Indirect carbon dioxide emissions from producing

bioenergy from forest harvest residues

ANNA REPO *, MIKKO TUOMI *w and J A R I L I S K I *

*Finnish Environment Institute, PO Box 140, FI-00251 Helsinki, Finland, wDepartment of Mathematics and Statistics,

University of Helsinki, PO Box 68, FI-00014 Helsinki, Finland

Abstract

Forest harvest residues are important raw materials for bioenergy in regions practicing

forestry. Removing these residues from a harvest site reduces the carbon stock of the

forest compared with conventional stem-only harvest because less litter in left on the

site. The indirect carbon dioxide (CO2) emission from producing bioenergy occur when

carbon in the logging residues is emitted into the atmosphere at once through combustion, instead of being released little by little as a result of decomposition at the harvest

sites. In this study (1) we introduce an approach to calculate this indirect emission from

using logging residues for bioenergy production, and (2) estimate this emission at a

typical target of harvest residue removal, i.e. boreal Norway spruce forest in Finland.

The removal of stumps caused a larger indirect emission per unit of energy produced

than the removal of branches because of a lower decomposition rate of the stumps. The

indirect emission per unit of energy produced decreased with time since starting to

collect the harvest residues as a result of decomposition at older harvest sites. During the

100 years of conducting this practice, the indirect emission from average-sized branches

(diameter 2 cm) decreased from 340 to 70 kg CO2 eq.MWh1 and that from stumps

(diameter 26 cm) from 340 to 160 kg CO2 eq.MWh1

. These emissions are an order of

magnitude larger than the other emissions (collecting, transporting, etc.) from the

bioenergy production chain. When the bioenergy production was started, the total

emissions were comparable to fossil fuels. The practice had to be carried out for 22

(stumps) or four (branches) years until the total emissions dropped below the emissions

of natural gas. Our results emphasize the importance of accounting for land-use-related

indirect emissions to correctly estimate the efficiency of bioenergy in reducing CO2

emission into the atmosphere.

Keywords: bioenergy, forest harvest residue, indirect emissions, land use, soil carbon, Yasso07

Received 14 May 2010 and accepted 30 June 2010

Introduction

Bioenergy, i.e. energy derived from renewable biomass,

is used to replace fossil fuels in energy production in

order to decrease greenhouse gas emissions into the

atmosphere. The rationale behind this practice is that

bioenergy does not cause any net carbon dioxide (CO2)

emissions since the amount of CO2 released into the

atmosphere in combustion is taken up again by the next

generation of growing plants (Wihersaari, 2005; Stupak

et al., 2007; Lattimore et al., 2009).

Following this idea, as a means to cut down greenhouse gas emissions, the Council of the European

Union (EU) adopted a directive on the promotion of

renewable energy, including bioenergy. This directive

set targets to produce 20% of the final energy consumption using renewable energy sources in the EU by

the year 2020. This target is higher for member states

already producing a lot of renewable energy, for example as a by-product of pulping industry. Consequently,

the national commitment is 38% for Finland and

49% for Sweden (Directive 2009/28/EC). During the

reference year of the directive 2005, renewable sources

represented already 28% of the total energy production

in Finland and 39% in Sweden, while the EU-average

was 11%.

These high targets for renewable energy are increasing the focus on biomass for energy production. Worldwide, this growing interest in bioenergy puts pressure

Correspondence: Anna Repo, tel. 1 358 400 148 652, e-mail:

anna.repo@ymparisto.fi

GCB Bioenergy (2010), doi: 10.1111/j.1757-1707.2010.01065.x

r 2010 Blackwell Publishing Ltd 1
on land use changes, including deforestation and

consequent conversion of the forest land to energy crop

cultivation (Melillo et al., 2009).

