Net Primary Productivity
INTRODUCTION:
The primary productivity of plants can be measured as an increase in dry
weight over a specified period of time. This increase in mass comes from the
carbon which the plants fix and convert to carbohydrates. See your text for more
information on productivity. In this lab you will let plant grass seed, let the grass
grow, clip it, and then let the grass plants grow (after clipping) for a certain
number of days, and determine the biomass produced during that time. You will
use this data to calculate net primary productivity.
PROCEDURE:
You will each plant a “plot” of grass in identical pots.
Determine the actual growing area (not volume)of your pot with a ruler (show
calculations, and keep significant figures). Label your pot.
Plant your pot using the amounts and method demonstrated by your teacher.
Place it in the tray.
When the grass seed has germinated and grown sufficiently, clip the grass
blades of each plant down to be level with the top of the pot. Discard the
clippings.
Allow the grass to continue growing in the pot for an additional week or time
period determined by your class. At the end of that time clip the grass in the pots
down to the same height again, and dry it in the drying oven. Keep the grass
from each pot in a separate pre-weighed drying container.
Keep track of all of your measurements and calculations in your lab notebook.
Helpful relationships:
100 Cm = 1 m, (100 cm)2 = (1m)2, 104cm2 = 1m2, 1 cm = .01 m
1000 m = 1 km, (1000 m)2 = (1 km)2, 106 m2 = 1 km2,, 1m = 0.001 km
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DATA: (give units for all measurements!)
Mass of grass seed planted _________________,
Date Planted __________
Date Trimmed first time_______
Date trimmed second time_________
Dimensions of pot _________________
Weight of weighing container ________________
Weight of dish + clippings after drying ____________
Biomass of clippings _____________
PRODUCTIVITY CALCULATIONS: show your work and keep the correct # of
significant digits throughout. Use scientific notation. Use your data to do the
calculations.
Biomass of clippings
Time period (days)
Biomass gained per day
kcal gained per day*
Area of pot in cm2
Area of pot in m2 **
Area of pot in km2
Net productivity
(kcal/day/square meter)
Class average for net
productivity
*There are 3.6 kcal/gram dry weight
**Hint: use unit analysis to do conversions, ask Mrs. Soos if you need help with
this!
Show your work for all calculations, use scientific notation, and include units.
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DISCUSSION/CONCLUSION:
1. Define net productivity and gross productivity.
2. Why are the results of this lab expressed as “net productivity” rather than
“gross productivity”?
3. Taking your own productivity calculation, use it to calculate the net
productivity for a grassland composed of your grass plants over a 12-week
summer period. Assume the grassland is one square kilometer. (Hint: apply
your knowledge of dimensional analysis you learned in chemistry to keep
track of your units. This is the same sort of process you used to convert cm2
to m2.) Show your work.
4. Repeat your calculations for the average biomass of clippings for the entire
class.
5. Compare your calculations to the data in the book for grassland productivity.
What is the % difference between your data and that given in the book?
(Show your work.) What is the difference between the class data and that
given in the book?
% difference = (accepted value - your value) X 100
accepted value
BONUS: Propose a method to measure grass gross productivity in this lab.
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