Fieldwork is based on random sampling of the grassy area. Quadrats are used to record which plant species are present and how much of each plant species there is.
A quadrat is a square frame used for sampling an area. An appropriate size for sampling in grassland is 50cm x 50cm. Although ready-made quadrats can be purchased from educational suppliers, they can easily be constructed from canes or plastic tubes.
Random sampling means that every part of the sample area has an equal chance of being sampled. If enough samples are taken, the results should be representative of the whole sample area.
The instructions for using quadrats with classes are notoriously variable. Throwing a quadrat is not truly random – pupils may aim for conspicuous patches of plants, and they are unlikely to sample ground close to their feet and areas immediately behind trees. You may also be conscious of the dangers of pupils throwing quadrats at each other. A useful strategy that we have used is outlined below.
- Prepare two sets of numbered pieces of paper from 1-30. Put these into a bucket, or similar.
- Put the pupils into groups of 3 or 4.
- Lay two 20m measuring tapes perpendicular to each other in the study area.
- Invite a pupil from one group to take a piece of paper out of the bucket. Once the paper has been replaced, he/she should walk along the tape until reaching the distance, e.g. if 14 is chosen, walk to 14m.
- Invite a second pupil from the same group to do the same for the other tape.
- Then they turn into the plot at a right angle to the tape and walk into the plot until they meet. They should place the quadrat here.
- Instruct the pupils to wait in a queue for random numbers, and to return to the back of the queue when they have finished sampling.
Sampling within a quadrat
The simplest way for pupils to process the results of quadrat surveys is to count the number of plants that they find in each quadrat. Where each plant is a discrete individual, such as dandelions and daisies, this is acceptable. When all the results have been collected, pupils can calculate the mean number of plants per quadrat for each of the sample areas. For a greater challenge, pupils can transform the figure for mean number per quadrat into mean number of plants per square metre.
Many grasses and other grassland plants, such as white clover, speedwell and creeping buttercup, cannot easily be counted in this way, because the edges of each plant are not immediately obvious. Instead it is easier to calculate the local frequency of the plant in each quadrat.
Frequency is easier to measure using a gridded quadrat. In the example shown to the left, the quadrat has been divided into 100 squares. Pupils count the number of squares in which the plant is found. Since the red paper is found in 56 squares, the local frequency of red paper is 56%. Once the frequency of plants has been calculated for each quadrat, the mean frequency can then be calculated in the usual way for the whole sample area.
Measuring soil hardness
Soil hardness (or soil compaction) can be measured in the field by using a metal stake or pin. These can easily be made by using knitting needles. Hold the stake out at arm’s length above the centre of the quadrat and let it fall through the fingers. Measure the depth of entry into the soil. This may seem unscientific, but it really does work if pupils take care to let the stake fall from the same height above the quadrat each time. Alternatively, after a dry period when the soil is hard, ask pupils to apply uniform pressure to the stake at each measuring site and measure the depth it reaches into the soil. Clean the stake of soil before it is used for the next measurement.
The fold-out guide Playing Field Plants can be used to help with identification. We have found that it is sometimes useful with classes to spend a few minutes creating a ‘species board’. Choose the 5 commonest grassland plants (not grasses) growing at the time of sampling, and ask each group of pupils to attach with sticky tape a labelled sample of each plant (including flowers and leaves) to a wooden board. They can use this board for reference while collecting data from quadrats.