Recently, indirect CO2 emissions from bioenergy

production associated with these land use changes have

caused concern (Searchinger et al., 2008, 2009; Melillo

et al., 2009). These indirect emissions occur when bioenergy production reduces the carbon stocks of biomass

or soil. These carbon losses may be remarkable, and it is

even possible that replacing fossil fuels with bioenergy

increases net greenhouse gas emissions into the atmosphere as a consequence of large indirect emissions. The

assumed CO2 neutrality of biofuels like ethanol and

their actual potential to mitigate climate change have

already been questioned because of large negative impacts on the carbon stock of soil (Fargione et al., 2008;

Searchinger et al., 2008, 2009; Melillo et al., 2009).

It is important to realize that the indirect emissions of

bioenergy production are not limited to the cases of

land use change but may also be caused by new

practices of ecosystem management within the same

land use. In countries with extensive forest cover and an

already high share of renewable energy, an appealing

way to produce bioenergy is to intensify biomass removals from forests. Forested countries Finland and

Sweden are pioneers in the field of using forest residues

for energy production (Ma ̈lkki & Virtanen, 2003). Still,

in order to meet the EU commitment of renewable

energy, Finland plans to increase the use of logging

residues for energy production from 3.6 Mm3 yr1 in

2006 to 12 Mm3 yr1 by 2020 (Ministry of Employment

and the Economy of Finland, 2008).

Until now, research on the effects of logging residue

removal has focused on nutrient balances (e.g. Wall,

2008; Luiro et al., 2009), socioeconomic impacts (e.g.

Bo ̈rjesson, 2000), profitability (e.g. Heikkila ̈ et al., 2007),

forest productivity (e.g. Peng et al., 2002) and properties

of wood fuel (e.g. Alakangas, 2005), whereas the indirect emissions have received little attention. Lattimore

et al. (2009) dealt with the indirect emissions to some

extent in their recent review. They concluded that, in

order to be sustainable, bioenergy production from

forest residues must not have adverse effects on soil

quality, hydrology and water quality, site productivity,

or forest biodiversity but also not on greenhouse gas

balances.

The indirect emissions from removing forest harvest

residues, and using them for energy production, result

from combusting the residues and releasing CO2 into

the atmosphere soon after harvesting instead of letting

them decompose slowly at the harvested site. As a

consequence of such practice, the amount of carbon

stored at the forest site decreases, possibly to a remarkable degree.

There are some field studies (Johnson et al., 2002;

Jones et al., 2008) and model-based calculations (Palosuo et al., 2001; Peng et al., 2002; A ̊ gren & Hyvo ̈nen,

2003; Eriksson et al., 2007) regarding the effect of logging residue removal on the carbon stock of soil. Some

of these studies show clearly that intensified removal of

harvest residues reduces the soil carbon stock (Johnson

& Curtis, 2001; A ̊ gren & Hyvo ̈nen, 2003; Eriksson et al.,

2007). Palosuo et al. (2001) estimated that the indirect

emissions from decreasing carbon stock are an order of

magnitude larger than the other emissions from an

energy production chain utilizing forest harvest residues. Despite the significant contribution of indirect

emissions to the estimate of total emissions per unit

of produced energy, Palosuo et al. (2001) calculate

emissions of approximately 50 kg CO2 eq. MWh1

,

which is 80–90% less than the emissions from various

fossil fuels.

The indirect emission of using logging residues for

energy production depend critically on the decomposition rate of the residues if they were left at the site.

Studies based on extensive sets of measurements have

been published recently making it possible to estimate

the decomposition rate of the harvest residues more

reliably than before (e.g. Tarasov & Birdsey, 2001;

Palviainen et al., 2004; Ma ̈kinen et al., 2006; Va ́vrˇova ́

et al., 2009). We have used these measurements plus

other measurements related to decomposition and carbon cycling in soil and developed a new soil carbon

model Yasso07 (Tuomi et al., 2008, 2009). The large

datasets used and advanced mathematical methods

applied make the Yasso07 model particularly suitable

for estimating the decomposition rate of woody litter in

boreal forests.

