1.3. The Carbon Budget of Terrestrial Ecosystems
The carbon sequestration potential of terrestrial ecosystems depends on the
type and condition of the ecosystem-that is, its species composition, structure,
and (in the case of forests) age distribution. Also important are site conditions,
including climate and soils, natural disturbances, and management. For the analysis
of a carbon budget, the fundamental differences between GPP, NPP, NEP, and NBP
must be recognized (see Figure 1-2). The justification
of the quantitative global flux estimates as defined below is given in the succeeding
sections of this chapter (see also Steffen et al., 1998).
Figure 1-2: Global terrestrial carbon uptake.
Plant (autotrophic) respiration releases CO2 to the atmosphere, reducing
GPP to NPP and resulting in short-term carbon uptake. Decomposition
(heterotrophic respiration) of litter and soils in excess of that resulting
from disturbance further releases CO2 to the atmosphere, reducing NPP
to NEP and resulting in medium-term carbon uptake. Disturbance from
both natural and anthropogenic sources (e.g., harvest) leads to further
release of CO2 to the atmosphere by additional heterotrophic respiration
and combustion-which, in turn, leads to long-term carbon storage (adapted
from Steffen et al., 1998).
Gross Primary Production denotes the total amount of carbon fixed in
the process of photosynthesis by plants in an ecosystem, such as a stand of
trees. GPP is measured on photosynthetic tissues, principally leaves. Global
total GPP is estimated to be about 120 Gt C yr-1.
Net Primary Production denotes the net production of organic matter
by plants in an ecosystem-that is, GPP reduced by losses resulting from the
respiration of the plants (autotrophic respiration). Global NPP is estimated
to be about half of the GPP-that is, about 60 Gt C yr-1.
Net Ecosystem Production denotes the net accumulation of organic matter
or carbon by an ecosystem; NEP is the difference between the rate of production
of living organic matter (NPP) and the decomposition rate of dead organic matter
(heterotrophic respiration, RH). Heterotrophic respiration includes losses by
herbivory and the decomposition of organic debris by soil biota. Global NEP
is estimated to about 10 Gt C yr-1. NEP can be measured in two ways: One is
to measure changes in carbon stocks in vegetation and soil; the other is to
integrate the fluxes of CO2 into and out of the vegetation (the net ecosystem
exchange, NEE) with instrumentation placed above (Aubinet et al., 2000).
The precision of both of these methods is improving.
Net Biome Production denotes the net production of organic matter in
a region containing a range of ecosystems (a biome) and includes, in addition
to heterotrophic respiration, other processes leading to loss of living and
dead organic matter (harvest, forest clearance, and fire, etc.) (Schulze and
Heimann, 1998). NBP is appropriate for the net carbon balance of large areas
(100-1000 km2) and longer periods of time (several years and longer). In the
past, NBP has been considered to be close to zero (Figure
1-2). Compared to the total fluxes between atmosphere and biosphere, global
NBP is comparatively small; NBP for the decade 1989-1998 has been estimated
to be 0.7 ± 1.0 Gt C yr-1 (Table 1-2)-about 1
percent of NPP and about 10 percent of NEP.