THE KEY FACTORS AFFECTING UNCERTAINTIES OF SINKS AND
STOCKS IN AN INVENTORY BASED FOREST CARBON BUDGET
Mikko Peltoniemi1, 2, Taru Palosuo1, Suvi Monni3, Raisa Mäkipää2
Introduction
Forest carbon model
The uncertainty of regional carbon budgets of forests is currently an issue of
special interest due to the requirements set for the greenhouse-gas reporting. In
order to report the uncertainties of national forest C budgets to UNFCCC one has
to prepare an uncertainty analysis for the results, and in order to improve system
one has to identify its weakest parts. We performed these analyses for a carbon
budget of Finnish forests using a simple inventory based carbon model.
CO2
TREES
VEG Ct
Li
· C%
Vt
(t, V1, V2,
GI, Dt)
· BEFi
· BEFi
· C%
Dt
· BEFi
Lgv
· BA
· C%
(t, A1, A2)
At
· BA
· TRGV
SOIL Ct-1/
At-1
· BEFTOT
· C%
SOIL Ct
(T, Drought,
soil model
parameters)
GROUND VEG.
· C%
Materials and Methods
The carbon model combines aggregated forest inventory data, models of biomass
and its turnover, and a dynamic soil carbon model, Yasso1. The estimated litter
is given as input to the soil model starting from a steady state with the input and
temperature of the first year. The time step for the system is one year, and the sinks
are calculated as differences between two consecutive stocks. The uncertainties
and key factors of this system were analysed with Monte Carlo simulations.
ACt,
· ∆At,t-1
The litter input (L) for the soil model is derived from growing stock (V), area (A), drain (D), and
growth variation (GI), using model parameters like biomass expansion factors (BEF) or (BA), turnover
rates (TR), and carbon density (C%).
Results
Approx. contribution to variance
0.2
0.4
0.6
0.8
0
1
*
0
*
*
*
1990
*
1992
1994
1996
Year
1998
2000
2002
Initialization of soil model lost its importance after some years had passed. Blue lines are from
analysis including an uncertainty of 20% (95% conf. limits) for soil initial state, red dash lines are
from analysis not assuming uncertainty for soil initial state. Bold lines represent the median, thin
lines represent the 2.5% and 97.5% confidence limits.
Uncertainty of forest C stock was dominated by soil model parameters, whereas these were less
important for the forest C sink.The sink uncertainty was contributed mostly by the soil initial state,
and inter-annually varying input variables or input variables that other-wise contributed to differences between consecutive stocks (*). The bars above the dash line represents the results when
all variables of the system were taken into the analysis.The bars below the dash line represent the
results when the soil model initial state and soil model parameters were fixed.
Effect of soil model parameters on soil C sink and stock
0.2
Forest C sink 1990-1989
Forest C Stock 1990
0.0
Soil model initialization
Soil model parametrization
Others total
Forest area estimates
Growing stock estimates
Turnover rate, ground veg.
Temperature sums
Growth indexes
BEF, fine roots
Drain estimates
Carbon density
Biomass, ground veg.(per area)
Soil model temp. dependence
Turnover rate, fine roots
Turnover rate, foliage
BEF, stand biomass
BEF, foliage
Others total
Importance of soil model initialization
Soil C sink (Tg)
5
10
15
Key factors affecting the uncertainty of forest C sink and stock
0
2
4
Conclusions
Uncertainty of forest sink (vegetation + soil) was also affected by the variables
that varied inter-annually. Their role will increase while the uncertainties in soil
system are decreased. In order to provide reliable estimates of the annual forest
sinks, the inter-annual variability of system parameters should be studied.
Precision of modelled soil carbon sink estimates may be improved substantially
using also data few years before the time the estimates are prepared for.
12
14
Precise modelling of soil
0.0020
0.0000
The large uncertainty in the results of soil carbon stock and sink, and the small role
of vegetation parameters in these uncertainties, implies that more work should be
focused into the improvement of methods to model soil on a national level.
6
8
10
Soil C sink 1990–1989 (Tg)
Default uncertainties
for soil parameters
500
1000
1500
2000
Soil C stock 1990 (Tg)
2500
3000
Given that soil processes could be modelled precisely (no uncertainties for the soil parameters),
there would still be left some uncertainty in the estimates of soil sink and stock. This uncertainty
is due to uncertainty in litter inputs, only.
Acknowledgements
The project Uncertainty Assessment of Forest Carbon Balance has been
financed by the Finnish Ministry of the Environment and the Finnish Ministry
of Agriculture and Forestry
References
1
Liski J, Palosuo T, Peltoniemi M, Sievänen R: Carbon and decomposition model Yasso for forest soils.
Ecological Modelling, accepted
Contact information: mikko.peltoniemi@metla.fi , +358 10 255 2506,
Finnish Forest Research Institute, PL 18, Vantaa, Finland
1
European Forest Institute, Torikatu 34, Joensuu, Finland
2
Finnish Forest Research Institute, PL 18, Vantaa, Finland
3
VTT Technical Research Center of Finland, 02044, VTT, Finland
www.efi.fi /projects/uncertainty
Metla/3399/SElo/03.2005