?

GOING FURTHER:

12. The most recent glacial advance was at its maximum about 11,000 years ago.  Since that time, oak tree have migrated northward approximately 500 miles.  If the minimum age for an oak to bear acorns is 30 years, what was the average distance of seed dispersal per generation for oak trees?  Suggest at least 2 different methods that might account for the dispersal of these large heavy seeds over this distance.

13. Wind is the agent of seed dispersal for many trees including ash, maple, cottonwood, and pine.  What structural adaptations do these species have to facilitate dispersal by wind?  What factors could account for the differing distances that such seeds travel?

SEED DISPERSAL DATA SHEET