Principles of terrestrial ecosystem ecology.pdf

from carbon fixation (a component of GPP).
Nonetheless, it is the closest thing to a
direct measurement of GPP that is currently
available.
A global network of sites measures NEE
continuously in many of the world’s ecosystems.
These measurements show that, in the
absence of disturbance, most temperate ecosystems
that have been measured are net sinks for
CO2 (Fig. 6.10) (Valentini et al. 2000).There are
at least four possible explanations for this
important finding: (1) Ecosystems may typically
be carbon sinks between episodes of disturbance,
and disturbance may be the factor that
brings NEP into balance at the regional scale.
(2) Recent environmental changes, such as
increased atmospheric CO2 and nitrogen deposition,
may have stimulated photosynthesis
more than respiration. (3) Midsuccessional
ecosystems with high NEP may have been overrepresented
in the sampling network relative to
the rest of the world. Many western European
forests, where these studies were done, are productive
midsuccessional sites that are developing
after agricultural abandonment. (4) Carbon
Carbon exchange (kg m -2 yr -1 )
1.5
-0.5
1
0.5
0
-1
C uptake
C loss
GPP
NEE
Respiration
40 45 50 55
Latitude
60 65
Figure 6.10. Latitudinal variation in annual gross
primary production, net ecosystem exchange, and
ecosystem respiration (R ecosyst) among 12 naturally
occurring European forests (Valentini et al. 2000).
The greater NEE of low-latitude forests reflected
their lower rates of ecosystem respiration. There was
no latitudinal trend in GPP.
Net Ecosystem Production 147
loss through leaching and other transfers may
be an important component of the regional
carbon balance.These nongaseous losses would
not be detected in measurements of NEE.
A second striking result of this study is that
latitudinal variation in NEE reflects variations
in ecosystem respiration rather than in GPP.
Recent high-latitude warming could contribute
to the greater respiration observed at high latitudes.
At the few sites where there are longterm
measurements of NEE, both respiration
and photosynthesis contribute to interannual
variations in NEE (Goulden et al. 1996, 1998).
Only recently has NEE been measured in
enough ecosystems to begin to identify regional
patterns in NEE and their likely causes.
Global Patterns of NEE
Seasonal and latitudinal variations in the CO2
concentration of the atmosphere provide a clear
indication of global-scale patterns of NEE
(Fung et al. 1987, Keeling et al. 1996b). At high
northern latitudes, conditions are warm during
summer, and photosynthesis exceeds total
respiration (positive NEE), causing a decline
in the concentration of atmospheric CO2
(Fig. 6.11). Conversely, in winter, when photosynthesis
is reduced by low temperature and
shedding of leaves, respiration becomes the
dominant carbon exchange (negative NEE),
causing an increase in atmospheric CO2. These
seasonal changes in the balance between photosynthesis
and respiration occur synchronously
over broad latitudinal bands, giving rise to
regular annual fluctuations in atmospheric CO2,
literally the breathing of the biosphere (i.e., all
live organisms on Earth) (Fung et al. 1987).
Latitudinal variations in climate modify
these patterns of annual carbon exchange. In
contrast to the striking seasonality of NEE at
north temperate and high latitudes, the concentration
of atmospheric CO2 remains nearly
constant in the tropics, because carbon uptake
by photosynthesis is balanced by approximately
equal carbon loss by respiration throughout
the year. In other words, NEE is close to zero
throughout the year. There is also relatively
weak seasonality of atmospheric CO2 at high
southern latitudes where oceans occupy most