• Open Access

Life cycle assessment tool for estimating net CO2 exchange of forest production

Authors


Antti Kilpeläinen, tel.+358 13 251 4411, fax+358 13 251 4444, e-mail: antti.kilpelainen@uef.fi

Abstract

The study describes an integrated impact assessment tool for the net carbon dioxide (CO2) exchange in forest production. The components of the net carbon exchange include the uptake of carbon into biomass, the decomposition of litter and humus, emissions from forest management operations and carbon released from the combustion of biomass and degradation of wood-based products. The tool enables the allocation of the total carbon emissions to the timber and energy biomass and to the energy produced on the basis of biomass. In example computations, ecosystem model simulations were utilized as an input to the tool. We present results for traditional timber production (pulpwood and saw logs) and integrated timber and bioenergy production (logging residues, stumps and roots) for Norway spruce, in boreal conditions in Finland, with two climate scenarios over one rotation period. The results showed that the magnitude of management related emissions on net carbon exchange was smaller when compared with the total ecosystem fluxes; decomposition being the largest emission contributor. In addition, the effects of management and climate were higher on the decomposition of new humus compared with old humus. The results also showed that probable increased biomass growth, obtained under the changing climate (CC), could not compensate for decomposition and biomass combustion related carbon loss in southern Finland. In our examples, the emissions allocated for the energy from biomass in southern Finland were 172 and 188 kg CO2 MW h−1 in the current climate and in a CC, respectively, and 199 and 157 kg CO2 MW h−1 in northern Finland. This study concludes that the tool is suitable for estimating the net carbon exchange of forest production. The tool also enables the allocation of direct and indirect carbon emissions, related to forest production over its life cycle, in different environmental conditions and for alternative time periods and land uses. Simulations of forest management regimes together with the CC give new insights into ecologically sustainable forest bioenergy and timber production, as well as climate change mitigation options in boreal forests.

Introduction

The growing concentration of atmospheric carbon dioxide (CO2) and its contribution to global warming has raised global concern about climate change (IPCC, 2007). Many mitigation and adaptation strategies to tackle this problem are set to reduce emissions or increase sequestration of carbon from the atmosphere. Forests and forest management are receiving special attention and can play an important role in reducing emissions and capturing and storing carbon in the forest biomass and ecosystems. Forest biomass also provides large potential for the energy industry to substitute fossil fuels in energy production The European Community has committed (EC, 2008) to reduce its greenhouse gas emissions by 20% and raise the share of renewable energy (including bioenergy) to 20% by 2020. This will increase the use of forest biomass in energy production, e.g., in Finland, the current use of 5 million m3 is expected to increase up to 12 million m3 by 2020 according to the National Climate and Energy Strategy. Novel forest management systems are needed in the future, because current management, aimed solely at timber production, is not necessarily appropriate in managing forest for the joint production of energy biomass (bioenergy) and timber, and at the same time increasing carbon storages of managed forests (Alam et al., 2010).

A large-scale harvesting of bioenergy (logging residues, small-dimensioned wood, roots and stumps) will raise questions about how sustainable energy systems based on biomass are, and what are the climatic effects when forest biomass is used in energy production? In fact, the carbon neutrality of renewable biomass has recently been questioned owing to high indirect greenhouse gas emissions, which are related to land use and land-use changes in producing bioenergy (Searchinger et al., 2008; Melillo et al., 2009). Regarding forest biomass, indirect carbon emissions are also important when assessing carbon sequestration in forest ecosystems and the role of the forest in mitigating climate change (e.g. Melin et al., 2010; Repo et al., 2010).

The ecosystem scale evaluations of environmental impact of the use of forest biomass in energy production are still few (Wihersaari 2005; Eriksson et al., 2007; Lindholm et al., 2010; Melin et al., 2010; Repo et al., 2010). On the other hand, suitable tools are needed in evaluating the capability of forests, forest management and forest biomass in the mitigation of climate change, and the substitution of fossil fuel in energy production. The holistic ecosystem level analysis of the carbon balance should include the carbon uptake in tree growth and the emissions of decomposition of soil organic matter controlling the sink/source dynamics of the ecosystem. Furthermore, carbon is emitted in management, harvesting and transport operations. All these phases in the production chain affect the carbon dynamics in forest–atmosphere interactions and the potential of forest biomass in reducing carbon emissions in energy production (Fig. 1). Assessments are needed over the whole life cycle of biomass production in order to identify the contribution of different factors on the sink/source dynamics of the forest ecosystem, and the role of forest biomass in mitigating climate change.

