What is the
difference between NPP and NEP?
NPP quantifies the carbon absorption by plants only, while
NEP includes carbon absorption by plants and carbon release
by soils. In other words, NPP is a component of the carbon
cycle, while NEP is net carbon exchange between the ecosystem
and the atmosphere.
What is the
significance of carbon budget in Canada's forests?
The global carbon mass budget balances carbon accumulation
in the atmosphere against fossil fuel release, land use change,
and oceanic uptake. At present, the global carbon budget has
an unaccounted residual component of about 2 Gt C y-1
(1 Gt = 1015 g), dubbed the "missing sink". Terrestrial
ecosystems that are not modified by land use changes, especially
those in mid-high latitudes of the northern hemisphere have
been suggested as the most likely candidates for the "missing
sink". This "missing sink" is about an order of magnitude
larger than the emission reduction targets set by the Kyoto
convention, i.e., the commitment that 38 industrialized nations
made to reduce fossil fuel carbon emission by an average of
5.2% from 1990 levels by 2012. Therefore, improving our understanding
of the spatial distribution and temporal variation of the
"missing sink" has implications not only for understanding
the global carbon budget, but also for policies related to
the global economy. Because of the high carbon storage capacity
and long carbon residence time, forest ecosystems are believed
to be the most likely candidate for the "missing sink". Canada's
forests cover 417.6 Mha of land surface or about one tenth
of the world's forests, and thus may contribute substantially
to explaining the global "missing sink".
How do we estimate the carbon budget of Canada's
forests?
InTEC, an Integrated Terrestrial Ecosystem C-budget
model, was developed to estimate the carbon budget of Canada's
forests by a government-industry scientific team from CCRS
and Intermap Technologies Ltd. (Chen et al., 1998a). The InTEC
model estimates the carbon budgets of forests from atmospheric,
climatic and biotic changes since the pre-industrial period
(Figure 1). The atmospheric changes
include the increases in carbon dioxide concentration and
nitrogen deposition. The climatic changes include inter-annual
variations in temperature and precipitation. The biotic changes
include forest area and age structural changes due to disturbances
caused by forest fires, insects and timber harvest. Based
on these changes, the effects of various biological and physical
processes on the carbon budget of forest ecosystems are modeled.
Processes considered in the model include nitrogen deposition,
fixation and deposition, carbon release due to disturbances,
forest regrowth, change in the length of the growing season,
CO2 fertilization, soil respiration, and changes
in carbon/nitrogen ratios of different components in biomass
and soil carbon pools.
What is new about our model?
There are two major reasons why uncertainties exist in determining
the regional C-budget. One is incomplete accounting of all
major components in the carbon budget (e.g. photosynthesis
and respiration) due to data limitation or simplistic assumptions
about which components should be considered (Greenough et
al., 1998). Greenough et al. (1998) demonstrated that by including
or excluding certain components, the estimated carbon budgets
of Canada's forests vary from a source of 0.089 to a sink
of 0.185 Gt C y-1. The other reason is that the
carbon budget is a small difference between several large
components that can only be measured or estimated to a certain
degree of accuracy. For example, if the global terrestrial
carbon sink is estimated as the difference between the global
net primary productivity (NPP) of 50-60 Gt C y-1
and the corresponding soil respiration R, then a 10% error
in the NPP, even if R is determined perfectly, will result
in a 5-6 Gt C y-1 uncertainty in the carbon budget.
This is 2-3 times larger than the "missing sink". We include
all atmospheric, climatic and biotic factors in the InTEC
model to address the first type of uncertainty. To address
the second type of uncertainty, we assume that the carbon
and nitrogen exchanges between terrestrial ecosystems and
atmosphere were in equilibrium under the mean climate conditions,
nitrogen deposition rate, and disturbance rates during the
pre-industrial period. The carbon budget is then the sum of
changes in all these component fluxes since that time.
The main characteristic of the model is the mechanistic integration
of two widely used models: the Century model for carbon and
nitrogen cycles in soils, and Farquhar's biochemical model
of leaf photosynthesis. The integration is implemented through
new spatial and temporal scaling algorithms developed in this
study. The spatial scaling from leaf to canopy is done with
a canopy radiation model with sunlit/shaded leaf separation.
