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Gallaun, H.; Zanchi, G.; Nabuurs, G.-J.; Hengeveld, G.; Schardt, M. & Verkerk, P. J., 2010. EU-wide maps of
growing stock and above-ground biomass in forests based on remote sensing and field measurements Forest
Ecology and Management, 2010, 260 Issue 3, 252-261
EU-wide maps of growing stock and above-ground biomass in forests based on remote sensing and field
measurements.
Heinz Gallaun
a,
*, Giuliana Zanchi
a
, Gert-Jan Nabuurs
b
, Geerten Hengeveld
b
, Mathias Schardt
a
, Pieter J. Verkerk
c
a
Joanneum Research, Steyrergasse 17, 8010 Graz, Austria
b
Alterra b.v., PO Box 47, 6700 AA Wageningen, Netherlands
c
European Forest Institute, Torikatu 34, 80100 Joensuu, Finland
Abstract
The overall objective of this study was to combine national forest inventory data and remotely sensed data to
produce pan-European maps on growing stock and above-ground woody biomass for the two species groups
“broadleaves” and “conifers”. An automatic up-scaling approach making use of satellite remote sensing data and
field measurement data was applied for EU-wide mapping of growing stock and above-ground biomass in forests.
The approach is based on sampling and allows the direct combination of data with different measurement units
such as forest inventory plot data and satellite remote sensing data. For the classification, data from the Moderate
Resolution Imaging Spectroradiometer (MODIS) was used. Comprehensive field measurement data from national
forest inventories for 98979 locations from 16 countries were used for which tree species and growing stock
estimates were available. The classification results were evaluated by comparison with regional estimates derived
independently from the classification from national forest inventories. The validation at the regional level shows a
high correlation between the classification results and the field based estimates with correlation coefficient r=0.96
for coniferous, r=0.94 for broadleaved and r=0.97 for total growing stock per hectare. The mean absolute error of
the estimations is 25 m³/ha for coniferous, 20 m³/ha for broadleaved and 25 m³/ha for total growing stock per
hectare. Biomass conversion and expansion factors were applied to convert the growing stock classification results
to carbon stock in above-ground biomass. As results of the classification, coniferous and broadleaved growing
stock as well as carbon stock of the above-ground biomass is mapped on a wall-to-wall basis with a spatial
resolution of 500m by 500m per grid cell. The mapped area is 5 million km², of which 2 million km² are forests, and
covers the whole European Union, the EFTA countries, the Balkans, Belarus, the Ukraine, Moldova, Armenia,
Azerbaijan, Georgia and Turkey.
Keywords:
Biomass, growing stock, carbon, remote sensing, forest inventory, MODIS
1. Introduction
The overall objective of this study was to develop EU-wide maps on growing stock and above-ground woody
biomass for the two species groups “broadleaves” and “conifers”, by combining national forest inventory data and
remotely sensed data.
Forests cover 44% of land area in Europe and are therefore of major ecological, political and economic importance.
Whereas accurate estimations of forest parameters are available at the local level, especially for growing stock,
European-wide assessments with high spatial resolution are not common.
The recent development of strategies for renewable energy production, the progressive globalisation of the
markets, including the market of wood products, and carbon stock estimation for climate change modelling, have
increased the demand for spatial explicit information on forest resources. Two important parameters that well
describe the spatial distribution of the forest resources are maps on growing stock and the biomass stock.
For the estimation of growing stock and above-ground biomass, different approaches have been developed, based
on field measurement, modelling, and remote sensing. A good overview is given by Lu (2006).
Field measurements are used for accurate estimation of growing stock and woody biomass from the allometric
relationship with measured diameter at breast height and tree height. Mostly, only small areas are measured, as
the field measurements are very labour- and cost-intensive. The typical size of plots measured in national forest
inventories is between 200m² to 500m². In general, remote sensing approaches for estimating forest parameters
are based on reference data such as e.g. plot data, for which the forest parameters are measured or estimated.
The reference data are then combined with the remote sensing data to derive statistical parameters that are used
for the estimation of the forest parameters for the whole area. However, this direct comparison requires that the
reference data and the picture elements of the remote sensing data cover the same area on the ground. The
remotely sensed imagery currently available for regional to global vegetation studies are often at a much lower
resolution than reference data (i.e. 25 ha minimum pixel size for MODIS satellite data and 200m² to 500 m² for field
plot measurements). Therefore, and because different land cover categories arc covered by single picture
elements (mixed pixels), the integration of sample data and remote sensing-derived variables is still difficult (Lu,
2006; Anaya et al., 2009).
