Introduction (written for students):


       Some years ago, one of the faculty in the Biology Department noticed that fewer seedlings and saplings of broadleaved woody species seemed to grow beneath the canopies of Eastern Hemlock trees than beneath the canopies of other species on the Hope College Nature Preserve property. Since that time, a number of student projects have confirmed this pattern, but none have been successful in determining the mechanism by which Eastern Hemlocks inhibit the other species.


       During this week in lab, we will conduct a brief orientation walk on the property, during which you will observe the spatial distributions of seedlings and saplings relative to Eastern Hemlock trees. We will then collect data to address the question: "Are the densities of seedlings and saplings of woody broadleaved plants lower beneath the canopies of Eastern Hemlock (Tsuga canadensis) than beneath those of broadleaved trees?"

       To address this question, each group of 3-4 students will sample seedlings and saplings beneath 3 Hemlock and 3 Sugar Maple canopies. So that the physical conditions and seed rain will be as similar as possible between the two canopy types, you should choose your Hemlocks at random (making sure that no other group has sampled them, of course), but choose your Maples such than they are the nearest canopy-level ones to each of the 3 Hemlocks. We will identify and count the seedlings and saplings within a 1 x 1 meter quadrat to the southwest of the trunk of each of the trees sampled. There's nothing special about that location, of course. In fact, that's the point: by choosing the site to sample before we even look at the trees, we reduce the possibility of biasing our results by choosing sample sites on the basis of their seedling density. So just use a compass to lay out each quadrat such that its northeast corner touches the tree trunk.

       Within each quadrat, count the total number of seedlings/saplings of woody plants (beech, maple, hemlock, oak, shrubs, etc.) separately. Don't count herbaceous plants like grasses, forbs, mosses, etc. Use the data collected by all groups in your lab section to test the null hypothesis that the mean density of woody seedlings and saplings does not differ between Hemlocks and Sugar Maples. The alternative hypothesis in this case is directional: mean density of seedlings and saplings is higher beneath Hemlocks than beneath Sugar Maples. So you'll need to do a one-tailed t-test. Note that when you're doing a one-tailed test the alpha levels for statistical significance are half of those for two-tailed tests, for which the tables of critical values are written. To get the correct critical value of t for a one-tailed test at alpha = 0.05, use the column for alpha = 0.10.

       Assuming that we find a difference in seedling/sapling density beneath hemlocks and maples in the first part, your group should discuss factors that might be responsible for it. After doing so, your group should propose to test two different working hypotheses to explain the difference. You're free (encouraged!) to come up with other hypotheses, of course, but a few that we can suggest include:

       * Light intensity is lower beneath Hemlock canopies, such that germination and/or growth of woody seedlings is inhibited.

       * The canopies of Hemlock trees act as "umbrellas" that shed seeds of broadleaved trees to the side, such that areas beneath Hemlock crowns receive fewer colonists.

       * Hemlocks alter soil pH in such a way that germination or growth of broadleaved plants are inhibited.

       * Hemlocks produce allelochemicals, which either leach from the stems or foliage or are exuded from the roots, and which inhibit either germination or growth, or both.

       Each lab group will formally propose 2 different working hypotheses to investigate, one of which must deal with allelopathy (chemical inhibition via compounds produced by Hemlock; alteration of soil pH doesn't count here). Each group will work independently of the others, and write a 1-2 page proposal that:

       1) Presents the statistical analysis and conclusion from the first part of the investigation (the analysis discussed above);

       2) Poses two hypotheses about the mode of inhibition (e. g., what is it about Hemlock trees that results in lower densities of other plants near them), one of which deals with allelopathy. The other hypothesis can focus on another factor of you own choosing, assuming that your instructor approves it;

       3) Briefly describes a series of experiments or other data collection procedures that you will use to test these hypotheses. Here again, be sure that you state explicitly what sorts of data you'll collect and how you'll analyze them, including statistical tests, etc;

       4) Contains a brief list of materials that you will need to do your study. You can use space in the greenhouse, pH meters, PAR ceptometers (which are fancy light meters often used in photosynthesis labs), and just about anything else within reason.

       Be aware that we can't realistically subject maple or other tree seeds from the forest to experiments you might think of to assess the effect of different pH's, soil extracts, etc., since this year's seeds won't germinate until next spring. Instead, if you plan a "bioassay," consider using seeds from a widely available and rapid germinating plant (such as sunflower, alfalfa, or lettuce - all of which are often used in these kinds of experiments).

The calendar of events for this study is as follows:

       Week 1: Collect initial data and report to instructor, who will compile it for all lab sections and distribute it via the course website. Groups will include statistical analyses of initial data in their proposals.

       Week 2: Proposals due, in lecture. Start collecting data as soon as your instructor has approved your proposal.

