Environmental Engineering Reference
In-Depth Information
and McDowell 2000
) representing on the order of 0.1% to 10% of NPP in terrestrial ecosys-
tems. Export can represent a much larger and even dominant fate of primary production
in streams, rivers, and estuaries. In these flowing-water ecosystems short water residence
time and periodic high discharge limit the development of phytoplankton and will often
scour attached algae and aquatic plants transporting NPP downstream. Finally, some of
the NEP can be oxidized back to CO
2
by abiotic processes such as by fire in terrestrial eco-
systems or exposure to ultraviolet light in aquatic ecosystems. Overall, the nonrespiratory
fates of primary production must be considered in an accounting of ecosystem carbon
budgets, particularly when considering whether an ecosystem is a net exporter or sink for
organic carbon as well as CO
2
.
A TALE OF SCALE
On a warm summer afternoon in a forest, scientists are busy studying primary produc-
tion. A physiological ecologist, working from a tower in the canopy, has enclosed a leaf in
a chamber and is measuring the leaf CO
2
exchange. On the same tower, but above the can-
opy, a micrometeorologist's eddy covariance system measures the CO
2
exchange of the
entire forest. Meanwhile, on the ground, a plant ecologist checks baskets that collect litter-
fall and measures tree diameters to compare with measurements made in the same stand
five years earlier. Nearby a geologist is surveying with specialized equipment looking for
primary production from an ancient fern forest that is now sequestered in coal seams
deep in the ground. The geologist suspects that storage of an era's worth of primary pro-
duction now compressed and converted to coal will provide a significant energy resource
for humans.
All of these measurements are attributes of primary production, but how are they
related to each other? The physiological ecologist is measuring net photosynthesis of the
leaf in her chamber, including the leaf's photosynthesis and respiration
.
To scale this up to
the annual NPP of the whole forest, she would have to measure (or model) the net photo-
synthesis of all the leaves in the canopy for the entire year, subtract the respiratory losses
for the nonphotosynthetic parts of the forest (e.g., stems, branches, and roots), and express
the results per unit ground area rather than per unit leaf area. Needless to say there is a
lot of uncertainty in such scaling.
The micrometeorologist is measuring NEE, which includes GPP and respiration of both
autotrophs and heterotrophs (
R
e
). Integrated over a year, the NEE is equivalent to the
annual NEP barring significant forms of exchange that are not measured. If the micromete-
orologist estimates ecosystem respiration from his nighttime measurements, he may be
able to calculate GPP, but not NPP. To compare his measurement with other GPP and
NEP measurements, he would have to know the upwind area over which the flux tower
integrates, and be certain that the other measurements are representative of the same area.
The plant ecologist is measuring accumulation of plant material in above-ground woo-
dy biomass and production of fine litterfall. To calculate above-ground NPP, he would
also have to account for tree mortality, coarse litterfall (e.g., fallen branches), and losses
due to herbivory and canopy leaching. To calculate total stand NPP, he would have to
account
for below-ground root biomass accumulation, root detritus production, and