Blackard et al. (2008) mapped forest biomass in the United States using nationwide forest inventory plot data and
moderate resolution information by modelling. The geospatial predictor variables included: MODIS-derived image
composites and percent tree cover, land cover proportions, topographic variables, monthly and annual climate
parameters and other ancillary variables. The estimations were performed separately for 65 strata. Only for 75% of
the strata, relative errors were below 1.0, indicating gains in the modelling process. Päivinen et al. (2009)
generated pan-European growing stock maps based on the forest proportion map of Häme et al. (2001) and
regional statistics derived from national forest inventories. The forest proportion map was derived from NOAA-
AVHRR satellite data which has a spatial resolution of 1.1 km by 1.1 km and CORINE 1990 land cover data. The
regional to national forest inventory estimates were distributed according to the forest cover proportion. They
transferred the mean growing stock volume for the region to the pixels by multiplying with the share of forest area
within a pixel. They analysed rounding errors but do not report on an independent accuracy assessment. Stümer et
al. (this issue, 2009) applied a self-organizing map algorithm for the assessment of carbon stock in forests for
Thuringia in Germany. The approach is based on national forest inventory data on plot level and remote sensing
data from Landsat. They compared this approach with a k-nearest neighbour approach as well as with results of
the present study. The results were compared with estimates derived from national forest inventory plots and all
three approaches show good agreement at the regional level (Stümer et al., this issue, 2009). Muukkonen and
Heiskanen (2007) combined ASTER and MODIS satellite data to estimate growing stock and above-ground
biomass in southern Finland. They used regression models based on stand-wise forest inventory data as
intermediate step between the field data and the MODIS data. The estimates obtained were close to the district-
level mean values provided by the Finnish National Forest Inventory with relative root mean square error of 9.9%.
Tomppo et al. (2002) combined Landsat Thematic Mapper and IRS-1C Wide Field Sensor (WiFS) data to estimate
tree stem volume and above-ground biomass in Finland and Sweden. The Landsat Thematic Mapper data were
used as an intermediate step between field data and WiFS data. The nonparametric k–nearest neighbour method
was used to analyse relationships between Landsat Thematic Mapper and field data, and nonlinear regression
analysis was used to develop models for predicting volume and biomass for WiFS pixels. Validation was performed
by comparing stem volume at the municipality level with the Finnish National Forest Inventory. Only WIFS pixels
which were totally on forestry land were used. The reported relative differences range from -24.9% to 28.5%.
Regarding growing stock, European-wide mapping was performed by Päivinen et al. (2009) as described above. A
main difference of the current study is that we use forest inventory plot data as reference data, whereas Päivinen et
al. (2009) use regional to national data. Furthermore, our maps are based on MODIS data with a spatial resolution
of 25 ha, whereas the work of Päivinen et al. (2009) is based on NOAA-AVHRR data with a spatial resolution of
120 ha. The area covered by the map of Päivinen et al. (2009) includes the European part of Russia, which is not
covered by our work, whereas we cover Turkey, Armenia, Azerbaijan and Georgia, which is not covered by
Päivinen et al. (2009).
EU-wide mapping of above ground forest biomass by direct combination of remote sensing and forest inventory
plot data has not been published up to now. The main reason for this is that appropriate ground measurement plot
data are mostly only available at the regional to national level, whereas for the current study, plot-level data from
16 national forest inventories, distributed over the main forest ecosystem regions of Europe, were provided by
national forest inventory institutes.
The work for this study was performed within the research project “CarboEurope-IP” which aimed to understand
and quantify the terrestrial carbon balance of Europe. The mapped area is 5 million km² of which 2 million km² are
forests.
2. Material and methods
For the production of the pan-European maps, comprehensive field measurement data from national forest
inventories were used (Nabuurs et al., this issue, 2009), which were provided by national forest inventory institutes.
An automatic up-scaling approach was applied to allow the direct use of the field inventory plots for the
classification of the MODIS data. The estimates refer to the year 2000 for which the MODIS data, the calibrated
inventory data at the regional level, and the mean biomass expansion factors at the regional level were available.