In this study, we used this model to estimate the

indirect emissions from using logging residues for

bioenergy production. The objectives of the current

study were to (1) introduce an approach to estimate

the indirect CO2 emissions associated with bioenergy

production from forest harvest residues, (2) estimate

these emissions in a forested boreal landscape during

the first 100 years after starting to produce bioenergy

from harvest residues, and (3) compare the total CO2

emissions per unit of bioenergy produced to the emissions caused by using other fuels.

Materials and methods

Modelling decomposition of forest harvest residues

To estimate the indirect CO2 emissions from producing

bioenergy from forest harvest residues in boreal conditions, we simulated the decomposition of logging residues using a user-interface of the dynamic soil carbon

2 A. REPO et al.

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model Yasso07 (Tuomi et al., 2008, 2009, http://

www.environment.fi/syke/yasso). The measurements

of the decomposition of woody litter used to develop

the model were taken in Finland and neighboring

regions in Estonia and Russia (M. Tuomi, R. Laiho,

A. Repo & J. Liski, unpublished results). These measurements used represent the majority of data on

woody litter decomposition in this region. The data

sets includes branches and stems ranging from 0.5 to

60 cm in diameter, and the mass loss of these woody

biomass components has been followed for 1–70 years

since the start of decomposition. In addition to these

data, the Yasso07 model is based on an extensive data

set on decomposition of nonwoody litter across Europe

and North and Central America (Tuomi et al., 2009)

plus data sets on the accumulation and stock of soil

organic carbon (Liski & Westman, 1995, 1997; Liski

et al., 1998). These additional measurements specially

provide information on the cycling of recalcitrant organic carbon compounds in soil that are relevant for the

long-term carbon balance of decomposing woody litter.

The parameter values of Yasso07 have been sampled

using a Markov chain Monte Carlo method (Tuomi et al.,

2009). This method has been used to make sure that,

first, the model is not over-parameterized given the data

and, second, there are unequivocal maximum likelihood

values for each parameter combination. This mathematical approach and the data available in the development

process of Yasso07 make this model suitable for this

study because the uncertainty estimates of the model

predictions are available and because the data covers the

simulated scenarios well without extrapolation.

Model simulations

Using Yasso07, we simulated the decomposition of

harvest residues at a typical site of harvest residue

removal in Finland, namely an even-aged mature 81–

100-year-old Norway spruce (Picea abies L.) forest stand

located in the Pirkanmaa region of Southern Finland.

Spruce stands cover some 40% of forest area in this

region (Korhonen et al., 2000). Clear-cut spruce stands

are favorable targets of harvest residue removals because there are more logging residues than in clear-cut

Scots pine stands, which are also common in the region.

Spruce stumps are also preferred over pine stumps

because they are easier to extract from the soil because

of a shallower root structure (Alakangas, 2005; Wihersaari, 2005).

To illustrate differences in decomposition rate between different harvest residues, we simulated decomposition of spruce branches varying from 1 to 5 cm in

diameter and stumps varying from 10 to 35 cm in

diameter for a 100-year period after the start of decomposition. The mean diameters of spruce branches and

stems in the study region, 2 and 26 cm, respectively

(Korhonen et al., 2000; Kantola et al., 2007), were chosen

for more detailed analyses of the indirect emissions

caused by using the logging residues for bioenergy

production. The other input variables of the Yasso07

model used in the simulations are shown in Table 1.

When estimating the indirect emissions we assumed

that needles were left at the site and only branches or

stumps were removed. We assumed also that there was

little or no delay in combusting the harvest residues at a

power plant and thus CO2 was released to the atmosphere at once.