Figure 1.

 Carbon exchange of forest production. Carbon uptake=photosynthesis−plant respiration.

In the above context, this study describes a life cycle assessment (LCA) tool for studying the net carbon exchange of forest production. Applications of the tool in emission calculation and allocating the emission for bioenergy and timber are presented, in order to demonstrate the performance of the tool under varying forest management and climate scenarios for the boreal forests in Finland.

Material and methods

LCA tool

The LCA tool calculates the net CO2 exchange (Cnet) of forest production on an annual basis (g CO2 m2 a−1) as indicated by Eqn (1). It includes components from both the ecosystem and the technosystem related to the forest production, within the system boundary shown in Fig. 2. Cnet is a sum of the carbon uptake in growth (Cseq) and carbon emissions. The emissions consist of carbon from management, harvest (Cman) and decomposition of soil organic matter (Cdecomp), and from the combustion of bioenergy and degradation of wood-based items manufactured from timber (Charv). In calculations, Cseq has negative values (carbon flows from the atmosphere to the forest) and the emissions Cman, Cdecomp and Charv have positive values (carbon flows from forests, energy biomass and wood-based items to the atmosphere).

image(1)
Figure 2.

 The forest production system boundaries utilized in this study.

In the calculations, bioenergy and timber are produced. In this context, all the main phases of forest production with relevant operations are included in the calculation tool. Thus, the life cycle of energy biomass and timber starts from seedling production in a nursery, proceeds through management and harvest, and ends up in the yard of a pulp mill (pulpwood), sawmill (saw logs) or power plant (bioenergy) (Fig. 2). The carbon emitted in management, harvest and logistic operations during the life cycle is included in the calculations through the consumption of fuel (diesel) or electricity. The parameters for productivity of operations and fuel consumption of machines are summarized in Table 1.

Table 1.   Emissions calculation parameters utilized in the emission calculation tool. Volumes are in solid m3
PhasesProductivityFuel consumption
Forest establishment
Seeding production 237.54 MJ seedling1000*
Seedling transportation (50 km) 0.40 L km1
Site preparation0.91 ha h118.20 L h1
Scarifier transportation12.10 km ha1§0.54 L km1
Forest operations
Thinning by harvester8.20 m3 h112.00 L h1
Final felling by harvester17.20 m3 h112.00 L h1
Stump removal (excavator)13.00 m3 h1**15.00 L h1††
Forwarder and harvester transportation0.16 km m3‡‡0.54 L km1
Wood transportation and chipping
Forwarding (thinning)11.80 m3 h18.50 L h1
Forwarding (final felling)15.90 m3 h18.50 L h1
Long distance transportation (truck) Transportation capacity: 40/25 tons 0.54 L km1
Chipping (drum chipper)150.00 m3 h160.00 L h1
Required commuter traffic (50 km) 0.07 L km1

The values for seedling production are those for container seedlings. Site preparation is assumed to be done with an excavator or scarifier. The parameter values for logging (cut-to-length method) and forest haulage are for a harvester and forwarder, with different values for thinnings and final felling (FF). The values for the transportation of machines (e.g. harvester, scarifier) from site to site are based either on productivity per area or per m3. The long-distance transportation of wood and biomass is assumed to be done with a truck. The way back with an empty truck is assumed to consume 70% of the fuel needed for a full load. In the case of energy biomass, chipping is assumed to be done at the yard of the power plant with a large-scale drum chipper or, optionally, at the road side with a chipper mounted on a truck. In addition, commuter traffic in various phases of production is included into the calculation by assuming working days of 8 h and travel using a personal car. The manufacturing and maintenance of working machines are excluded from the calculations.

Timber (pulpwood, saw logs) is converted into useable wood-based products, and the carbon emissions from the items no longer in use are calculated applying Eqn (2) (Karjalainen et al., 1994):

image(2)

where PU is the proportion (0–100) of products in use and a, b, d are fixed parameters (120, 5, 120, respectively). C (a−1) is 0.5; 0.15; 0.065 and 0.03 for short, medium-short, medium-long and long lifespan of a product, respectively. In the calculations, the pulpwood represents the items with the medium-short lifespan and the saw logs the items with the medium-long lifespan. The carbon released from products no longer in use was assumed to convert completely into CO2.

