Challenges to Anticipate and Solve:
We have identified several challenges that commonly arise:
- Challenge 1 - The experiments carried out by each team may involve different techniques or approaches aimed at
aimed at answering similar questions. Because of differences among groups in experimental focus and design, differences
sometimes also arise in results and data interpretation. Students are encouraged to critically evaluate their own work and
that of other teams. They will learn, through doing this experiment, that the way they set up their experimental design in the
first place, has everything in the world to do with what they find out. Consistently, we find that students need to gain experience
in understanding the difference between “thinking” they know what they are testing when they design an experiment, and actually
“knowing” what they have tested. This invariably comes with hindsight, but that’s an integral part of the learning experience in this lab.
- Challenge 2 - We don’t give the references to the students in the student handout, since that is one of the things we
require from them as part of the grading.
Comments On the Lab Description:
Introducing the Lab to Your Students.
First, we take our students to the field site, identify 4 or 5 species of trees, including the ones most pertinent to the upcoming
investigation, give a little bit of information on history of the site, and then ask them to walk around in pairs and
look at the woody seedlings and saplings in the understory. We ask them to notice particular density patterns, if any.
Typically, at our site, students will notice patterns derived from light gaps, terrain, soil moisture, and very obviously…a
relative paucity of seedlings and saplings in distinct “circles” emanating from around Eastern Hemlock trees!
Then, we stand or sit in the forest and talk about what some of these patterns are, and how we would TEST for a hypothesized
pattern, focusing on the hemlock pattern: Is the density of woody seedlings and/or saplings beneath Eastern Hemlock
canopies lower than under the adjacent canopies of other common species in this forest, such as American Beech and
Sugar Maple? We ASK the students HOW they can investigate this (in the next hour!). As a group, we discuss alternative
suggestions from students themselves, followed up a consensus on what seems to be the best approach. For example,
students need to decide on which plants to sample, (seedlings, saplings, dbh criteria, height criteria)? for what area
around each tree? What constitutes a sample? A replicate? What constitutes a paired observation (non-Hemlock canopy)?
Possible exclusions (e.g. dead trees, trail influences)? We spend perhaps 30 min. deriving a quick, but consistent means of
acquiring our first data set, and then students set out in pairs to collect this information. It is this data set that forms the basis
of independently derived inhibition hypotheses for the independent project portion of the lab.
Comments On the Activities in the Lab.
This is a field and laboratory project that we use during September - November in southwest Michigan. As a guided inquiry (instructor
chooses problem, but students are expected to design methods of tackling the problem) it has been very appropriate for sophomore-and-up
undergraduates who are ready to begin instructor-facilitated independent investigations. Students are introduced to the concepts of perception
and scale while exploring the potential allelopathic interactions that take place within a temporal or spatial framework with which they have
little previous familiarity. They gain experience in articulating their own hypotheses and alternative hypotheses, designing well-controlled
experiments, identifying dependent, independent and controlled variables, and choosing appropriate sample sizes and treatments.
Because all teams investigate the same general questions (Do hemlocks inhibit the establishment of other wood plant species, and if
so what are the mechanisms?) but from different approaches (soil chemistry vs. germination rates vs. plant distribution patterns), students
also gain experience in the critical evaluation of their work and that of others.
We first came up with the idea for this project when we observed distinctive patterns of establishment (or lack of establishment) of
broadleaved tree seedlings beneath Eastern Hemlock trees as compared with that beneath other species (e.g. Sugar Maple,
American Beech) in the same forest. Encouraging our students to make observations that will allow them to discover the same patterns
has become the introduction to the "lab". Once the patterns seem evident, students are challenged to gather evidence to support or refute
this hypothesis, and then to follow up with experiments that will help elucidate the underlying reasons for the observed differences in
seedling establishment. There is a wealth of primary literature available on the topic (including potential mechanisms for allelopathy), and
thus a good area to use to instruct students in literature-searching skills and the value of integrating other research results with their own.
Visit easily accessible field sites, preferably forests that have been minimally disturbed over the past few decades, and examine the
understories for the same kind of evidence of inhibitory effects that we observed at our site. If Eastern Hemlock does not occur in your
area, find out what local tree species are known to exert inhibitory effects on other plants (e.g. Ailanthus), and then determine a nearby
site where those trees are part of the community. That’s all you really need!
An excerpt from the assignment sheet follows:
18 September - Field trip and preliminary observations at the study site (3 hrs. includes 25 mins transport time each way, 1 hr to collect
preliminary data on seedlings distributions, 1 hr for discussion and overview of the project assignment.
25 September - Written proposals from each team of 4 students are due. Students may start collecting data when the proposal is approved
by the instructor.
