Name: ______________________________
Date: ________________
APES
Block: _____
LAB - Measuring Primary Productivity in Grass Plants
(adapted from Dr. Angela C. Morrow, University of Northern Colorado and
Molnar, Laboratory Investigations for AP Environmental Science)
Abstract
Grass plants grown over a one week period in the classroom are
used to calculate productivity rates using two methods of
collection. The concepts of net productivity, respiration rates,
gross productivity, and standing biomass are introduced in this
laboratory.
Objectives
 Measure net primary productivity (NPP) of rye grass, comparing NPP measurement
methods for reliability
 Explain the concepts of standing biomass, net productivity versus gross productivity, and
the importance of dry versus wet weight
 Apply the NPP concept to problems of crop growth and higher trophic level support
Introduction to Laboratory
Net primary productivity (NPP) is defined as the amount of carbon from the atmosphere that gets
added to green plants per unit time. It is a rate, the quantity of new vegetable matter added per
unit area, per day, per week, or per year. NPP is usually expressed as grams of carbon per
square meter per year (gC/m2/year). NPP is calculated as the total gain of biomass from
photosynthesis minus the losses due to plant respiration. It is this net gain that is available to
other organisms as food. The higher the NPP is in a region the higher the overall biomass and
diversity.
During the growing seasons tropical and temperate regions receive approximately 8,000 to 10,000
kcal/m2 each day. Of this energy, only a small amount (about 1-3 percent in the most productive
zones) will be trapped by green plants through the process of photosynthesis.
6CO2 + 6H2O + light energy => C6H12O6 + 6O2
Photosynthesis results in the production of glucose which can later be converted into other
products in the plant and provide for the growth of the plant. This results in an increase in
biomass. Another term, which is used to describe this process more quantitatively, is gross
primary productivity (GPP) – the amount of biomass produced by photosynthesis per unit area
over a specific time period.
Gross productivity can be measured indirectly using grass plants. Why indirectly?
The answer is due to the metabolic needs of the plant itself. That is, as the plant is producing
glucose through photosynthesis at least one-half of this glucose is used to meet the plants own
energy needs (cell respiration). So what is directly measured in this laboratory exercise is the net
primary productivity (NPP). In order to establish the GPP, another quantity must be determined –
that is the respiration rate of the plants.
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You will measure the new primary productivity (NPP) as an increase in dry weight over a one
week period.
Materials
 Flats (approximately 20 x 40 cm) for each laboratory group
 Flats should be sown with grass seeds and grown to approximately 2-3 cm (about 2 weeks)
in height before the experiment. Grass should be watered thoroughly at least one hour
before each data collecting session
 Potting soil
 Grass seed
 Balances
 Scissors
 Fertilizer (?)
 Light Sources
 Water
 Drying oven
 Aluminum foil
 Spoons or indoor garden spades to remove grass plants
Procedure
Your assigned task is to calculate the net primary productivity of one week of grass growth using
two methods: (A) complete plant removal and (B) clipped grass. The basic instructions for each
method are listed below. Note that classroom experimentation will prevent your group from
collecting all the grass in a 1m2 plot, therefore, the data you collect will need to be converted.
Your raw data will be in units of grams of carbon from a few plants over a few days; this data will
need to be converted into units of gC/m2/year.
A. Complete Plant Removal (CPR) Method
1. Trim your grass plots to approximately 2-3 cm. Discard the grass clippings.
2. Take five complete plants, roots and all, from the soil, removing as much soil as possible.
3. Place the plants on an open container made from aluminum foil, and put it in the drying oven
for 48 hours at a temperature between 90C and 95C
4. In the section of the tray from which you took the samples, count the number of plants in a 5
cm by 5 cm area. Record this count in your lab notebook.
5. After the plants are dry, weigh and record the mass. This is the starting dry mass. Record
this measurement in your lab notebook.
6. Allow the remaining grass to grow for one week more, watering when necessary.
7. Remove five more plants and repeat the drying and weighing procedure. This is the final
dry mass. Record this measurement in your lab notebook.