The allocation of carbon emissions (or uptake) for bioenergy (per MW h) and timber (per m3) was done from the net carbon exchange, and according to the produced biomass proportions over the rotation. Charv of bioenergy (combustion) was not allocated for timber and vice versa. The management related emissions were also allocated according to the produced biomass proportions of bioenergy and timber over the rotation.

In the case of growth, soil decomposition and harvested bioenergy or timber, the LCA tool has flexibility in using them as input data from various sources. This may include empirical data of such information, or simulation results performed with a growth and yield model, or an ecosystem model outputting this information. The benefit of the utilization of an ecosystem model comes from the fact that it usually has detailed information, e.g. on decomposition processes in soil and on other carbon fluxes, and alternative management and climate scenarios can be utilized in simulations.

Calculation of Cseq and Cdecomp based on an ecosystem model

Outlines of the ecosystem model. When demonstrating the performance of the LCA tool, we utilized the Sima forest ecosystem model (Kellomäki et al., 1992a, b; Kolström, 1998) and linked its output to the LCA tool. The details of the model are described in several previous papers (Kellomäki et al., 1992a, b, 2005, 2008; Kellomäki & Kolström 1994; Talkkari & Hypén, 1996; Kolström, 1998; Alam et al., 2008, 2010, unpublished data). Therefore, only the main outlines are given here, with the focus on decomposition of soil organic matter and the earlier validation and performance of the model.

The dynamics of the ecosystem in the Sima model is driven by the dynamics of the number and mass of trees, as regulated by their regeneration, growth and death. All these processes are related to the temperature conditions, the availability of light, soil water and nitrogen, and the CO2 concentration in the atmosphere. The availability of light, soil water and nitrogen are, in turn, regulated by the dynamics of the gaps in the canopy of the tree stand.

Dead trees and the litter from different organs of living trees (foliage, branches, fine roots) are decomposed in the soil system and converted to humus. In decomposition, litter and humus are treated as cohorts. This enables the separation of the carbon emissions from new litter and old litter, as well as humus, which may exist in the site. The decomposition rate of litter and humus is related to the evapotranspiration and the quality of litter and humus (content of nitrogen, lignin and ash). Whenever the nitrogen concentration of the decaying litter of a particular cohort exceeds the critical concentration, the organic matter and nitrogen of the cohort are transferred to matter and nitrogen in humus. The decomposition of humus controls the mineralization of nitrogen, which makes nitrogen bound in humus available for tree growth.

The model is parameterized for Scots pine, Norway spruce and birch growing between the latitudes 60°N and 70°N and longitudes 20°E and 32°E in the territory of Finland. In this context, the performance of the model has been discussed in detail by Kolström (1998) and Kellomäki et al. (2008). Furthermore, Routa et al. (unpublished data) used the parallel simulations with the Sima model and the Motti model (Hynynen et al., 2002) to analyse the validity of the Sima model in growth yield studies. This comparison showed that there is a good correlation between the corresponding simulated values with the Motti and Sima models for different tree species, but the Sima model underestimated the growth by 10–20% compared with that obtained with the Motti model. Furthermore, the Sima model was used to calculate the mean growth (m3 ha−1 a−1) of different tree species for the 10 Forest Centres in southern Finland, based on National Forest Inventory data (Peltola, 2005). The calculation showed a close correlation between the measured and simulated values of growth for the regions representing the Centres (Routa et al., unpublished data).

Layout of simulation examples. In the calculation examples, the Sima model produces the annual growth (stem, branches, foliage, coarse roots and fine roots) (Cseq) and the amount of biomass harvested (Charv) in energy wood thinning (EWT) (energy biomass in terms of foliage, branches and stems), in the commercial thinnings (timber) and in the FF (energy biomass, timber). Furthermore, the model simulations produce the annual litter fall for the decomposition and consequent emissions of carbon from soil (Cdecomp) to be used in the LCA tool. Timber refers to saw logs (stem part with diameter >15 cm) and to pulpwood (stem part with diameter 6–15 cm). Other parts of the tree, such as branches, tops of the stem, needles, stumps and large roots are logging residues. In the case of needles, the loss in harvesting was assumed to be 30%. Wood density of 400 kg m−3 was utilized and carbon content was assumed to be 50%. The energy content of 3.2 MW h in Mg (dry biomass) was utilized.