The estimations were performed on a wall-to-wall basis for the whole area with a spatial resolution of 500 meters
by 500 meters. The general workflow is outlined in Figure 1.
Fig. 1: General workflow
2.1 Field data from national forest inventories
The data were selected from a collection of National Forest Inventory (NFI) plot data (Nabuurs et al., this issue,
2009). From this database, 98979 locations in 16 countries were selected where data on tree species and growing
stock were provided by national forest inventory institutes. The data were classified and aggregated to the classes
”broadleaved” and ”conifer”. For each location the volumes were reported in m
3
/ha. All locations were geo-
referenced.
2.2 Remote sensing and ancillary data
We used four datasets from remote sensing: MODIS satellite data, meteorological data, CORINE Land Cover 2000
data and MODIS Vegetation Continuous Fields. We only considered data that are freely available for map
production. Repeated application of this approach therefore allows a cost effective monitoring of the forest
resources over time.
MODIS satellite data
The Moderate Resolution Imaging Spectroradiometer (MODIS) was launched on board Terra satellite in 1999
followed by Aqua satellite in 2002. It is a 36-channel visible to thermal infrared sensor with a scene width of 2230
km and a temporal resolution (repeated coverage) of 1-2 days. For the classification, the product “Nadir BRDF-
Adjusted Reflectance 16-Day L3 Global 500m” (MCD43A4) was used. This product is based on multi-date,
atmospherically corrected, cloud-cleared input data measured over 16-day periods. For the classification, the six
spectral bands green (0.545–0.565 µm), red (0.620–0.670 µm), near-infrared (0.841–0.876 µm) and middle-
infrared (1.628–1.654 µm and 2.105–2.155 µm) were used. Band 3 in the blue spectral region (0.459–0.479 µm)
was not used because calibration uncertainty can lead to large errors in this band. For the classification, data from
9
th
June to 24
th
June 2000 (days of year 161 to 176) were used. For small areas that are not properly covered by
this dataset, e.g. cloud covered region in Scotland, data from July were additionally incorporated. The so-called
Collection 5 was used as it reflects the recent (2009) improvements to the MODIS science algorithms and has a
spatial resolution of 500 m rather than 1 km as in previous collections.
Meteorological data
In addition to the MODIS reflectance data, meteorological data from Hijmans et al. (2005) were used as geospatial
predictor layers. Hijmans et al. (2005) interpolated climate surfaces for global land areas at a spatial resolution of
30 arc seconds, often referred to as 1-km spatial resolution. The mean temperature of the warmest quarter and the
precipitation of the warmest quarter were incorporated in the classification.
CORINE Land Cover 2000
CORINE Land Cover 2000 (CLC2000) data were used for stratification for the unsupervised clustering approach
and as a basis to generate the forest/non-forest map. CORINE Land Cover 2000 data is an update for the
reference year 2000 of the first CORINE Land Cover database which was finalised in the early 1990s as part of the
European Commission programme to coordinate information on the environment (CORINE). It provides consistent
information on land cover and land cover changes during the past decade across most European countries.
MODIS Vegetation Continuous Fields
The Vegetation Continuous Fields (VCF) data contains proportional estimates for vegetative cover types: woody
vegetation, herbaceous vegetation, and bare ground. The product is derived from all seven bands of the Moderate
Resolution Imaging Spectroradiometer sensor (Hansen et al., 2003). For the current study, the vegetative cover of
woody vegetation was taken as input for the generation of the forest/ non-forest map.
2.3 Forest/ non-forest map
An EU-wide forest/ non-forest map was generated, consistent with the Temperate and Boreal Forest Resource
Assessment –TBFRA 2000 (UNECE-FAO, 2000) at the national level. For areas where CORINE land cover data
are available, the CORINE dataset was aggregated from the original 100 meters to 500 meters spatial resolution.
Firstly, the number of forest pixels within each 5 by 5 pixel aggregation unit was calculated. Secondly, a threshold
with the minimum number of forested pixels within the aggregation units was determined for each country. This
threshold was selected accordingly, to generate a forest map in agreement with the total forest area given by
TBFRA 2000 at the national level.
For areas not covered by CORINE data, a similar approach was applied with Vegetation Continuous Fields data.
The area covered with woody vegetation in the VCF data is given in percent. A percentage threshold of the
minimum area covered by woody vegetation was defined for each country in order to produce the forest/ non-forest
map. Also in this case, the total forest area is coherent with TBFRA 2000 data.
