Environmental Engineering Reference
In-Depth Information
A Temperate Hardwood Forested Ecosystem
Except for being remarkably well studied, the small, forested watersheds at the
Hubbard Brook Experimental Forest are quite typical of forests in northeastern North
America. This forested landscape had a history of cutting and clearing that ended in the
early 1900s. For the past century the forest has been growing back, or aggrading, and in
the most recent years the net growth of trees has ceased (
Fahey et al. 2005
). As with many
upland forests there is little import of organic C into the ecosystem: Rainfall, the only
significant input, which typically has DOC concentrations of approximately 1 mg C/L,
contributes 1.6 g C m
2
2
y
2
1
(
McDowell and Likens 1988
; Likens 1992). The major input to
this system is from gross photosynthesis, about 1230 g C m
2
2
y
2
1
(
Fahey et al. 2005
). A lit-
tle more than half of GPP (645 g C m
2
2
y
2
1
) is consumed by the respiration of the plants
themselves (R
a
). Thus, NPP (NPP
R
a
) is 585 g C m
2
2
y
2
1
. NPP is the organic C
that is potentially available to consumers.
Fahey et al. (2005)
estimate that the lion's share
of heterotrophic respiration at Hubbard Brook occurs within the soil, and the lion's share
of this is attributable to soil microbes (460 g C m
2
2
y
2
1
or 79% of NPP). Above-ground
insects, birds, and mammals respire only about 10 g C m
2
2
y
2
1
, less than 2% of NPP. The
portion of NPP that is not respired by heterotrophs (R
h
) is either stored in new biomass in
the system, stored as detritus in the soils, or exported. If we take these numbers at face
value, NPP
5
GPP
2
NEP. Export in stream flow as DOC (1.7 g C m
2
2
y
2
1
)
and POC (0.56 g C m
2
2
y
2
1
) accounts for only 2.5% of this NEP. Thus, about
112 g C m
2
2
y
2
1
, more than 90%, is sequestered in this forest either as new biomass or soil
detritus. As there has been essentially no net addition in the biomass of living trees in the
past 20 years at this site, the inference is that sequestration must occur either on the forest
floor or in the soil. Direct measurements of the organic matter in the surface organic hori-
zons of the soil (O
ie
and O
a
layers) show no net change in repeated measurements since
1975 (
Fahey et al. 2005
), leading to the inference that the deeper, mineral soil is the location
of most of the C sequestration.
Clearly, NEP is small in comparison to GPP and most sequestration is into detrital
rather than living pools. The soil organic C pool is very large, about 15,740 g C/m
2
, and
varies with both depth and space. One thing that is important to note is that it is not possi-
ble to detect an annual or even decadal increment to this pool.
115 g C m
2
2
y
2
1
R
h
5
2
5
FIGURE 6.4
Carbon storage and export for the past
12,000 years at the Hubbard Brook Experimental Forest.
(Redrawn from
Cole and Caraco 2001
.)
3
Mineral soil
2
DOC
loss
Forest floor
Woody debris
Roots
Living trees
1
0
Net accumulation
Export