The simulations were run for Norway spruce (Picea abies L. Karst.) trees growing in stands in southern (Joensuu region: 62°40′N, 29°30′E) and northern (Rovaniemi region: 66°34′N, 25°50′E) Finland. The site was of Myrtillus type (MT) (Cajander, 1949). In the initialization of the runs, the amount of soil organic matter (SOM, litter and humus) was 67 Mg ha−1 for Joensuu and 65 Mg ha−1 for Rovaniemi (Kellomäki et al., 2008). The initial SOM represents the old carbon in the sites. The calculations were done under the current (CU) and changing climate (CC). The CC was based on the A2 scenario (IPCC, 2007), where CO2 concentration will rise up to 840 ppm by 2100. At the same time, the temperature in January will increase by 7.6 °C and in July by 3.4 °C over the whole of Finland. Precipitation will have increased by 30% in the winter and 10% in summer by 2100 (Ruosteenoja & Jylhä, 2007; Ruosteenoja et al., 2007).

In the forest management, the current recommendations for privately owned forests were used (Tapio, 2006). In both sites, the initial stand density was 2500 seedlings per hectare, with the initial diameter of 2 cm. In the energy wood thinning, EWT, the stand density was reduced down to 1000–1200 trees per hectare. Thereafter, the thinning rules based on the development of basal area and dominant height was applied. The FF was done when 80 years had elapsed from the start of a simulation (the rotation period of 80 years). The simulations were done separately for traditional timber production (TP) and for the integrated production of timber and bioenergy (BP). In the latter case, the energy biomass was harvested in the EWT and FF. In thinnings between the EWT and FF, no energy biomass was harvested.

Results

A computational example of net CO2 exchange of forest production

Southern Finland. In southern Finland, the average carbon uptake in the Norway spruce stand was −1138 g CO2 m−2 a−1 over the rotation (Table 2; Fig. 3a) in TP in the current climate. On the annual basis, the carbon emissions turned into the carbon uptake fairly early (after 20 years of simulation) and the uptake was maximized in the middle of the rotation with the value −1767 g CO2 m−2 a−1. Towards the end of the rotation, the CO2 uptake decreased gradually and was −1283 g CO2 m−2 a−1 just before the FF. Over the rotation, the mean losses of CO2 owing to the decomposition of litter and humus were 728 g CO2 m−2 a−1 (Table 2; Fig. 3a). The emissions from management were fixed for the establishment of the tree stand (15 g CO2 m−2), but they varied from the first thinning (78 g CO2 m−2 a−1) to FF (239 g CO2m−2 a−1) (Fig. 3a). The CO2 emissions from wood-based products changed with the time [see Eqn (2)], reaching 183 g CO2 m−2 a−1 at maximum. Over the uptake and emissions of carbon, the average net CO2 exchange was −319 g CO2 m−2 a−1 over the rotation period, with the sink of −1107 g CO2 m−2 a−1 just before the first thinning (Fig. 3a), and −138 g CO2 m−2 a−1 at the end the rotation.

Table 2.   Mean values (g CO2 m2 a−1) for LCA tool components [see Eqn (1) for abbreviations] for Norway spruce over a 80-year rotation period in southern and northern Finland in alternative management and climate scenarios [corresponding those in Figs 3 and 4]
Management and climateCseqCdecompCharv (pulp)Charv (saw logs)Charv (Eb)CmanCnet
  1. TP, traditional timber production regime; BP, integrated timber and bioenergy production regime; CU, current climate; CC, changing climate; Eb, bioenergy.

Southern Finland
TP, CU−1138728701505−319
BP, CU−111272526222227−110
TP, CC−1040791731504−157
BP, CC−10537512982116−49
Northern Finland
TP, CU−974671711304−214
BP, CU−9416164002286−51
TP, CC−1198753812205−337
BP, CC−113472150182177−120
Figure 3.

