Fleming, 2014, plant ecology research lesson, S2 Lab 8

Lab:Competition and Biomass Production, pt. 2

  1. Retrieve your flats. Look carefully at the plants and familiarize yourself with the appearance of different species. Refer to your notes to remind yourself of what you actually planted. For the high-diversity treatments, have each group member identify individual plants separately, then make sure you all agree on what you’re actually seeing.
  1. Observe each flat from the side. How many layers of plants are there? (A “layer” occurs when leaves are relatively dense and form a canopy of sorts.) Is there a low layer of leaves near the soil surface? A medium layer of leaves higher up? A top canopy layer? Each member of your lab group should estimate the number of layers when viewed from the side. Then calculate and record the average of the group’s observations on the data sheet.
  1. Observe the plants from above. Can you see any bare soil, or do leaves obscure most of it? Each member of your lab group should visually estimate the percent of soil covered by leaves when viewed from above. Then calculate and record the average of the group’sobservationson the data sheet.
  1. Count the number of individual plants of each species. For each flat, harvest the plants by cutting them off at the roots. If they are present, include the taproot swellings that will become radish or carrot. Grasses can a bit tricky because multiple stems can grow from a single set of roots. Pull up these plants before counting individuals, and then remove the roots. Do not count dying leaves that have dropped off as separate individuals, but do add the leaves to your measurement of biomass. Divide the plants into piles by species, wrapping each pile in a damp paper towel and place it in a paper bag labeled with the treatment and species name (ex: A4polyculture, carrot). Count the number of individuals in each pile and record these numbers in the data sheet. Add up the numbers for all species in a flat to calculate total number of germinants in that flat.
  1. Measure the above ground biomass (fresh weight) of each species. Be sure to “tare” the balance by zeroing it with a weigh boat on the balance. Transfer a pile of plants to the weigh boat and wait for the numbers on the balance to stabilize. Record the biomass (in grams) in the data sheet. Transfer the pile back to the paper bag and repeat for as many species as you have for each flat. Add up the biomass for all species in a flat to calculate total biomass for the flat.***SAVE YOUR PLANTS under a damp paper towel until all your data are recorded!
  1. For later analysis, you might calculate the average biomass per plant for each species in a flat. Two other measurements or calculations mayalso prove useful. Average cover by species and the germination rate are probably useful statistics to compare later. These values might vary across treatments.
  1. Reporting your data:Transfer your data to the template file in MS Excel and email it to Dr. Fleming, who will make a master file for the class. It will then be possible to plot data for the entire class. First, however, think about graphical presentation of the data. Remember that in this lab session we are interested in the relationship between biodiversity (plant species richness) and ecosystem function (biomass production).

What is the independent variable?
What is the dependent variable?

A best-fit line through this cloud of points is one that makes the distances between each point and the line as small as possible, and predicts the average change in “y” as a function of the change in “x”. You should display such “best fit lines” and theirrespective r2and significance values.What is your general sense of these relationships? Positive, negative or no trend?

State the expected result(s) of this experiment.

Analysis and Discussion

It’s easy to see why adding individuals (more seeds per pot) would result in more biomass, at least up to a point. What do you suppose eventually limits plant growth, so that even carpeting a pot with seeds results in no more total biomass than a gardener’s best spaced planting? It’s a bit trickier to figure out why adding species would result in more production (assuming that it does!). Some hints: Are interactions between members of the same species equivalent to interactions between members of different species? Might this outcome depend of how many functional groups are present? Which interactions might be more negative? Are there possibilities for positive interactions, such that plants stimulate each others’ growth? What is more important, species richnessor the number of functional groups?

Clean-up Checklist

After you harvest plants, stack the flats on the front table of the greenhouse (by the empty pots and garbage cans).

In the lab room, put harvested plants and paper bags in the bucket for composting.

Don’t leave anything on balances. Be a good steward of the greenhouse and lab room, and leave it cleaner than you found it.

Testing Hypotheses About Plant Species Richness and Biomass Production: The Roles and Interactions of Particular Species

Each lab group must choose a hypothesis to test using the master data set. Discuss this hypothesis with each other and, after discussing your ideas, fill out and turn in the form on the following page. As an aid to thinking about the kinds of questions you might ask, I have included a sample of the final spreadsheet in the section on Analyzing Your Data.

Recall that there are multiple replicates of functional groups of the pots you counted and measured. In addition, there are numerous replicates in terms of number of species (achieved through different compositions). Select the relevant cases, and use these to test your hypothesis (e.g. clovers in polycultures will have greater biomass per individual than clovers grown in monoculture). You will have work time today to create graphs and interpret how they bear on your hypothesis. Each lab group will have 10-15 minutes next week to present their findings to the rest of the class.

