Geoscience Reference
In-Depth Information
Measuring the carbon balance of a
boreal forest ecosystem
NEW DEVELOPMENTS
The role of different ecosystems in the global carbon balance - past, present and future - is still a matter of
considerable debate. Boreal forests and associated wetlands represent the largest terrestrial reservoir of carbon (IPCC
2000), as well as being located in a region especially sensitive to climate change. Some studies suggest a large sink
for anthropogenic CO
2
within the boreal forest biome, whereas others argue that this is unlikely on observational
and theoretical grounds. Significant changes in the climate of sub-arctic boreal forests since the 1980s are well
documented, and involve changes in air temperature, duration and extent of snow cover, intensity and amount of
precipitation, and type, frequency and amount of clouds. Long-term changes in photosynthesis and respiration will
already be occurring. As the net carbon balance of any ecosystem represents a subtle balance between photosynthesis
and respiration, it is not easy to predict whether changes in climate and CO
2
exchange patterns will result in net gain
or loss of carbon in ecosystems. However, there is concern that increases in temperature may result in considerable
sources of carbon due to higher rates of respiration, for there is evidence that in warmer years boreal forests are
sources of CO
2
to the atmosphere.
Carbon flows in ecosystems can be measured by the eddy covariance technique. A special anemometer is installed
at 6 m height on an aluminium eddy covariance tower (
Plate 21.3
).
Measurements of CO
2
are made on air drawn
from the top of the tower into a gas analyser (above-canopy eddy covariance) and also at ground level (below-canopy
eddy covariance), thus measuring all CO
2
going into and out of the canopy. Also, there is instrumentation to record
associated environmental parameters such as water vapour flux, downward and upward radiation, light, air
temperature, soil temperatures, precipitation and wind speed.
Plate 21.3
appears similar to
Plate 5.5
,
but, in addition
to recording conventional micrometeorological parameters, it is more focused on the carbon cycle, measuring CO
2
and methane (CH
4
) above and below the canopy. On-site instrumentation is used to monitor dissolved organic carbon
(DOC), Net Primary Productivity (NPP), Gross Primary Productivity (GPP) and ecophysiological characteristics of the
vegetation such as stomatal resistance.
Figure 21.6
shows the fluxes needed to quantify the
Net Ecosystem Exchange (NEE)
per unit of time (hours, days,
or years). Net Ecosystem Exchange is the net flux of CO
2
across the sensor on the tower; it has a negative sign
because it is a net downward flux towards the surface. It is related to Net Ecosystem Productivity (NEP), the rate
at which carbon is being accumulated by an ecosystem, by the equation:
NEE
= - NEP
= Respiration - Photosynthesis
= R(A) - P
The strong seasonal variation in NEE, with the lowest values in the summer months, reflects seasonal variations in
light, temperature, precipitation and other climate elements, as do any year-to-year variations. Sophisticated
experimental set-ups can now measure soil respiration, stem respiration and foliar respiration to give Total Ecosystem
Respiration (TER).
Studies of net ecosystem carbon fluxes are bringing new insights to our knowledge of the carbon cycle. The study
of the taiga spruce forest in European Russia illustrated in
Plate 21.2
found that the forest was a significant source
of carbon to the atmosphere (Milyukova et al. 2002). The total ecosystem respiration flux was 130 mol C m
-2
yr
-1
,
similar to boreal forests in Scandinavia but much higher than in Canadian and Siberian boreal forests. Photosynthetic
rates increased with both light and temperature, but the temperature response was less than for ecosystem
respiration. The conclusion is that the forest is a source of carbon to the atmosphere on warm summer days, and a
net sink only on sunny days with daily air temperatures below 18
C. The data suggest that many boreal forests may
be releasing substantial amounts of CO
2
to the atmosphere. Thus they are a net source of carbon and are accelerating
climate change. The source or sink status is not a permanent characteristic of this biome, and can change over time
as result of changes in forest composition (i.e. age-class structure), disturbance by fire and insects, and human
resource use.