 Annual CO2 flows (g CO2 m−2 a−1) in Norway spruce in traditional timber (TP) and bioenergy production (BP) regimes in current (a, c) and changing climate (b, d) in southern Finland [see Eqn (1)].

In BP under current climate, the average carbon uptake in the Norway spruce stand was −1112 g CO2 m−2 a−1 over the rotation (Table 2; Fig. 3c). On the annual basis, the carbon uptake was at its highest in the middle of the rotation period, −1706 g CO2 m−2 a−1. The CO2 uptake decreased gradually towards the end of the rotation, and was −1234 g CO2 m−2 a−1 just before the FF. Over the rotation, the mean losses of CO2 owing to the decomposition of litter and humus were 725 g CO2 m−2 a−1, and as a result of the combustion of bioenergy 222 g CO2 m−2 a−1 (Table 2; Fig. 3c). The emissions from management were again fixed for the establishment of the tree stand (15 g CO2 m−2), but were 88 g CO2 m−2a−1 in EWT and 345 g CO2 m−2 a−1 in FF, for example (Fig. 3c). The CO2 emissions from wood-based products were at maximum 111 g CO2 m−2 a−1. The average net CO2 exchange was −110 g CO2 m−2 a−1 over the rotation period (Table 2).

In the CC, the uptake and emissions patterns for Norway spruce in TP were similar to the current climate (Fig. 3b). However, the carbon uptake values decreased by the end of the rotation period and the values for decomposition increased. The maximum uptake of CO2 into trees was −1750 g CO2 m−2 a−1. Over the rotation, the average uptake into trees was −1040 g CO2 m−2 a−1, the average CO2 loss from decomposition 791 g CO2 m−2 a−1, and the net CO2 exchange −157 g CO2 m−2 a−1 (Table 2).

In BP, changed climatic conditions also decreased the carbon uptake values by the end of the rotation period. The maximum uptake of CO2 into trees was −1745 g CO2 m−2 a−1. Over the rotation, the average uptake into trees was −1053 g CO2 m−2 a−1, the average CO2 loss from decomposition 751 g CO2 m−2 a−1 and from combustion of bioenergy 211 g CO2 m−2 a−1 (Table 2; Fig. 3d). The net CO2 exchange was −49 g CO2 m−2 a−1 (Table 2).

The decomposition of new (litter fall build-up during this rotation) and old humus (prevailing at the beginning of the simulation) is shown for TP and BP, under CU (Fig. 5a) and CC (Fig. 5b), in Fig. 5. The decomposition of the old humus was not much affected by change in climatic conditions or management, unlike the new humus. Changed climatic conditions increased and BP regime decreased the decomposition of new humus in Norway spruce stands in southern Finland compared with current climate and TP, respectively.

Figure 5.

 Annual decomposition of new and old humus (g CO2 m−2 a−1) in traditional timber (TP) and bioenergy production (BP) regimes in Norway spruce in current and changing climate in southern (a, b) and northern Finland (c, d).

Northern Finland. In northern Finland, the average carbon uptake in the Norway spruce stand in TP was −974 g CO2 m−2 a−1 over the rotation (Table 2; Fig. 4a). The mean losses of CO2 owing to the decomposition of litter and humus were smaller than in southern Finland; 671 g CO2 m−2 a−1 (Table 2; Fig. 4a). The emissions from management were fixed for the establishment of the tree stand (15 g CO2 m−2), but varied from the first thinning (78 g CO2 m−2 a−1) to FF (171 g CO2m−2 a−1) (Fig. 4a). The CO2 emissions from wood-based products were, at maximum, 203 g CO2 m−2 a−1. The average net CO2 exchange was −214 g CO2 m−2 a−1 over the rotation period (Table 2). At the end of the rotation, the net CO2 exchange was −154 g CO2 m−2 a−1 (Fig. 4a).

Figure 4.

 Annual CO2 flows (g CO2 m−2 a−1) in Norway spruce in traditional timber (TP) and bioenergy production (BP) regimes in current (a, c) and changing climate (b, d) in northern Finland [see Eqn (1)].

