TERRESTRIAL PRIMARY PRODUCTIVITY
Introduction
All of the energy that enters living systems, with minor exceptions, is incorporated first
by photosynthesis. A measure of the amount of energy thereby made available to
consumers is the amount of plant biomass produced, or the net primary productivity.
(This is the total amount of energy fixed by producers minus the energy they require for
respiration.)
This exercise describes a method for estimating net primary production in a terrestrial
ecosystem in which the producer organisms are annual or perennial herbaceous plants.
The estimate obtained by this method assumes no significant decomposition of plant
material that dies during the interval between the initial and final sampling. (This is a
valid assumption, according to some researchers, for weedy fields or grassland
communities for periods of up to about a month.) The estimate is uncorrected for losses
of root material by death and decomposition, and for losses of shoot or root material as a
result of herbivory. Thus, the method works best if measurements are made during a
portion of the growing season in which a continuous increase of biomass of living plant
material is occurring, and in situations in which herbivore grazing is minimal.
Our biomass study will be done in two locations - at a grassy field, and in a stand of
kudzu or some other rapidly growing plant.
To prepare for this lab:
1. Read the entire lab.
2. Devise (in groups)a prediction concerning the primary productivity of the two study
areas relative to each other.
3. Devise data tables for recording your raw data.
Procedure
This experiment will be carried out in two stages.
Stage 1 During the first week we will mark, in each habitat (field and kudzu patch), two
plots that are 0.1 m2. At each site, remove dead shoot material from plot #1 and weigh it,
but leave all live material standing. From plot #2, living shoot and root material is
collected separately, and weighed. Root material is washed before it is weighed.
Once each group has weighed and recorded the wet-material data, combine material of
each type (by site, by plant part sampled) with that of other groups. Separate out 100 g of
the total wet material of each type and oven dry for about 48 hr at 100 0C, then obtain dry
weights.
6.1
Stage 2
At both sites, at the #1 plots, living shoot material, dead shoot material, and root material
are collected and weighed for wet-weights as before. As before, 100 g sub-samples are
oven dried and dry weights are obtained after 48 hrs.
Calculations
Vegetation varies in the amount of water it contains. The best way to compare primary
productivity between sites, then, is to compare the changes in the dry-weights rather than
in the wet-weights. The first task, then, is to transform wet-weight data into dry-weight
data. You know what 100 grams of each material when wet becomes when it dries. By
setting up proportions, you can calculate the dry-weight of each of your samples.
d = x
100 ww
where d = dry-wt. of 100 gm sample
ww = wet-weight of sample
x = dry-weight of sample
Calculate the dry-weights for each of the samples you collected.
For each site, calculate estimated Net Primary Productivity (NPP):
NPP = (b1 - b0) + d + (r1 - r0)
where,
b1 is the estimated dry-weight of living shoot material from the #1 plots (taken in 2nd
sample);
b0 is the estimated dry-weight of living shoot material from the #2 plots (1st sample);
d is the estimated dry-weight of dead material from the #1 plots
(2nd sample);
r1 is the estimated dry-weight of living root material from the
#1 plots (taken in the 2nd sample);
r0 is the estimated dry-weight of living root material from the
#2 plots (taken in the 1st sample).
ANALYSIS
Refer back to your initial prediction in discussing your results. Be sure to point out
possible sources of error, and how your use of certain initial assumptions may have
influenced your findings.
6.2