NOTE: Hypotheses may be general (ex: growing clovers in monocultures versus polycultures will result in differing levels of biomass of clovers), directional (ex: clovers in polycultures will have greater biomass per individual than clovers grown in monoculture), or articulated to test specific variables/mechanisms (ex: clovers in polycultures will have greater biomass per individual than clovers grown in monoculture because of facilitation of clovers by grass-like plants). In this lab we are not testing specific mechanisms by which plants are more productive when grown in polycultures. Rather, we are simply testing the hypothesis that this relationship exists.

Students in Group ______

Question that you wish to explore
Hypothesis
How does your hypothesis relate to any paper we have read in class, or any lecture material you can recall?
Predictions that you make from your hypothesis.
What tests of data do you plan to use?

What graphed data should look like, if your hypothesis is supported?

Analyzing the Data

How to organize the data:

Your hypothesis probably falls into one of two basic categories. First, you may be interested in the relationship between continuous variables (species richness; density per species or total density; biomass per individual, per species, or total biomass, number of functional groups, etc). Second, you may be interested in comparing different treatments (species or functional group combinations) with biomass/productivity.

All the data will be sorted in an Excel spreadsheet. The sample below shows a possible selection of the data available (some columns omitted).

Leafy herbs / Nitrogen fixers
Lab group / Plant
/flat / Lettuce # / Lettuce biomass / Alfalfa
# / Alfalfa
biomass / Total # of plants / # fxnl groups / # species / Total biomass / % cover / Layers
D / 2 / 0 / 0 / 29 / 7.44 / 63 / 2 / 2 / 28.77 / 46 / 2
D / 2 / 0 / 0 / 22 / 11.13 / 53 / 2 / 2 / 52.5 / 65 / 2
D / 4 / 18 / 26.86 / 15 / 7.21 / 72 / 2 / 4 / 95.04 / 85 / 3
D / 4 / 20 / 12.68 / 9 / 3.89 / 63 / 2 / 4 / 48.53 / 60 / 3
D / 8 / 8 / 9.28 / 8 / 1.47 / 63 / 3 / 8 / 83.21 / 72 / 3
D / 8 / 9 / 8.67 / 7 / 2.35 / 57 / 3 / 9 / 76.08 / 60 / 4
D / 1a / 0 / 0 / 0 / 0 / 60 / 1 / 1 / 95.47 / 81 / 2
D / 1a / 0 / 0 / 0 / 0 / 72 / 1 / 1 / 94.01 / 70 / 1
D / 1b / 0 / 0 / 58 / 42 / 58 / 1 / 1 / 42 / 63 / 2
D / 1b / 0 / 0 / 55 / 42.9 / 55 / 1 / 1 / 42.9 / 55 / 1

You may use Excel or any other software you know to organize, analyze and graph your data. Think about ways to summarize data that you have learned in class. Can you present significance results and report p-values, show r2 values of regressions, and use ANOVA effectively? Let’s see about your stats skills!

Presentation Guidelines (due next lab session)

You will have 10-15 minutes to present your data analysis to the class. Each person in the lab group should participate in the presentation; you will all receive the same number of points.

Hypothesis / What question are you seeking to answer? Why were you interested in this question?
Data / Which data did you select to examine the hypothesis? How did you analyze the data? Describe the results of your analysis.
Discussion / Interpret the results in terms of their interactions between and within species (if applicable). How do they inform ecological thinking about mechanisms leading to positive relationships between biodiversity and ecosystem function?

Name ______

Biodiversity and Ecosystem Functioning

Lab presentation: Each lab group will present a short oral report that summarizes their interpretation of lab results. Include all graphs generated that are pertinent to your interpretation. The following template may prove useful to help organize your thoughts.

Hypothesis:
Prediction:
Data used:
Results: Describe your results and include relevant graphs.
Interpretation of results: What do the results tell you about species interactions? How do they bear on mechanisms for a positive relationship between biodiversity and production? How do your results compare and contrast with results from other groups? How do your results compare and contrast with lecture content from class, and/or papers we have read?

Lab Group Names ______

Lab Group ______

ONE SPECIES
(Name) / % cover / # layers / # of individuals / Biomass (g) / Evidence of Herbivory (describe)
1a.
1b.
TWO SPECIES
1.
2.
TOTAL
FOUR SPECIES
1.
2.
3.
4.
TOTAL
EIGHT SPECIES
1.
2.
3.
4.
5.
6.
7.
8
TOTAL

Turn in one copy of this page with the combined data for your group.