8. Find the difference in dry mass. Record this measurement in your lab notebook. This
number is in grams of added carbon per five plants. Scale this value up by using the
number of plants growing in 25 cm2. Then convert units of cm2 to m2 and convert the
number of days between samples to years. Use your lab notebook to record and calculate
these values.
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9. Calculate the net primary productivity in units of gC/m2/year. Record this value in your lab
notebook.
A. Clipped Plant (CP) Method
1. Trim your grass plots to approximately 2-3 cm. Discard the grass clippings.
2. Harvest 15 plants, all from the same area in the tray, by cutting the plants 0.5 cm above the
soil.
3. Place the plants on an open container made from aluminum foil, and put it in the drying oven
for 48 hours at a temperature between 90C and 95C
4. In the section of the tray from which you took the samples, count the number of plants in a 5
cm by 5 cm area. Record this count in your lab notebook.
5. After the plants are dry, weigh and record the mass. This is the starting dry mass. Record
this measurement in your lab notebook.
6. Allow the remaining grass to grow for one week more, watering when necessary.
7. Harvest an additional 15 plants by clipping the plants to 0.5 cm above the soil. Repeat the
drying and weighing procedure. This is the final dry mass. Record this measurement in
your lab notebook.
8. Find the difference in dry mass. Record this measurement in your lab notebook. This
number is in grams of added carbon per 15 plants. Scale this value up by using the number
of plants growing in 25 cm2. Then convert units of cm2 to m2 and convert the number of
days between samples to years. Use your lab notebook to record and calculate these
values.
9. Calculate the net primary productivity in units of gC/m2/year. Record this value in your lab
notebook.
Data and Observations
Data will consist of:
1. Measurements
2. Calculations
3. Graph or tables
4. Written description of the data
Introduction, Analysis/Discussion & Conclusions Questions
The following questions will help with your introduction as well as the analysis/discussion &
conclusions:
1. What is the definition of net primary productivity? Why is this rate such an important
piece of information for ecologists? (Introduction)
2. Compare the NPP figures from the entire plant method to those of the clipped plant
method. Explain why the figures are the way they are. Are they as you would
expect? Why or why not? (Discussion)
3. Which method appears to give the most reliable data? Why? How could you find a
valid way to compare data from the two methods? (Discussion)
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4. Why are the results expressed as net productivity instead of gross productivity?
What is gross productivity? (Introduction &Discussion)
5. Grass seed could have been replanted on the plot from which the grass was
removed in the CPR method and the amount of new plant growth compared and
recorded as productivity. Comment on the reliability of such data. (Discussion)
6. Suppose the rye grass plants continued to grow at the same rate over a six month
growing season. What would be the net productivity for a field that is 1 km 2?
(Conclusions)
7. Comment on the availability of energy from the plant, is if is consumed by herbivores
after the six month growing season. Explain how energy availability affects the
timing of farmers’ harvest of their grain crops. (Conclusions)
8. In the eastern United States, in particular from Massachusetts to Virginia, climax
hardwood forests are being rapidly replaced with large, single-family homes and
townhouse complexes. Based on the outcome of this investigation, explain one
reason why the deer herds in those states have drastically increased. (Conclusions)
9. Design an experiment by which you could measure the net primary productivity of a
field in nature. Describe the experiment. (Conclusions)
a. How could NPP be used to determine the number of herbivores that could be
supported by the field? For example, how many cows could feed there?
b. How would you estimate the number of trophic levels that could be sustained in
the field?
10. Describe how remote sensing could be used to determine the NPP of an area.
(Conclusions)
11. Standing biomass is the organic matter of the living organisms in an area. Due to the
movement of animals, this term is most often just associated with the plants of an
area. The terms net productivity and standing biomass are often mistakenly used
interchangeably. Why would these terms not be interchangeable?
Background research information links
PhysicalGeography.net: Primary Productivity of Plants
http://www.physicalgeography.net/fundamentals/9l.html
Additional Reading Sources
Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center’s (DAAC)
http://www-eosdis.ornl.gov/
Net Primary Productivity Estimation Using Remote Sensing
http://wiki.landscapetoolbox.org/doku.php/remote_sensing_methods:net_primary_productivity
Net Primary Productivity Database
http://www-eosdis.ornl.gov/NPP/npp_home.html
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