Energy production estimation

The indirect CO2 emissions from using the harvest

residues for bioenergy production were taken to be

equal to amount of carbon remaining in the harvest

residues if the residues were left to decompose at the

site harvested. These emissions were also related to the

amount of bioenergy produced. The cumulative indirect emissions caused by combusting the harvest residues until year i were calculated by summing up the

amounts of carbon left in the harvest residues until this

year (i) and relating these emissions to the cumulative

amount of bioenergy produced. In other words, we

Table 1 The values of input variables used in the Yasso07

model

Climate variables

Mean annual temperature 3.2 1C

Temperature amplitude 11.6 1C

Precipitation 681 mm

Chemical composition of woody

litter

Branch  SD/stump  SD

(%)

Acid hydrolysable compounds 59  4.3/70  5.0

Water soluble compounds 1  0.3/1  0.2

Ethanol soluble compounds 1  0.3/1  0.2

Klason lignin (neither

hydrolysable

nor soluble compounds)

37  1.0/28  0.8

The climate values represent averages for southern Finland

between 1971 and 2000 (Drebs et al., 2002) and the chemical

composition averages of several individual studies (Hakkila,

1989). The standard deviation (SD) values of the chemical

composition are based on coefficients of variation calculated

from a data base of foliage litter (Berg et al., 1991). The

temperature amplitude means a half of the difference between

the mean temperatures of the warmest and the coldest month

of the year.

INDIRECT EMISSIONS OF HARVEST RESIDUE BIOENERGY 3

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simulated a case where the practice of removing the

harvest residues and using them for bioenergy production was started and continued on a harvest area of

similar size year after year. We applied biomass-compartment-specific net calorific heating values to estimate the amount of energy obtained by combusting

the harvest residues, i.e. 19.30 MJ kg1 for dry Norway

spruce branches and 19.18 MJ kg1 for dry stumps

(Nurmi, 1997). These values of dry logging residues

range commonly from 18 to 20 MJ kg1 (Alakangas,

2005). The carbon content of the harvest residues was

assumed to be equal to 50% of dry wood (m/m).

In order to estimate the full fuel cycle emissions from

using the logging residues for energy production, we

added other emissions from a typical wood chip fuel

production chain using harvest residues to the calculated indirect emissions. These emissions result from (1)

collecting, chipping, and transporting the harvest residues, (2) emitting methane (CH4) and nitrous oxide

(N2O) from combustion, (3) fertilizing the forest to

compensate for nutrient loss, and (4) recycling ash,

and they range typically from 5 to 18 kg CO2 eq. MWh1

produced (Palosuo et al., 2001; Wihersaari, 2005). We

used a central value of this range, 12 kg CO2 eq. MWh1

produced, in our calculations. Other estimates for direct

emissions from wood fuel chain range from 5 to 20 kg

CO2 eq. MWh1 (Korpilahti, 1998; Ma ̈lkki & Virtanen,

2003; Berg, 2010) depending on background information and included operations in the estimates. Fossil

fuel comparison values 280, 306, and 395 kg CO2 eq.

MWh1 were estimates of entire fuel cycle emissions

of natural gas, oil, and diesel, and coal (Statistics Finland, 2006; Ecoinvent Centre, 2007).

Results

Branches lost mass at a remarkably higher rate than

stumps because the simulated decomposition rate

of woody litter was dependent on the initial diameter

(Fig. 1). For example, after 10 years of decomposition,

the branches 1–5 cm in diameter had 30–55% of the

initial mass still remaining (Fig. 1a) while stumps 10–

35 cm in diameter had 63–81% (Fig. 1b). The simulated

rate of mass loss decreased over time, and after 100

years of decomposition there was still 2–16% of the

initial branch mass remaining and 19–28% of the initial

stump mass remaining.

The indirect CO2 emissions, caused by combusting

logging residues after harvesting instead of letting them

decompose at the harvested site, were equal to the CO2

emissions from combustion, 340 kg CO2MWh1, when

the practice was started but these emissions decreased

over time as a result of decomposition of the harvest

residues (Fig. 2). The indirect emissions of using

branches for bioenergy decreased faster than

the emissions of using stumps because the branches

decomposed faster (Figs 1 and 2). After the first 10 years

of conducting this practice, the indirect emissions from

branches were equal to 200 kg CO2MWh1 and those

from stumps 310 kg CO2MWh1 (Fig. 2). After the first

100 years, these emissions from the branches and

stumps were equal to 70 and 160 kg CO2MWh1

,

respectively.

