Teaching Approaches

There are three subsections for each dataset: 1) Methods and rationale, 2) Open-ended questions, and 3) Detailed graphing instructions. The first subsection describes the main methods and why these measurements are of interest. The instructions for making figures are for Excel 2003.

For the second section, a set of open-ended questions are provided. These are designed to give students the opportunity to look at the data with questions in mind, as scientists do. Too often students blindly make graphs without thinking about the types of figures that would be most meaningful. Students could also examine the data and ask their own questions; you can use the ones provided to prompt further thought. In either case, you will likely need to keep asking "how can you use these data to get at that question?" and "what type of graph bests presents the data to address that question"? After students have outlined the types of graphs they would like to make, you can provide the detailed instructions for making figures.

Students should work in groups. See "cooperative groupwork" links in the Teaching section. Beginning students will benefit from first sketching out ideas for figures; this way they will more carefully think about "what goes on what axis," for example. One challenge with this exercise is that students' abilities with Excel vary and therefore some will need more time than others to work with these data. You could have a separate Excel session, perhaps with teaching assistants if you have them, to bring everyone up to the same level.

The Student Instructions begin with a very basic overview; this is written for beginning ecology students. Before providing the specific graphing instructions you might suggest that students hand draw sketches of patterns they might expect to see in this experiment. This would be appropriate for students with little experience working with Excel figures. More advanced students should skip this step and the basic overview.

The time needed depends in part on student experience. Students in a plant ecology class could work with all four data sets in a lab session plus homework assignment. For beginning students, the first two data sets might be sufficient.

The Data and Experimental Design

The Excel directions are for the 2003 version of the spreadsheet. The resource section below provides directions for older versions.

The data selected for this exercise illustrate how chronic N additions have impacted a range of ecosystem properties and processes, including both plant (tree productivity and photosynthetic capacity) and soil (microbial biomass and soil respiration) responses. There are four datasets with which students can work. Each dataset is located on an individual worksheet page within the Excel spreadsheet (see Datasets section). The faculty dataset includes the figures generated by following the detailed graphing instructions in the student section (see Student Instructions). While each dataset can be used as a stand-alone activity, the students will gain the greatest understanding of how excess N impacts forest ecosystems by working through the activities for all four datasets. The final activity (written report or PowerPoint presentation) is designed to help students synthesize all of the information presented in the datasets and to think more broadly about how northeastern forests are being impacted by N enrichment.

This experiment provides a great opportunity to discuss the issue of replication and "pseudoreplication" in ecology. Over 20 years ago Hulbert (1984) published his well known paper titled "Pseudoreplication and the Design of Field Experiments" (Ecological Monographs 54: 187-211) which stimulated a good deal of discussion among ecologists. Central to the argument is the scale needed for replication — in the case of the Chronic N Amendment Study, you might ask your students whether taking measurements within one N treatment plot is sufficient or whether it is necessary to establish replicate plots in different forest stands. Again, the depth of discussion about this issue will depend on your time constraints and the experience of your students.

You may be surprised to learn this, but reasons for replication are a major conceptual challenge for many students; therefore discussion about this issue is especially important. The main points to bring out are: why replication is important, the relationship between reliability of conclusions and replication, why replication of large plots (30 x 30 m in this case) is not always feasible (e.g., cost and logistical constraints), and the design of this particular experiment and why scientists might disagree about replication in this case. The goal is not to criticize the Chronic N Addition researchers (some students who have not read the core papers will be very quick to do this!) but to use this dataset to stimulate this important discussion.

Dataset 1: contains average values for aboveground net primary productivity for 1988 through 2002 as g m-2. NPP for the pine stand was highest in the control plot and decreased 31% and 54% relative to the control for the low N and high N plots, respectively. The hardwood stand responded differently with the fertilized plots showing higher NPP relative to the control.

Dataset 2: gives the average values and standard errors for the photosynthetic capacity of red pine needles collected from the control and high N plots. Photosynthetic capacity of needles from the high N treatment was significantly lower than that of the control trees.

Dataset 3: contains average values and standard errors for cumulative soil respiration (g C m-2) during the growing seasons of 1988, 1989, and 2001. In the hardwood stand, soil respiration was higher in the fertilized plots compared to the control. However, in the subsequent two years (1989 and 2001), respiration was lower in the high N plot relative to the low N and control plots. In the pine stand, soil respiration was lower in both fertilized plots for 1989 and 2001.

Dataset 4: presents the raw data (as opposed to averages as in the first three datasets) for bacterial and fungal biomass in soils collected in fall 2002. Students are first instructed to calculate the means and standard errors before preparing the graphs. This provides them with a better understanding of how data are typically analyzed. Fungal biomass was 27-61% and 42-69% lower in the fertilized compared to control plots in the hardwood and pine stands, respectively. Bacterial biomass was not greatly affected by N additions.

Synthesis Activity

Students are given a choice of synthesizing the information in the four datasets either in a written report or by making a 15 minute PowerPoint presentation. Some students excel at writing, while others are more comfortable with oral communication. Both skills are of course important, but giving the students a choice will allow them to focus their attention on data comprehension and interpretation rather than on the presentation format. Choice also gives students a sense that they have some control over the evaluation process. Be clear at the outset as to how each of these activities will be evaluated. Below I provide one example of how the written report can be evaluated. This can be easily modified for the PowerPoint presentation. For either, give students the criteria (rubric) ahead of time that you will use to judge their work. See also "rubrics" in the TIEE Glossary.

The written report will be graded according to the following criteria:Points
Does the introduction present the problem and objectives clearly?_____(15)
Do the figures present the data clearly?_____(20)
Does the written portion of the results section accurately reflect the data as presented in the figures?_____(25)
Are the interpretations and overall conclusions supported by the data?_____(25)
Is the report well organized and well written?_____(15)

Student Evaluation

Students can be evaluated on a single dataset in terms of the questions they pose and figures they suggest, their understanding of the question being addressed, their ability to properly prepare (e.g., correctly labeled axes) and to accurately interpret the graphs, and their understanding of the main conclusions that can be drawn from that particular dataset. Evaluation can be based on either written or oral communication, or some combination of both. The written report or PowerPoint presentation (synthesis activity) will provide information on all of the above, in addition to evaluating the studentís general understanding of N deposition, why the Chronic Nitrogen Addition Experiment was conducted, and his/her ability to synthesize several disparate pieces of information and to draw conclusions about how N enrichment has impacted two different forest stands at Harvard Forest. Students should be presented with the evaluation criteria at the time the assignment is made.


Excel Tutorials

Please note — there are many versions of Excel; the sites below include tutorials for older versions.