The temporal scaling from minutes to a year is conducted using
statistics of variance and covariance of various meteorological
and biological factors involved in photosynthesis. The statistics
are obtained from the measurements of two boreal species from
1994-1996 (Chen et al., 1998b; Goulden, et al., 1998). This
temporal scheme allows the estimation of annual total photosynthesis
using annual mean meteorological measurements such as temperature
and precipitation and yet is able to take into account the
effects of diurnal and seasonal variability of the photosynthetic
processes on the integrated carbon budget. A similar temporal
scaling method was used for inter-annual variations in NPP
and in other factors such as nitrogen deposition, climate,
CO2 and disturbance rates. Disturbances are considered
explicitly in this model as processes that directly release
carbon and nitrogen into the atmosphere, and modify the total
NPP by changing forest area and age class. The carbon cycle
in forest products is also included as a process which stores
a fraction of harvested wood in products that slowly oxidize.
Sub-models of respiration and canopy photosynthesis are verified
and parameterized using BOREAS data at the Old Black Spruce
(1995 and 1996) and Old Aspen (1994 and 1996) sites. In particular,
we determine the time fraction of photosynthesis dominated
by the electron transport control or Rubisco activities after
separating a canopy into sunlit and shaded leaf groups, thus
avoiding a large uncertainty in using Farquhar's model to
determine canopy photosynthesis and interannual variation
in NPP. Remote sensing derived data of leaf area and NPP,
network measurements of N deposition, and historical data
of climate and disturbance rates are used in the InTEC model.
How has the carbon budget of Canada's forests
varied from 1895 to 1996, based on the new approach?
InTEC results indicate that in the past 100 years, Canada's
forests as a whole were a small carbon source of about 30
Mt C y-1 in the period 1895-1905 due to large disturbances
near the end of the 19th century; a large carbon sink of about
170 Mt C y-1 (1930-1970) due to forest regrowth
in previously burnt areas; and moderate carbon sinks of about
50 Mt C y-1(1980-1996) see Figure 2. The 1980-1996 sink is a
net balance between the negative effects of increased disturbances
and the positive effects of other non-disturbance factors.
In order of importance, the non-disturbance factors, were
found to be
- atmospheric nitrogen deposition measured by a national
monitoring network,
- net nitrogen mineralization and fixation estimated from
temperature and precipitation records,
- CO2 fertilization estimated from CO2
records using a leaf level photosynthesis model,
- growing season length increase estimated from spring air
temperature records.
The positive effect of these non-disturbance factors is experimentally
supported by the recent BOREAS measurements. Increased disturbances
(mostly fire and insects) in recent decades caused a loss of
about 60 Mt C y-1 from the forests in 1980-1996.
If disturbance rates had remained unchanged, Canada's forests
in 1980-1996 would have been a sink of about 150 Mt C y-1
in size. The large amount of carbon accumulated during 1930-1970
with low disturbance rates caused an additional loss of about
40 Mt C y-1 through decomposition in 1980-1996. From
the modeling perspective, it is evident that:
- attention must be given to nutrient cycles as they are
particularly closely coupled with carbon cycle in the boreal
environment,
- estimating the carbon budget from historical changes in
nitrogen and carbon cycle components is the preferred approach
since the outcome is not sensitive to inaccuracies in estimates
of soil respiration and NPP
- the absolute magnitude of current sinks per unit area
is small (about 0.35 t C/ha or 13% of current NPP) and exceedingly
difficult to measure directly.
To reduce the uncertainties in our estimates, improvements are
therefore needed in the comprehensive modeling of all processes
involved in the terrestrial nutrient and carbon cycles using
biological and physical principles which can be experimentally
verified.
Figure 1. Structure of
the Integrated Terrestrial Ecosystem C-budget model (InTEC)
which synthesizes the interactive effects of disturbances,
nitrogen deposition, climate change, and CO2 fertilization
on the carbon budget of boreal forests. Dashed arrows indicate
influences, and solid arrows show C-N flows.
Figure 2. Carbon budget
of Canada's forests from 1895 to 1996. Five-year means are
plotted, the last point being only the average from 1995 to
1996. Also included is the record of Canada's green-house
gas carbon emission rate.
References
Chen, W.J., Chen, J.M., Liu, J., and Cihlar, J.: 1988a, "C
budget of Canada's forests estimated from transient changes
in disturbances, N, climate, and CO2. I: An integrated
terrestrial ecosystem C-budget model", Global Biogeochem.
Cycles (submitted).
Chen, J.M., Chen, W.J., Liu, J., Cihlar, J., and Gray, S.:
1988c, "C budget of boreal forests estimated from changes
in disturbances, N, climate, and CO2. II: Results for Canada
from 1985-1996", Global Biogeochem. Cycles (submitted).
Chen, W.J., Black, T.A., Yang, P.C., Barr, A.G., Neumann,
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Goulden, M.L., S.C. Wofsy, J.W. Harden, S.E. Trumbore, P.M.
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