The estimated greenhouse gas emissions from the rest

of the bioenergy production chain, i.e. the emissions

from collecting, transporting, chipping, and combusting

the harvest residues plus the emissions from fertilizing

the forest and recycling the ash, were equal to 12 kg

CO2 eq. MWh1 (Fig. 3). At the time of starting this

practice, these direct emissions represented only 3% of

the total emissions caused by using the harvest residues

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90 100

Stump mass remaining (relative units)

Time since the start of decomposition (year)

35 cm

26 cm

20 cm

10 cm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90 100

Branch mass remaining (relative units)

5 cm

2 cm

1 cm

Time since the start of decomposition (year)

(a)

(b)

Fig. 1 Mass remaining of decomposing Norway spruce

branches (diameter 1–5 cm) and stumps (diameter 10–35 cm)

over a 100-year period after the start of decomposition in southern boreal conditions as simulated using the Yasso07 model

(model input values in Table 1).

4 A. REPO et al.

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for bioenergy. After the 100-year period of conducting

this practice, this share increased to 15% if bioenergy

was produced from branches and to 7% if bioenergy

was produced from stumps (Fig. 3). The increased

contribution of the direct emissions was a result of the

decreased indirect emissions (Fig. 2).

At the time of starting to use the harvest residues for

energy production, the total emissions were comparable

to the emissions caused by using fossil fuels (Fig. 3).

After 10 years of producing bioenergy from branches,

the total emissions caused were 210 kg CO2 eq. MWh1

.

These emissions are 24%, 30%, or 46% lower than the

emissions caused by producing the energy from natural

gas, oil, or coal, respectively. If bioenergy was produced

from stumps, it took 22 years for the average emissions

to decrease below the emissions of producing the

energy from natural gas or 14 years for the emissions

to decrease below the emissions of oil. After 100 years of

producing energy by combusting branches, the emissions were lower by 71%, 74%, or 79% than the emissions caused by producing the energy from natural gas,

oil, or coal, respectively. For bioenergy produced by

combusting stumps, these percentages of emission reductions were 40%, 46%, or 58%, over 100 years, compared with the emissions of natural gas, oil, or coal,

respectively.

Discussion

Using logging residues for energy production decreases

the amount of carbon stored in forest and causes thus

indirect CO2 emissions into the atmosphere. These

indirect emissions occur because combustion releases

carbon of logging residues to the atmosphere at once,

otherwise the residues would form a slowly decomposing carbon stock at the harvest site. The indirect emissions are an order of magnitude larger than the direct

emissions from the rest of this bioenergy production

chain (Fig. 3), not accounting for the CO2 emissions of

combusting the harvest residues. The indirect emissions

depend on the decomposition rate of the harvest residues, with the decomposition rate being lower with an

increasing size of woody litter (Fig. 1, Harmon et al.,

1986; Janisch et al., 2009; M. Tuomi, R. Laiho, A. Repo &

J. Liski, unpublished results). The use of bigger-sized

stumps for energy production causes therefore larger

indirect emissions than the use of smaller-sized

branches (Fig. 2). The average indirect emissions per

unit of energy produced decrease over time since the

start of this form of bioenergy production (Fig. 2) but

during the first few years, or a couple of decades in the

case of stumps, the total emissions caused by energy

production from harvest residues are comparable to the

emissions of fossil fuels (Fig. 3).

The reliability of the current results depends critically

on the decomposition estimates of the harvest residues.

