First, you should start by asking students questions about how to set up the experiment. Suggested questions include:
- Where should we locate our experimental plots?
The experiment should be located in an area where the plots will not be disturbed by students or others. It also should be located in as flat an area as possible to minimize water runoff into other plots.
- Do you want any distance between controlled and watered plot?
If the plots are physically side-by-side, water from the experimental plots could easily run into your control plots. If your grassy area is hilly, water may run from the watered plots into the control plots. In either case, the additional water in the control plots would bias your data.
- What is the purpose of the control plots? Why are they so important?
Control plots are for comparison. Controls serve as the “normal” condition against which you measure the experimental condition. In this case, the control grassy plots will grow as they “normally” would without any additional water. The experimental grassy plots will receive more water than normal. Your students will measure any changes between plant growth in the control and experimental plots.
Next, determine the experimental layout and how to run the experiment (see Figure 1).
- Determine layout
- Determine watering regime (who waters the plots, how often, how much, for how long, etc.)
- Number your plots 1-12 for data collection
Figure 1: One Potential Experimental Layout
- Run the experiment according to the watering regime you have determined
- Collect data
- You should assign 2-3 students to be in charge of the data collection for one plot.
- Have students clip the plants from their plot with scissors at dirt level (students are only collecting the portions of the plants above ground).
- The students should separate their clipped plant material for each individual plot into two piles: grass vs. non-grass. The students should then place each pile into a small, brown paper bag and label each bag with the plot number and if the bag contains grass or non-grass material. If the suggested layout is used, there should be 24 bags at the completion of the sorting.
- You should dry all of the plant material in the bags in a drying oven. If you do not have access to a drying oven, the bags can be placed in a warm, sunny spot and the material can dry naturally for about two weeks.
- When the plant materials are dry, the students should weigh their samples—both grass and non-grass—and record their measurements. The students should share their data with the rest of the class so that everyone has data from all of the plots.
- Have the students graph the collective data (the weight of the grass/control, grass/watered, non-grass/control, and non-grass/watered) and make comparisons in terms of how the water addition affected growth overall, as well as potential grass/non-grass interactions (competition, etc.).
Variation on a Theme
If no grassy area exists at your school or you cannot get permission to use your school grounds to run this project, you could run the experiment in pots outdoors or in a greenhouse. This could be done in several ways.
- Twelve clay pots, plastic pots, or some type of growing container could be arranged using the layout highlighted on this sheet. If you are doing this outdoors, you could use garden soil. However, in a greenhouse, sterile potting soil would probably be best. If using garden soil, mix the soil thoroughly before placing it in pots. The soil in the pots should be as uniform as possible. Sprinkle a 50%/50% mixture of grass and clover seed, purchased from a local garden center, into the pots. Plant the same amount of seed in each of the 12 pots. Allow the plants to become established (plants are at least 2 cm high). Run your experimental watering regime and collect data as described above.
- Seed from other plants could be used for this experiment too; however, we would suggest a grass species (annual or perennial, early or late season, native or domesticated) in combination with a nitrogen fixer (clover, beans, peas, native species). These are two important functional plant groups (see the video for more information).
- The seed mixture percentage used could be varied, too. If you have a local grassy area, check the grass/nitrogen fixer ratio and plant your seed in the pots based on the observed ratio.
- Pot arrangement can vary. Rather than use the layout described, arrange your pots using a randomized, block design. This would give you the opportunity to discuss experimental design with your students.
- Repeat these experiments on a yearly basis. Collect data from each year’s class for comparison.
This teacher sheet is a part of the Simulating Climate Change Research in Grasslands