The result is an EU-wide forest/ non-forest map in agreement with the Temperate and Boreal Forest Resource
Assessment –TBFRA 2000 on total forest areas at the national level.
2.4 Growing stock estimation
Growing stock or stem volume is defined in this study as volume of standing trees, above stump measured over
bark. The data sources described above were used for the EU-wide estimation of the growing stock of the two
species groups “conifers” and “broadleaves”. The comprehensive field measurement plot data were divided in two
parts: one part was taken for training and the other part was used for an independent accuracy assessment.
An automatic up-scaling approach was applied to allow the up-scaling of plot measurements and remote sensing
data of a different spatial resolution, e.g. up-scaling of forest inventory plot data with typical measurement area
below 1 ha and MODIS satellite data with a pixel size of 25 ha. As opposed to conventional classification methods
such as k-NN classification (Tomppo et al., 2002) which relate the plot measurements directly to remote sensing
data, this approach has the main advantage that it is not sensitive to scale mismatch between the area covered by
the individual plots and the area covered by each pixel.
The main processing steps of the automatic up-scaling approach are as follows:
1. Clustering is performed which separates the remote sensing data into classes such that the between-
class variance of the specified number of classes is maximized. The determination of the clusters is not
performed for the whole area, but separately for selected land cover types, based on ancillary data.
2. For each class, fractional cover maps or posterior probability of membership are calculated for each
pixel.
3. For each class, a training set consisting of “pure” pixels which show a high posterior probability of
membership to the respective class is selected.
4. The sampling plot or point measurements located within each training set are used to estimate the
respective parameter, e.g. mean growing stock for each class.
5. The classification of the whole area is then performed by weighting the class mean values with the
fractional cover maps or posterior probability of membership of the respective class.
The detailed processing steps as applied for the generation of the EU-wide mapping of growing stock are
described in the following paragraphs (compare also Figure 1, General workflow).
Step 1 – Clustering
For clustering, the Iterative Self-Organizing Data Analysis Techniques (ISODATA) algorithm was applied. CORINE
land cover data was used as ancillary information to avoid selection of clusters which represent composed land
cover types (e.g. cluster which represent mixed agriculture / forest land cover). For each selected land cover
category, only those MODIS pixels were utilised which show a homogeneous land cover (all CORINE pixels within
the respective MODIS pixel belong to the same land cover type).
Step 2 – Calculation of fractional cover maps for each category
To measure the fractional coverage of the different land cover categories within each pixel, the posterior probability
of class membership was calculated (Foody et al., 1992; Foody and Cox, 1994; Häme et al., 2001). The posterior
probability was calculated according to Strahler (1980):
The result of this processing step is one layer for each land cover class with posterior probability of membership to
the respective class.
Step 3 – Automatic selection of training sets
For each class, the pixels selected for training are the ones that include NFI sample plot data and that show a high
posterior probability of membership to the respective class (Foody and Arora, 1996). Contrary to conventional
classification, where homogeneous training areas are often used as reference data, this approach produces one
training set for each class composed of dispersed pixels.
Step 4 – Calculation of mean growing stock for each class
The NFI plots located within each training set are used to calculate mean growing stock for each class, separately
for the species groups “conifers” and “broadleaves”. Since the mean values are determined by sampling, the
method is not sensitive to mismatch in spatial scale between the plot measurements and the pixel size. This is the
main advantage of this approach compared to conventional classification methods.
Step 5 - Classification of the whole area
The classification of the growing stock is performed by weighting the class mean values with the posterior
probability of membership of the respective class (Foody et al., 1992; Foody and Arora, 1996; Häme et al., 2001).
The classification procedure was performed separately for the three strata: Northern, Central and Southern Europe.
The result of these processing steps is an estimation of the growing stock for each pixel, separately for the species
groups “conifers” and “broadleaves”.
Accuracy assessment
For assessing the accuracy of the results, 24 regions were selected that included at least 250 field measured plots
located within each region. To allow an accuracy assessment independent of the classification, these plots were
not used in the previous classification process. For each region, the mean growing stock per hectare forest for
conifers and broadleaves was calculated with the field measured plot data. These estimates were compared with
the mean growing stock per hectare of forest derived from the classification. Areas at the forest borders were
excluded for the calculation of the mean values from the classification, because they are composed of forest and
non-forest areas. The results of the comparison are presented in Table 1 and in Figure 2. The accuracy
assessment is representative for those regions for which appropriate ground reference plot data were provided by
the national forest inventory mapping agencies (Belgium, Croatia, Estonia, Finland, France, Germany, Italy,
Lithuania, Netherlands, Norway, Portugal, Slovak Republic, Slovenia, Spain, Sweden).