       Week 4: progress report: you'll need to show your instructor your experiments if they're in the greenhouse, show her/him the data you have collected thus far, etc. Set up an appointment with your instructor that can be attended by as many of your group members as possible.

       Week 8: Formal (individual) lab report due, in lecture.

       Week 9: Group oral presentation of results using PowerPoint (more on this later), and written outline for the presentation (hard copy or electronic copy of the PowerPoint presentation preferred).



Materials and Methods (written for faculty):

Study Site(s).

       Study sites are located in the Hope College Biology Nature Preserve, which is a 50 acre parcel of beech-maple "dune" forest about 1-2 km from Lake Michigan.

Overview of Data Collection and Analysis Methods.

Data Collection Methods.

       During the first lab in the field, students are instructed in the identification of the major tree species at the field site, including seedlings and saplings. Then they are asked to break up into small groups and to observe patterns in the distribution of those young plants with respect to the distribution of 3 of the dominant tree species : Eastern Hemlock, Sugar Maple, and American Beech.

       As the instructor circulates to assess the progress of each group's observations, s/he encourages those groups that have noticed a difference (fewer seedlings and saplings beneath Eastern Hemlock, for example), to begin to record some preliminary observations/data documenting that difference. The students are asked to begin to think about how they would test such an hypothesis. After an hour, all groups convene to discuss their findings and their hypotheses. The instructor presents the question for the independent project investigation and the written, 1-2 page proposal as an assignment.

Guidelines for Research Proposals.

       Students are asked to include the following information in their proposal:

       1) A description of how they will test the hypothesis that woody plants of other species are distributed differently beneath hemlock trees than they are beneath other species of trees. They are asked to explicitly state what kind(s) of data they will collect, how they will do the data analysis, and the statistical tests they will use.

       2) Assuming that the students do find evidence of inhibition, they are asked to pose at least two hypotheses about the mode of inhibition (e. g., what is it about Hemlock trees that results in lower densities of other plants near them). At least one of these hypotheses has to involve allelopathy (chemical inhibition via compounds produced by Eastern Hemlock). The other hypothesis can focus on another factor of the group's choosing, with instructor approval.

       3) A description of a series of experiments or other data collection procedures that the group will use to test these hypotheses. Again, students are asked to state explicitly what sorts of data will be collected, how the data will be analyzed, including statistical tests.

       4) A brief list of materials needed for the study.



Questions for Further Thought and Discussion:

  1. Based on the group presentations, summarize what you think is the most likely explanation for the decreased abundance of woody saplings beneath Eastern Hemlock?

  2. What criteria have you used for giving this particular hypothesis your greatest support?

  3. Have the research investigations, on the whole, suggested additional explanations for the plant community pattern that were not initially considered? If so, what are they, and how could they be investigated?

  4. Does support for one hypothesis necessarily negate other hypotheses? Give an example of a case in which evidence for two or more hypotheses are or are not mutually exclusive. Could you design an experiment in which the hypotheses ARE mutually exclusive? Briefly, describe the design of such an experiment.

  5. What selective advantages are associated with a plant's ability to inhibit competing plants of a different species? Offer other examples from different plant communities where similar inhibition processes occur, and explain the mechanisms of inhibition that are used in your examples.

  6. If inhibition helps increase a plant's fitness, why do we not find evidence that all plants are doing it? For example, why don't we see similar patterns for American Beech or for Sugar Maple at the biology field station?

  7. How does competition among plant species at this forest site contribute to the structure of the plant community there?



References and Links:

Potentially Useful References for the Hemlock Project



Tools for Assessment of Student Learning Outcomes:

       Each team receives a grade for the proposal, for the progress meeting, and for the oral presentation. A minimum of 5 primary literature citations are required as a supplementary handout for the oral presentation.

       We use a basic Grade Sheet for scoring their Lab Reports (available at HemlockGradeSheet.pdf). We give this page to the students in Week 1.

       In addition to the oral and written reports, students in this course have 2 laboratory practicals each semester. During the lab practicals, students are asked to DO various tasks that derive from the course experiments, including the hemlock lab. For example, students are given a sample data set, and asked to complete a paired-samples t-test using SYSTAT, on the computer. Construction of computer graphics are graded similarly. Other items on lab practicals derived from this lab include: plant identification, use of light ceptometers, measuring devices, a portable photosynthetic system, and evaluation of various hypotheses using hypothetical data sets and results.



Tools for Formative Evaluation of this Experiment:

       At the end of the semester, students are asked to give each lab investigation a letter grade (A, B, C, D or F) and a brief explanation of why that grade was given. We use this feedback from the students to determine which labs should be retained, improved, modified (e.g. extended, shortened, more time on statistics, new field site, etc.), or simply “tossed” in preparation for the next year’s class.

An extensive discussion on Evaluation appears in the Teaching section of this site.