In BP, the average carbon uptake was −941 g CO2 m−2 a−1 over the rotation (Table 2; Fig. 4c). On the annual basis, the carbon uptake was at its maximum in the middle of the rotation period with a value of −1538 g CO2 m−2 a−1. Over the rotation, the decomposition of litter and humus were 616 g CO2 m−2 a−1 and the combustion of bioenergy constituted an average emission of 228 g CO2 m−2 a−1 (Table 2; Fig. 4c). The emissions from management were again fixed for the establishment of the tree stand (15 g CO2 m−2), but were 74 g CO2 m−2 a−1 in EWT and 357 g CO2 m−2 a−1 in FF, for example (Fig. 4c). The CO2 emissions from wood-based products (only pulpwood was produced) were at maximum 155 g CO2 m−2 a−1. The average net CO2 exchange was −51 g CO2 m−2 a−1 (Table 2).

In the CC, the carbon uptake values were higher more quickly after the beginning of the simulation, both in TP and BP, compared with the current climate, but they decreased by the end of the rotation period (Fig. 4b). On the contrary, the values for decomposition increased by the end of the rotation period. In TP, the maximum uptake of CO2 into trees was −2040 g CO2 m−2 a−1. Over the rotation, the average uptake into trees was −1198 g CO2 m−2 a−1, the average CO2 loss from decomposition 753 g CO2 m−2 a−1 and the net CO2 exchange −337 g CO2 m−2 a−1 (Table 2).

In BP and under the changed climate, the maximum uptake of CO2 into trees was −1538 g CO2 m−2 a−1. Over the rotation, the average uptake into trees was −1134 g CO2 m−2 a−1, the average CO2 loss from decomposition was 721 g CO2 m−2 a−1, and from combustion of bioenergy 217 g CO2 m−2 a−1 (Table 2; Fig. 4d). The net CO2 exchange was −120 g CO2 m−2 a−1 over the rotation period (Table 2).

The decomposition of new and old humus is shown for TP and BP, under CU in Fig. 5c and under CC in Fig. 5d. Also in northern Finland, the main changes were found in the new humus. Changed climatic conditions increased and BP regime decreased the decomposition of new humus. The peaks in the new humus decomposition owing to the decomposition of logging residues left at the stand in thinning were also absent in EWT.

Emissions for timber and bioenergy

In southern Finland, CO2 emissions allocated for pulpwood production ranged from −95 to −337 kg CO2 m−3 for TP and BP (Table 3). This means that pulpwood has net sequestrated carbon during the calculated rotation period. Similarly, in both management regimes (TP and BP), saw logs had net sequestrated carbon over one rotation period according to the emission allocation. The values calculated per m3 ranged from −5676 to −13 888 kg of CO2 (Table 3). In northern Finland, pulp production net sequestrated carbon of ranging from −114 to −325 kg CO2 m−3, except from BP in CC, in which the net loss of carbon, 40 kg CO2 m−3, was found over the rotation period. Saw logs also had net sequestrated carbon, similar to southern Finland, during the calculated rotation period, ranging from −7276 to −10 630 kg CO2 m−3 (Table 2). However, as the thresholds for timber production were not met in the simulations, only pulpwood was produced in BP under CU.

Table 3.   CO2 emissions allocated for bioenergy (Eb) (kg CO2 MW h−1) and timber (Pulp and Sawlogs) (kg CO2 m−3) for Norway spruce grown over a 80-year rotation period in southern and northern Finland and in alternative management and climate scenarios
Management and climateSouthern FinlandNorthern Finland
EbPulpSawlogsEbPulpSawlogs
  1. Emissions are allocated to timber and bioenergy from net ecosystem exchange (i.e. Cseq+Cdecomp) and from Cman according to their harvested biomasses over the rotation [see Eqn (1) and Table 2].

  2. TP, traditional timber production regime; BP, integrated timber and bioenergy production regime; CU, current climate; CC, changing climate; Eb, bioenergy.

TP,CU−330−13888−259−10630
BP,CU172−236−5676199−114
TP,CC−95−7395−325−9910
BP,CC188−337−962815740−7276

The CO2 emissions allocated for bioenergy production in southern Finland were 172 and 188 kg CO2 MW h−1 in CU and CC, respectively. The emissions for bioenergy were lower under CU than in CC in southern Finland (Table 3). In northern Finland, CO2 emissions were lower under CC compared with CU (Table 3).