Fig. 2: Comparison of coniferous, broadleaved and total growing stock estimates based on remote sensing, with
estimates based on field measurements for n=24 selected regions. The correlation coefficient R is between 0.94
and 0.97.
Table 1: Comparison of remote sensing based growing stock estimates (m³/ha) and field based growing stock
estimates for n=24 selected regions.
Total growing
stock Broadleaved
growing stock
Coniferous
growing stock
Correlation coefficient 0.97 0.94 0.96
RMSE
a
32 m³/ha 25 m³/ha 32 m³/ha
MAE
b
25 m³/ha 20 m³/ha 25 m³/ha
MAE
rc
11 % 23 % 17 %
Despite the high correlations, the accuracy assessment shows a slight underestimation of the growing stock in
regions characterised by high growing stock volume. This may be caused by saturation effects, as optical sensors
such as the utilised MODIS bands measure only the upper layers of vegetation cover, and is in agreement with
analysis of others, e.g. Tomppo et al. (2002). As the comparison was performed at the regional level, this
assessment shows the accuracy at the regional level, but not at the pixel level. The accuracy assessment was
performed prior to the calibration at the regional level, which is described below.
Calibration based on regional EFISCEN data:
At the regional level, the accuracy of the mean growing stock estimates from field based forest inventories is
generally high. The estimates of growing stock from forest inventories are usually included in a 5% range of the
mean (Liski et al. 2006). As shown in Table 1, the relative mean absolute error of the remote sensing based
growing stock estimate is 11 % at the regional level. The inventory-based regional data from EFISCEN were
therefore used to calibrate the remote sensing based estimates in those areas which are covered by EFISCEN
data.
EFISCEN is an inventory-based model that projects the development of forest resources under different
management and wood demand scenarios (Sallnäs 1990; Schelhaas et al., 2007). The model results are projected
on a five-year time scale at the regional, national and European level (Eggers et al., 2008; Nabuurs et al., 2007).
National forest inventories are used as input data sources to define the state of forests and their future
development. In the model, the forest is structured as a matrix in which the forest area is distributed over age and
volume classes for each tree species. Growth dynamics are simulated by shifting forest area between matrix cells
and the growth is driven by functions based on inventory data or yield tables. The development of forest resources
is also influenced by the management regimes (thinnings and fellings) and by the increase or decrease of forest
area. Some of the outputs of the model are the projection of forest area, growing stock, increment, harvest level,
age class distribution and biomass carbon stock for different tree compartments. The results are aggregated at the
regional level covering 27 European countries (EU27, Greece and Malta excluded and Switzerland and Norway
included).
Growing stock and above-ground biomass were estimated for the year 2000 for the European Union (excluding
Cyprus, Greece and Malta), Norway and Switzerland. For Bulgaria, the Czech Republic, Estonia, Latvia and
Lithuania the initial inventory data in the EFISCEN dataset represented the year 2000. For the other countries with
forest inventory data, we simulated forest resource development until 2000 based on (i) conventional forest
management regimes (Nabuurs et al. 2007), (ii) historical roundwood production converted to overbark volumes
(UNECE-FAO, 2000) from coniferous and broadleaved species separately, and (iii) used harvest residues. To
capture regional differences in management intensity, we regionalised the harvest volumes for Austria, Finland,
France, Germany, Norway, Sweden and Switzerland based on national reports. We scaled the Forest available for
wood supply in our database to the values reported by MCPFE (2007) to correct for small deviations in the area.
To calibrate the classification results, the EFISCEN data on the growing stock in 2000 were aggregated in the two
species groups “broadleaves” and “conifers”. The regional mean values on growing stock derived from the
classification results were then adjusted to correspond to the regional EFISCEN data by multiplication. For areas
not covered by the EFISCEN model, no calibration was performed.