The Yasso07 decomposition and soil carbon model used

in this study is based on a large collection of mass loss

measurements taken on woody litter in boreal forests

across Finland and neighboring countries plus large

0

50

100

150

200

250

300

350

400

0 10 20 30 40 50 60 70 80 90 100

Indirect emission (kg CO2 MWh−1)

Time since starting the practice of bioenergy

production (year)

Spruce stump

Spruce branch

Fig. 2 Indirect CO2 emission per unit of energy produced from

using Norway spruce stumps (diameter 26 cm) or branches

(diameter 2 cm) for bioenergy over a 100-year period after

starting this practice. This indirect emission is equal to the

carbon stock of woody litter lost in combustion per energy

obtained until each year.

0

50

100

150

200

250

300

350

400

450

0 10 20 30 40 50 60 70 80 90 100

Time since starting the practice of energy production (year)

Total emission (kg CO2 eq. MWh−1)

Fig. 3 The total greenhouse gas emission per unit of energy

produced from using Norway spruce branches (diameter 2 cm)

or stumps (diameter 26 cm) for bioenergy over a 100-year period

after starting this practice and the total emissions from various

fossil fuels. The emission estimates of bioenergy production

include both an indirect (see Fig. 2) emission resulting from

decreasing carbon stock and a direct wood fuel chain emission

(equal to 12 kg CO2 eq. MW h1

) resulting from collecting, chipping and transporting the harvest residues, CH4 and N2O

emissions from combusting the residues, fertilizing the forest

to compensate for nutrient loss, and recycling of ash. The

estimates of the fossil fuels represent entire fuel cycle emissions

(Statistics Finland, 2006; Ecoinvent Centre, 2007).

INDIRECT EMISSIONS OF HARVEST RESIDUE BIOENERGY 5

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sets of other relevant measurements from across the

world (see ‘Materials and methods’). This model has

been shown to give unbiased estimates for the decomposition of woody (M. Tuomi, R. Laiho, A. Repo &

J. Liski, unpublished results) and nonwoody litter

(Tuomi et al., 2009). Compared with other studies on

the decomposition of woody litter carried out under

comparable conditions, the estimates of this study are

similar except for the end of the 100-year study period;

for this late phase of decomposition, the current estimates of mass remaining are higher. Ma ̈kinen et al.

(2006) estimated that Norway spruce stems lost about

20% of their initial mass during the first 10 years of

decomposition and the stems disappeared completely

in some 60–80 years of time. On the other hand,

according to the residuals reported by Ma ̈kinen et al.

(2006), their model seems to underestimate the mass

remaining after 30 years of decomposition. Melin et al.

(2009) compared several decomposition models developed for Norway spruce logs, snags, and stumps. They

concluded that after 10 years of decomposition some

60–75% of the initial mass was still remaining whereas

after 100 years of decomposition practically none or

o10% of the initial mass was still left. Our higher

estimates of mass remaining during the late phases of

decomposition can be explained by a difference between the methods used. The two earlier studies were

based on measurements of woody litter mass remaining. These measurements may not capture the formation and translocation of well-decayed soil organic

matter originating from woody litter. The Yasso07 model, on the other hand, is additionally based on measurements of formation of soil organic matter (Tuomi et al.,

2009). For this reason, we think that the estimates of

Yasso07 model are probably more realistic for the late

phases of woody litter decomposition.

In addition to the decomposition estimates, the reliability of the current results depends also on the other

parameters of our calculations such as the reference

energy systems or combustion techniques chosen as

well as variation in the chemical composition of litter

(affecting the decomposition rate estimates of the harvest residues) and calorific values. Furthermore, the

simulations were done for climate conditions prevailing

in Southern Finland today. The results do not thus

account for the effects of climate change or those of

increased atmospheric CO2 concentration on forest

growth. In addition, forest soil disruption associated

with stump removal may release additional CO2 into

the atmosphere. Currently, empirical research on the

magnitude of these emissions is few in number (Jandl

et al., 2007; Walmsley & Godbold, 2010). Hope (2007)