21 October - Written progress report. Instructor meets briefly with each group to view experiments set up in the greenhouse, raw data
collected thus far, discuss statistical analysis, etc.
2 December - Group oral presentations occur during a 3 hr lab slot (about 15 minutes/group). We have a one-page grade-sheet that is
distributed to the students well in advance of the oral presentation so that students will know exactly what is expected.
When do the students work on their projects? Typically, we allow the time from at least 1 additional 3 hr lab to work on projects, but
most teams devote more than this on their own. Often, if additional labs have been canceled because of poor weather, we designate
that as "time to work on group projects" as well.
Comments On Questions for Further Thought:
This lab could be a good connection for lecture coverage of interspecific competition in an ecology or evolution course. From this familiar
starting point (students have now seen evidence of competition among species in a self-designed investigation), build concepts such as
competitive exclusion, species interactions, invasive species and biodiversity, coexistence of competing species, resource partitioning, and
the role of competition in structuring communities.
Comments On the Assessment of Student Learning Outcomes:
We try to stagger graded events to help students learn valuable skills in time management of large tasks (e.g. progress
interview, up-front literature cited list, etc.).
In some semesters, we provide students with a re-write option for the written report. However, to keep students from making
the first submission a “rough draft” (and consequently placing the burden of editing on us), we do not call it a draft - it is the first
submission of the final paper and is worth 75 points. Students who elect to re-submit can only earn UP TO an additional 25 pts.
For students who do NOT re-submit, the original grade is pro-rated to an equivalent score out of 100 possible.
Peer review can be incorporated easily into the oral presentations. One mechanism we have used to evaluate peer review is
to have students write their questions that they ask other groups, submit those to us, and then we grade the quality of those
questions. In one semester, we asked all groups to grade each other, using the same rubric sheet that the instructor’s used.
We think this is a good exercise, but in our case, found that students tend to be exceptionally lenient in grading oral presentations,
so we have not actually used the student scores as a part of the final grade since then.
Comments On the Evaluation of the Lab Activity:
The greatest value to us comes in knowing WHY students enjoyed or hated a particular lab. The hemlock lab is consistently
rated highly because: (a) it gives students a great deal of independence and a sense of personal ownership and accomplishment,
and (b) it gives students an opportunity to spend lots of time outdoors in the forest, and the vast majority of them really appreciate this.
In addition, extensive notes on how to conduct
formative evaluation are in the Teaching Resources sector
of TIEE under the keyword "Formative Evaluation."
Comments On Translating the Activity to Other Institutional Scales:
This investigation could be modified as a more structured class project to fit into 1 or 2 lab periods (all students do the
same investigation, or groups work on assigned sub-topics); it can also be used with a different “focal” tree species other than
To adapt this investigation for larger institutions (100 or more students involved), we would suggest the following modification,
intended to provide a more manageable structure. Normally, with larger classes there are multiple lab sections meeting more
than one day of the week. Instead of allowing every student group to come up with their own independent experiments, link
particular experiments across 2 or 3 different lab groups that meet on different days of the week (e.g. a Monday and a Tuesday
group will work on soil pH; a Monday and a Tuesday group will work on the light environment, etc.). Allow small amounts of time
during lecture (2 or 3 times during the course of the project should be sufficient), when all students are together, for counterpart
teams (the Monday and Tuesday groups working on the same experiment) to meet, share notes, agree on protocols, etc.
This will double the amount of data that is generated for each experiment, while still encouraging full participation by all members
of all groups because the working group size, per se, is not changed (Monday teams collect data on Mondays and Tuesday teams
collect data on Tuesdays, etc.). Having teams double-up on experiments, however, creates a much smaller drain on the amount
and variety of resources that will be used, and the number of different experiments that the instructor must supervise. Another
benefit of this approach comes in the experience that counterpart groups will gain in collaborating with each other by discussing
protocols that are acceptable to both teams, coordinating work schedules, maintaining consistency in measurement techniques,
For high school students, or other classes in which the students have had less preparation in designing their own experiments,
we suggest that the instructor add more structure to the experimental portion. That is, the instructor may want to create a list of
possible questions to investigate, and allow student groups to select from that list. In this way, the students still have a choice in
what question topic may interest them the most, but the instructor will determine the relevant questions and the level of ambition
and experimental rigor required to answer them.
For institutions with little or no access to forested sites, consider a similar investigation on the impact of invasive plant species on
the growth and distribution of native plants in a more disturbed plant community (regenerating farmland, overgrown fields, etc.).
Possibilities for focal invasive plant species might include Autumn Olive (Eleagnus umbellate), Tree of Heaven (Ailanthus spp.),
Myrtle (Vinca minor), or Purple Loosestrife (Lythrum salicaria).