2.5 Carbon stock of the above-ground woody biomass
Biomass conversion and expansion factors (BCEFs) were applied to convert the growing stock results from remote
sensing to the carbon stock in above-ground woody biomass. Mean BCEFs were calculated for conifers and
broadleaves at the regional level based on EFISCEN results. In EFISCEN, age- and species-dependent BCEFs
are applied to convert the growing stock to biomass over bark for the different tree compartments (stem, branches,
foliage, coarse roots and fine roots). The factors are calculated on the basis of species-specific allometric
equations and yield tables (Vilén et al., 2005; Mokany et al., 2006; Gasparini et al., 2006). For this study,
aggregated BCEFs for broadleaves and conifers were calculated by dividing the mean above-ground woody
biomass (stem and branches) at the regional level for the mean growing stock in the same region.
In the countries not covered by EFISCEN projection, the expansion factors were calculated in a similar way based
on the data published in the Temperate and Boreal Forest Resource Assessment – TBFRA 2000 (UNECE-FAO,
2000) or by applying the aggregated EFISCEN BCEFs from neighbouring countries where no data were available.
The woody biomass was converted to carbon stock (tC ha
-1
) by applying a constant carbon fraction of 50% (IPCC,
2003).
3. Results and discussion
3.1. Growing stock
Maps of retrieved coniferous, broadleaved and total growing stock per hectare are presented in Figures 3, 4 and 5.
The forest type map, based on the share of coniferous growing stock in the total growing stock is shown in Figure
6. The spatial distribution of the assessed data is determined with a pixel size of 25 ha.
The higher growing stocks are concentrated in the Central European countries, in particular in mountain areas.
Coherently with the ecological distribution of tree species, the growing stock of broadleaves increases at lower
latitudes, while conifers have higher stocks at high latitude and altitude.
Fig. 3: Growing stock, coniferous per hectare of forest.
Fig. 4: Growing stock, broadleaved per hectare of forest.
Fig. 5: Total growing stock per hectare of forest.
Fig. 6: Forest type map, based on share of coniferous growing stock in total growing stock as defined in the legend.
The accuracy assessment shows a high correlation between the remote sensing based estimates and the
estimates based on field inventories at the regional level with correlation coefficients of r = 0.96 for coniferous,
r=0.94 for broadleaved and r=0.97 for total growing stock per hectare of forest. The mean absolute error of the
estimations is 25 m³/ha for coniferous, 20 m³/ha for broadleaved and 25 m³/ha for total growing stock per hectare.
Despite the high correlations, the accuracy assessment shows a slight underestimation of the growing stock in
regions characterised by high growing stock volume. This may be caused by saturation effects, as optical sensors
such as the utilised MODIS bands measure only the upper layers of vegetation cover. For those regions for which
EFISCEN data was available for recalibration of the classification result, this underestimation is corrected. The
accuracy assessment was performed at the regional level, but not at the pixel level. For future applications the
accuracy should also be assessed for smaller reference units, up to the pixel level. However, this necessitates
comprehensive data with area-wide coverage of reference sites, e.g. derived from laser scanning, which were not
available for the current study.
The results give area-wide spatial explicit information on the forest resources for the whole European Union, EFTA
countries, the Balkans, Belarus, the Ukraine, Moldova, Armenia, Azerbaijan, Georgia and Turkey with a high spatial
resolution of 25 ha per grid cell which was not available up to now.
3.2. Carbon stock in the above-ground biomass
Carbon stock in the above-ground forest biomass was calculated separately for broadleaves and conifers with a
spatial resolution of 500 meters, according to the spatial resolution of the MODIS satellite image data. Aggregated
biomass conversion and expansion factors (BCEFs) were applied to convert the remote sensing based growing
stock classification results to carbon stock of the above-ground forest biomass. As the accuracy of the aggregated
BCEFs is not known for all regions, no separate validation of the carbon stock estimates was performed. The
uncertainty of the biomass carbon stock using EFISCEN, which includes the uncertainty of BCEFs, was estimated
to be quite low. The calculated coefficient of variation in 4 pilot countries was equal to 2-5% (Meyer et al. 2005).
The biomass expansion factors in Great Britain, converting stem biomass to above-ground biomass, show a
coefficient of variation of 4-14% (Levy et al. 2004). In Lethonen et al. (2004) a relative standard error of 3-21% was
calculated for the BCEF converting growing stock to total biomass for different species and age classes. The higher
uncertainties refer only to BCEFs for young age classes, while the relative standard error is about 3-5% for factors
of mature stands. Similar ranges are also reported for the Czech Republic (Lethonen et al. 2007).
