APES Measuring Net Primary Productivity Laboratory
Gross Primary Productivity (GPP) is a measure of the rate at which an ecosystem incorporates
carbon from the atmosphere into biomass in a given length of time (day, week, month, etc.) In
contrast, the Net Primary Productivity (NPP) is a measure of the rate at which all plants in an
ecosystem produce net useful chemical energy. This net useful energy is the difference in the
GPP and the energy used for cellular respiration, or basically what is left for primary consumers.
NPP = GPP – cellular respiration
The overall NPP of an ecosystem is dependent upon many variables including climate,
biodiversity of consumers and at the very least, human activity. The higher the NPP, the higher
the overall biomass and diversity of an ecosystem. Biomass is the sum of all living organisms
in a given area or at a given trophic level, usually expressed in terms of living or dry mass.
Your experiment report should include results with a written explanation, calculations,
graphs and answers to the discussion questions.
Materials:
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Grass Plants in flats
Aluminum Foil
Filter paper
Drying area (heat lamp in fume hood)
Scissors
Ruler
Balance
Procedures:
1. Using a flat of grass, separate out three uniform samples and label each as A, B and C.
Measure the area of the plots and record. Measure and record the grass length from the
top of the soil to the tip of the grass in each of the plots.
2. In plot A, cut the grass to a height of 1 cm. Weigh the grass clippings, record and place on
filter paper in the drying area. Cover the grass plants in plot A with an aluminum foil cover
and leave to grow.
3. In plot B, cut the grass to a height of 1 cm. Weigh the grass clippings, record and place on
filter paper in the drying area. Leave the grass plants in plot B uncovered and leave to
grow.
4. In plot C, simply leave the grass untrimmed and uncovered.
5. Allow all samples to grow for up to 1 week.
6. Uncover the plants in plot A. Observe and record the appearance of the plants. Measure
the height of the grass. If it has grown, trim the grass to a height of 1cm. Weigh the
clippings, record and place on filter paper in the drying area.
7. Measure the height of the grass in plot B. Trim the grass to a height of 1cm. Weigh the
clippings, record and place on filter paper in the drying area.
8. Measure and record the height of the plants in plot C. Trim the grass to a height of 1cm.
Weigh the clippings, record and place on filter paper in the drying area.
9. Allow the clippings to dry for 24-72 hours, reweigh and record the mass. Complete the
calculations and questions below.
Calculations:
For each of the plots, use the following formulas to calculate NPP. Use units and show ALL of
your calculations.
Dry weight of Final clippings from Plot B
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Dry weight of initial clippings
Net Primary Productivity
Dry weight of Final clippings from Plot A
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Dry weight of initial clippings
Respiration
Net Primary Productivity + Respiration = Gross Primary Productivity
*Recalculate the NPP and GPP of your samples using the untrimmed sample (Plot C) instead of
Plot B.
Questions:
1. What do the plots with the foil covering represent?
2. How would variations in respiration rates change your results? Under what conditions
would you expect the plants’ respiration rate to increase? Decrease?
3. Compare the difference in appearance between the foil covered and uncovered grass. If
there is a difference in appearance, explain the difference.
4. If there was growth in the covered plants, why would growth have taken place in the
absence of sunlight?
5. What units should be used to express productivity?
6. In your calculations, was there a significant difference between the dry weights and the wet
weights? Which of the two calculations would provide you with a more correct rate for
gross productivity? Why?
7. How would you minimize this difference in the field?
8. Using the data from your entire class, draw a graph showing the results of the respiration
rate, net and gross primary productivity experiments from Plots A, B and C.