Environmental Engineering Reference
In-Depth Information
substantial effect that a single species, either native or introduced, can sometimes have on
the food web structure and patterns of biogeochemical cycling in an ecosystem.
A Temperate Grassland
The Great Plains covers about 2.6
10
6
km
2
, making it the second largest ecozone in
North America. The vast size of the North American Great Plains and those of other grass-
lands around the world make them important for the global C budget. Some have argued
that grasslands, globally, might account for as much as 0.5 Pg C/y of the so-called missing
terrestrial C sink (
Scurlock and Hall 1998
). Because of the large values of both GPP and R
in grasslands, small changes in the balance between GPP and R can affect C sequestration
into soils at a globally significant scale (
Schimel et al. 1994
). Physically, grasslands are well
suited for measurement of gas exchange with the atmosphere because of their relatively
uniform plant height. The use of net ecosystem gas exchange (NEE) was discussed as a
technique for measuring primary production in Chapter 2 as is particularly suited to this
kind of ecosystem. NEE is for nearly all purposes identical to NEP (see Chapter 2).
NEE would include both the biotic (respiration and photosynthesis) as well as abiotic
(weathering, carbonate precipitation) reactions, and so it is slightly different from NEP.
Several grasslands are part of a network of ecosystems with exchange towers called
Ameriflux (
http://public.ornl.gov/ameriflux/
)
. At one of the network sites (in Alberta,
Canada), the dominant grasses are in the genus Agropyron and the dominant forb,
Tragopogon (
Flanagan et al. 2002
). Using eddy covariance during both daylight and night,
researchers were able to calculate GPP and total ecosystem respiration (R) nearly continu-
ously for a several-year period (
Figure 6.7
). As in many grassland regions, rainfall is
highly variable and exerts profound controls over plant biomass, gross productivity, and
respiration. In this system leaf area index (the total area of leaves above a unit area of
ground) ranged from 0.2 m
2
/m
2
when soil moisture was low, to as much as 8 m
2
/m
2
.
Both GPP and NEE were tightly correlated to leaf area index. The system was a small net
source of CO
2
to the atmosphere when soil moisture was low and a sink for CO
2
when
moisture was high (
Figure 6.7
). In 1999, a year with average precipitation, NEE showed a
net loss of 18 g C/m
2
; the system achieved a peak, above-ground biomass of 51.2 g C/m
2
;
and stored 4.9 g C/m
2
in below-ground pools. For the driest year, 2000, NEE was a much
smaller net gain (21 g C/m
2
); peak biomass was much smaller (39.4 g C/m
2
); and the sys-
tem lost 16.5 g C/m
2
from below-ground pools. Studies of the export term are uncommon
for grassland environments: typically, runoff is very low due to relatively low precipita-
tion (40 cm/y at this site) and high evaporation. We can say only that export is probably
small. The import term would be entirely by DOC in rainfall, which we did not consider.
Assuming a DOC content of 1 mg C/L, the input would be about 0.4 g C m
2
2
y
2
1
.
Averaging the two years, NEE was 1.5 g C m
2
2
y
2
1
(
Figure 6.7
).
3
Small Mesotrophic Lakes
In lakes, whole-system changes in dissolved gases are used in much the same way that
eddy covariance is used in terrestrial systems. In the aquatic system, we measure the