The total sum of carbon stocks in above-ground forest biomass within a raster of 10km by 10km is shown in Figure
7. Whereas Figures 3 and 4 show the mean values of growing stock per hectare forest within 500m by 500m grid
cells, Figure 7 shows the spatial distribution of the carbon stock aggregated within 10km by 10km grid cells.
Fig. 7: Total carbon stock of above-ground forest biomass within a 10km by 10km raster.
As for the growing stock, the highest densities of tree carbon stock in Europe can be found in mountain areas and
in the southern part of Fenno-Scandia. A relevant amount of carbon stock also exists in some parts of the Baltic
countries and Belarus. The lowest carbon stocks were calculated for the Mediterranean countries (Greece, Spain
and Turkey) because of the low productivity of forests and in Great Britain and Ireland because of the limited extent
of forest area.
4. Conclusions
This study presents the estimates of coniferous and broadleaved growing stock as well as carbon stock of the
above-ground biomass with a spatial resolution of 500m by 500m per grid cell. The mapped area is 5 million km² of
which 2 million km² are forests. It covers the whole European Union, the EFTA countries, the Balkans, Belarus, the
Ukraine, Moldova, Armenia, Azerbaijan, Georgia and Turkey. The classification results were evaluated by the
comparison with regional estimates derived from national forest inventories independently of the classification. The
validation shows a high correlation between the classification results and the field based estimates with correlation
coefficients of r = 0.96 for coniferous, r=0.94 for broadleaved and r=0.97 for total growing stock per hectare. The
mean absolute error of the estimations is 25 m³/ha for coniferous, 20 m³/ha for broadleaved and 25 m³/ha for total
growing stock per hectare. Biomass conversion and expansion factors were applied to convert the growing stock
classification results to carbon stock in above-ground biomass.
An automatic up-scaling approach was applied to allow the up-scaling of forest inventory plot measurements with a
typical area below 1 ha and MODIS satellite data with a pixel size of 25 ha. Contrary to conventional classification
methods such as k-NN classification which relate the plot measurements directly to remote sensing data, this
sampling based approach has the main advantage that it is not sensitive to scale mismatch between the area
covered by the field measurements and the area covered by each pixel. This approach can be applied when
sampling based reference data are used for automatic classification of remote sensing imagery.
In the current study, the estimates are for the year 2000. In the future, the method will be applied to evaluate the
growing stock and the carbon stock of European forests on a yearly basis. The development of similar estimates
for different points in time will give the opportunity to analyse forest dynamics all over Europe by using satellite
images and to fill information gaps in areas or in years where/when no inventory data exist. In addition the maps
will provide a harmonised data source to be compared with other international datasets like forest statistics from
FAO with the advantage of adding spatial information on forest resources.
Acknowledgements
The work was financed by the European Union within the project “CarboEurope-IP - Assessment of the European
Terrestrial Carbon Balance”, Framework Progamme 6 (GOCE-CT-2003-505572), and by JOANNEUM RESEARCH
Forschungsgesellschaft mbH., Austria.
We’re greatly indebted to the Forest Focus programme and the national forest inventory institutes’ correspondents.
Special thanks go to the many NFI field crews in the countries. NFI plot data were received from: Jacques
Rondeux and Martine Waterinckx, Belgium; Juro Cavlovic, Croatia; Veiko Aderman, Estonia; Kari Korhonen,
Finland; Thierry Bélouard, France; Heino Polley, Germany; Marino Vignoli, Remo Bertani, Giorgio Dalmasso &
Maurizio Teobaldelli, Italy; Andrius Kuliesis, Lithuania; Wim Daamen and Henny Schoonderwoerd, Netherlands;
Stein Tomter, Norway; Susanna Barreiro & Margarida Tomé, Portugal; Olivier Bouriaud, Romania; Vladimir Seben,
Slovak Republic; Gal Kusar, Slovenia; J. Villanueva and Antoni Trasobares, Spain; Göran Kempe, Sweden; Bill
Mason & Shona Cameron, United Kingdom; Igor Buksha, Ukraine. The collection of national forest inventory plot
data was carried out under Eforwood-IP, Carbo Europe IP, ADAM-IP and in connection with COST E43. Gert-Jan
Nabuurs and Geerten Hengeveld were partially funded under the BSIK-IC2 Programme.
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