UNIVERSITY OF COPENHAGEN
DEPARTMENT OF GEOSCIENCES AND NATURAL RESOURCE
MANAGEMENT
Peter Aarup Iversen
NS
MEMORANDUM
Re:
Uncertainty of carbon pool and changes related to forests
and harvested wood products
Authors:
Thomas Nord-Larsen, Erik Schou, Kjell Suadicani, Vivian
Kvist Johannsen
Input for the meeting in Working Group 5: "Implementation of the
LULUCF Decision and policy development of the land use, land use change
and forestry sector" under the Climate Change Committee, Tuesday, 14
April 2015
13 APRIL 2015
FOREST, NATURE AND BIOMASS
ROLIGHEDSVEJ 23
1958 FREDERIKSBERG C
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+45 35331728
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+45 2030 0969
[email protected]
REF: VKJ
Re •Technical approaches for managing uncertainty and inter-annual
variation - with a focus on forest carbon pools
Sampling history
From 1881 to 2000, a National Forest Census has been carried out roughly
every 10 years based on questionnaires sent to forest owners (Larsen and Johannsen, 2002). Since the data was based on questionnaires and not field
observations, the actual forest definition may have varied. The basic
definition was that the tree covered area should be minimum 0.5 ha to be a
forest. There were no specific guidelines as to crown cover or the height of
the trees. Open woodlands and open areas within the forest were generally
not included. All values for growing stock, biomass or carbon pools based
on data from the National Forest Census were estimated from the reported
data on forest area and its distribution to main species, age class and site
productivity classes. The two last censuses were carried out in 1990 and
2000.
The 1990 National Forest Census was based on reported forest statistics
from 22 300 respondents. The number of respondents in the 2000 National
Forest Census was 32,300, or 44 % more than in the 1990 survey. The
change in the number of respondents probably contributed to the observed
increase in forest area and growing stock between the 1990 and 2000
census. The estimated forest area in 2000 was 486,000 ha or 11.3 % of the
land.
In 2002, a new sample-based National Forest Inventory (NFI) was initiated
(Nord-Larsen et al., 2008), which replaced the National Forest Census. This
type of forest inventory is very similar to inventories used in other countries,
e.g. Sweden or Norway.
The NFI is a continuous sample-based inventory with partial replacement of
sample plots based on a 2 x 2 km grid covering the Danish land surface. At
each grid intersection, a cluster of four circular plots (primary sampling unit,
PSU) for measuring forest factors (e.g. wood volume) are placed in a 200 x
200 m grid. Each circular plot (secondary sampling unit, SSU) has a radius
of 15 meters. When plots are intersected by different land-use classes or
different forest stands, the individual plot is divided into tertiary sampling
units (TSU).
About one third of the plots is assigned as permanent and is re-measured in
subsequent inventories every five years. Two thirds are temporary and are
moved randomly within the particular 2x2 km grid cell in subsequent
inventories. Based on analysis of aerial photos, each sample plot (SSU) is
allocated to one of three basic categories, reflecting the likelihood of forest
or other wooded land (OWL) cover in the plot. All plots in the two
categories are inventoried in the field. Based on this the overall forest area is
estimated to be 615,000 ha, or 14.3 %.
The annual estimates of carbon pools are based on a full five year sampling,
covering the entire land. The last year of the inventory is taken as the index
year.
Elements of uncertainties
Area:
The assessment of the forest area has varied over time, due to changing
sampling methodology. The current assessment is based on field sampling
with a manual first phase classification based on visual classification of
orthophotos. The uncertainty based on the sampling error is giving a forest
area of 13.8-14.8 pct. with a 95% certainty.
Carbon pools above ground and below ground:
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The total volume of wood in the Danish forests is 128 mio. m3, equal to an
average of 208 m3/ha and corresponding to total pool of carbon of 38 mio.
ton C or 62 ton C/ha. Based on boot strap sampling, the standard error of the
estimate is 0.9 %. Corresponding to a confidence interval of the mean (95 %
certainty) of 205-212 m3/ha.
The uncertainty of pool of dead wood is larger, as dead wood only is present
on only about 1/3 of the plots and highly variable. However, the pool of
dead wood is small compared to the other pools carbon pools, constituting
only 1.1 ton C/ha
Change based on stock change, annual reporting and uncertainty
The fact that the uncertainty of the stock is 0.9 % for each year, and the
expected net annual changes due to increment, mortality and harvesting is
less than 0.7 % (net change 1.5 m3/ha out of 208 m3/ha), it is not surprising
that the annual reporting may indicate results of merely sampling error more
than human induced changes. This at least goes for forestry, where pools are
large and changes in relation to this is small.
Article 3.3 and article 3.4 areas
Dividing the forest area according to the forest mask of 1990, causes some
additional uncertainty, especially for the art. 3.3 area, which by history is
the smallest. Hence the uncertainty of both area and carbon pool estimates
for this subset of the forest area is larger than for the forest area in total.
Uncertainty for HWP
The subject is treated in general in the 2013 KP Guidelines, and the figures
stated there are used as a basis for the description of uncertainty in the HWP
reporting (see Schou et al., 2015).
Uncertainty on half-life/sensitivity of carbon pool changes
Regarding the uncertainty on half-life and the influence on the estimated
annual pool changes, the dynamics of the pool are crucial. For Denmark the
pool of wood-based panels has been continuously increasing, whereby the
uncertainty plays a larger role, compared to the pool of sawnwood, which in
general has been decreasing in later years, and thus is relatively insensitive
to the choice of half-life (se Figure 1).
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70.000
PAGE 4 OF 5
Outflow 2013 [Mg C]
60.000
50.000
40.000
30.000
Sawnwood
20.000
Wood panels
10.000
0
0
20
40
60
80
Half-life [years]
Figure 1. Sensitivity of 2013-carbon pool changes to the choice of half-life.
Uncertainty on activity data
The HWP questionnaire on the production of the Danish wood industry has
probably caused a reduction in uncertainty on activity data compared to the
figures mentioned in the 2013 KP Guidelines. The survey covers approx. 95
% of the sector of sawnwood production and 100 % of the sector of woodbased panel production measured by the generated revenue – thus the survey
almost represents a complete inventory of the primary wood industry. A
comparison of the reported consumption of roundwood in the questionnaire
with figures from Statistics Denmark yields a deviation of 4-11 % for 20112013 (see Table 1). The calculated utilization rates for the production of
semi-finished wood products lie quite close to what may theoretically be
expected, except for a few of the minor companies.
Table 1. Domestic consumption (DC) of roundwood from domestic harvest (DH) 2011-2013.
3
Roundwood (DH DC) – total [m ]
2011
2012
2013
Questionnaire
937
873
956
Statistics Denmark
852
840
864
Deviation
10%
4%
11%
References
Larsen, P.H., Johannsen, V.K., 2002: Skove og plantager 2000 [Forest and
plantations 2000]. Statistics Denmark, Forest & Landscape and The Danish
Forest and Nature Agency, Ministry of Environment, 171 p. ill.
Nord-Larsen, T., Johannsen, V.K., Jørgensen, B. B., Bastrup-Birk, A., 2008:
Skove og plantager 2006 [Forests and plantations 2006]. Forest &
Landscape, University of Copenhagen, Hørsholm, 186p. ill.
Schou, E., Suadicani, K., Johannsen, V.K., 2015: Carbon Sequestration in
Harvested Wood Products (HWP) – Data for 2013-reporting to the
UNFCCC. Department of Geosciences and Natural Resource Management,
University of Copenhagen, Frederiksberg 2015, 50p.
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