found that stump removal together with forest floor

scarification reduced soil carbon stocks in the first year

of stump harvesting and 9 years later. This conclusion is

apparent only if during stump removal the forest soil is

completely scarified by removal or mixing with mineral

soil. Although the quantitative data on stump extraction

and emissions associated with this practice is scarce,

generally it is known that soil disturbance can change

the microclimate and stimulate the decomposition of

litter (Johansson, 1994). In a Finnish study site preparation after a clear-cut with mixing organic matter with

mineral soil increased CO2 efflux from soil but this effect

leveled off rapidly (Pumpanen et al., 2004). Despite of

these uncertainties, we think that the current estimates

are reliable enough to demonstrate the magnitude of

indirect emissions associated with producing bioenergy

from harvest residues in boreal coniferous forest.

The indirect emissions, caused by the reduced carbon

stock of decomposing harvest residues, represented 85–

97% of the total emissions of this bioenergy production

chain, not accounting for the CO2 emissions from

combusting the harvest residues. These emissions are

thus highly significant for the full fuel cycle emissions

of logging residues. When comparing the present

results to earlier ones it is important to acknowledge differences in system boundaries, energy usetechnology, reference energy system, conversion

technology and type and management of raw material

(Cherubini et al., 2009). Still, it is possible to conclude

that a common outcome of the earlier studies is that the

greenhouse gas emissions from logging, collecting,

chipping, and transporting harvest residues are relatively low (Bo ̈rjesson, 1996; Palosuo et al., 2001; Ma ̈lkki

& Virtanen, 2003; Wihersaari, 2005). These estimates

have ranged from 4 to 20 kg CO2MWh1 depending

on the details of the bioenergy production chains studied. The current study shows that if indirect CO2

emissions are accounted for, the total CO2 emissions

are at least an order of magnitude higher than these

emissions which have been considered to represent the

total emissions (cf. Ma ̈lkki & Virtanen, 2003).

The decreasing effect of logging residue removal on

soil carbon stock has been demonstrated earlier but this

has not been considered to be problematic as long as

this removal practice does not jeopardize the carbon

sink of soil (Bo ̈rjesson, 2000; A ̊ gren & Hyvo ̈nen, 2003;

Ma ̈lkki & Virtanen, 2003; Petersen Raymer, 2006; Eriksson et al., 2007; Sieva ̈nen et al., 2007; Eriksson & Gustavsson, 2008). Sieva ̈nen et al. (2007) calculated that

increasing the removals of logging residues from 4 to

15 Mm3 yr1 in Finland will not turn the Finnish forests

from net carbon sinks to net sources. However, the

intensified removals of the logging residues would

decrease the annual carbon sink of these forest soils

by 3.1 million tons of CO2 eq. (Sieva ̈nen et al., 2007).

Assuming that 1 m3 of harvest residues gives 2 MWh of

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energy (Alakangas, 2005), the indirect emission from the

decreasing carbon stock of soil is equal to about 100 kg

CO2 eq.MWh1

. This estimate is somewhat lower than

the estimates of the present study because Sieva ̈nen et al.

(2007) applied an earlier version of Yasso soil carbon and

decomposition model in their study (Liski et al., 2005).

This earlier model version gave less reliable, higher

estimates for the decomposition rate of woody litter

because it was based on a substantially smaller number

of measurements. Nevertheless, these figures demonstrate that it is important to relate the indirect and direct

emissions of bioenergy production to the amount of

energy obtained in order to get a correct picture on the

efficiency of using different energy sources in decreasing

greenhouse gas emissions to the atmosphere. The current results demonstrate that if the indirect CO2 emissions are counted in, the total fuel chain emissions from

using spruce branches or stumps for energy production

may cause even bigger CO2 emissions during the first

years or decades of starting this practice than producing

energy from oil or natural gas.

The indirect greenhouse gas emissions resulting from

using logging residue for bioenergy production are

highest per unit of energy produced immediately when

the practice is started. The average emissions per energy unit decrease, however, over time. As a result of

this temporal pattern, this form of bioenergy production

is not efficient in decreasing emissions to the atmosphere in the near future. Our results stress the importance of considering time perspective when assessing

the potential of different bioenergy options to mitigate

climate change (Schlamadinger & Marland, 1996; Petersen Raymer, 2006). The issue of which temporal

approach is appropriate depends on the management

and policy strategies and whether the selection of energy systems is made to meet long-term or short-term

greenhouse gas reduction objectives (Schlamadinger

et al., 1997).

It is possible to reduce the indirect emissions of

logging residue removals by collecting quickly decomposing harvest residues for bioenergy production, for

example branches instead of stumps. However, it may

be still tempting to extract stumps from harvest sites

because the gain of primary energy per hectare may be

twice that compared with collecting branches (Eriksson

& Gustavsson, 2008). Leaving the needles at the harvest

site, which helps to avoid nutrient loss, has a marginal

effect on the carbon balance at clear-cut sites, although

needle and fine root litter produce more than two-thirds

of the soil carbon stock in growing forests (A ̊ gren &

Hyvo ̈nen, 2003).

The indirect emissions have a remarkable effect on

the total greenhouse gas emissions from some systems

of bioenergy production, as demonstrated in this study

for harvest residues of boreal forests, and emphasized

for several other systems by Johnson (2009) and Searchinger et al. (2009). For this reason, to account for the

actual greenhouse gas effect of various alternative bioenergy options, it would be essential to include the

indirect emissions adequately in guidelines of greenhouse gas inventorying and reporting. Currently, the

rules of carbon accounting applied under the Kyoto

Protocol do not count all indirect emissions, which

among other things distorts the accounting of net

emissions (Johnson, 2009; Searchinger et al., 2009). A

particular shortcoming of the accounting rules under

the Kyoto Protocol is that a party to this protocol may

choose not to account for changes in one or several of

the agreed carbon pools (aboveground biomass, belowground biomass, litter, dead wood, and soil organic

carbon) as long as it can reliably show that the pool is

not decreasing. Owing to this threshold, some of the

indirect emissions caused by using logging residues for

bioenergy production may be excluded from the inventory figures. On the other hand, in inventory reports of

greenhouse gases under the United Nations Framework

Convention on Climate Change, the carbon release

resulting from forest harvesting must be counted and

reported as a land-use emission or as a energy emission

but not both (IPCC, 2000). Today, the land-use-related

greenhouse gas emissions from bioenergy production

systems are recognized and investigated (IPCC, 2000;

Melillo et al., 2009) but one of the practical problems is

that measuring methods for the indirect effects and

feasible means to bring these impacts to regulatory

policies are still lacking (Mathews & Tan, 2009).

Conclusions

Using logging residues as a source of bioenergy causes

net CO2 emissions into the atmosphere and a great

majority (85–97%) of these emissions are indirect emissions resulting from a decline in the carbon stock of

harvest residues in forest. The amount of the indirect

emissions increases with a decreasing decomposition

rate of the harvest residues. Norway spruce stumps

decay at slower rate than branches and consequently

the energy use of the stumps causes 1.5–2 times larger

indirect emissions than the use of branches. Production

of bioenergy from forest harvest residues causes emissions that are comparable to the emissions of fossil fuel

over the first few years (branches) or first few decades

(stumps) of the practice. After 50 years, bioenergy

produced of Norway spruce stumps decreases average

emissions per unit of energy produced by some 20%

and bioenergy from branches by some 60% compared

with entire fuel cycle emissions of natural gas.

INDIRECT EMISSIONS OF HARVEST RESIDUE BIOENERGY 7

r 2010 Blackwell Publishing Ltd, GCB Bioenergy, doi: 10.1111/j.1757-1707.2010.01065.x
Acknowledgements

This study was partially funded by the European Commisson

through research project ‘Greenhouse gas management in European land use systems’ (grant agreement no. 244122). We thank

two anonymous reviewers for their comments and suggestions.

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