Discussion

Forests act both as a sink and a source of CO2, with impacts on the atmospheric CO2 concentration and consequent changes in the global climate. This emphasizes the importance of understanding how the dynamics of the forest ecosystem and management interact in controlling the carbon balance in forests. The impacts of different management regimes on the uptake and emissions of CO2 are important when simultaneously considering various ecosystem goods and services in forest production. The calculation of CO2 exchange for different management regimes also provides information on the environmental impacts of regimes. In this respect, the feasibility of using forest biomass in energy production has been questioned, as a result of indirect carbon emissions, which are often excluded in assessing the potentials of bioenergy in mitigating climate change (Searchinger et al., 2008; Mathews & Tan, 2009; Melillo et al., 2009).

This paper describes a LCA tool for studying the net carbon exchange in forest production. The tool integrates the carbon uptake and emissions, which result from the ecosystem dynamics and the operations needed in management, harvest and logistics in different phases of timber and energy biomass production. In this context, we integrated the LCA tool with a gap-type forest ecosystem model, to assess how the production of timber and energy biomass affects the carbon exchange in the boreal conditions under varying forest management and climate scenarios. The simulations for southern Finland showed that the average carbon uptake in the Norway spruce stand was −1138 g CO2 m−2 a−1 over the rotation, if the current climate was used in simulations. When applying CC, the growth of Norway spruce decreased (9%) in southern Finland, but increased (23%) in northern Finland, compared with the current climate. This increase compensated for the increased carbon loss owing to the enhanced decomposition of soil organic matter and the use of energy biomass in northern Finland, but not in southern Finland. In southern Finland, more frequent droughts reduced growth under a CC (cf. Kellomäki et al., 2008). In southern Finland and under the current climate, the net exchange values calculated with the LCA tool were −319 and −110 g CO2 m−2 a−1 in timber production and in the combined production of timber and bioenergy, respectively. The corresponding values for northern Finland were −214 and −51 g CO2 m−2 a−1.

The decomposition of soil organic matter is among the main sources of indirect emissions when assessing the environmental impacts of the production of timber and bioenergy (Wall, 2008; Luiro et al., 2009; Repo et al., 2010). Generally, the simulations showed that the decomposition was enhanced under the CC (cf. Davidson & Janssens, 2006; Karhu et al., 2010). In addition, the emissions were enhanced most in the case of new organic matter. This was partly due to the enhanced decomposition, increasing growth and consequent increase in litter fall. On the other hand, the production and utilization of bioenergy, in terms of harvesting residues, reduced the soil organic matter and increased emissions, which further increased the climatic impacts of bioenergy use (Johnson & Curtis, 2001; Ågren & Hyvönen, 2003; Eriksson et al., 2007).

In the simulations, the management scenarios and rotation lengths were fixed, since the aim was to demonstrate the performance of the LCA tool in terms of the sink/source dynamics. The simulations showed that the management may have strong effects on the uptake and emissions of carbon, and that the proper choice of management regime is among the key questions in mitigating climate change in biomass production (Schlamadinger & Marland, 1996). In this context, the emissions from the decomposing wood-based products are also of importance. Pulpwood and saw logs acted as carbon sinks over the whole rotation period, but this was not always the case. The management related emissions in our study were quite low compared with ecosystem emissions; i.e. their range was 4–8 g CO2 m−2 a−1 depending on the management regime. Management emissions per energy unit for energy biomass were well in line with the findings of Palosuo & Wihersaari (2000) and Wihersaari (2005), who estimated the management emissions to be 4–14 kg CO2 Eq MW h−1 for the whole production chain. This implies that the main part of the carbon emissions per energy unit originated from the decomposition of soil organic matter and from the combustion of biomass. The emissions per energy unit were 172 and 188 kg CO2 MW h−1 in current and CC, respectively, in southern Finland, and the corresponding values in northern Finland were 199 and 157 kg CO2 MW h−1.

As a conclusion, the developed tool is suitable for estimating the net carbon exchange of forest production. The tool also enables the allocation of direct and indirect carbon emissions related to forest production over its life cycle, in different environmental conditions, and for alternative time periods, biomass resources, management regimes and land uses. These give new possibilities to evaluate the ecological sustainability of forest bioenergy and timber production in the context of climate change mitigation.

Acknowledgements

This work is supported by funding from the University of Eastern Finland (Project: Ilmastonmuutoksen ja terveyshaittojen torjunta bioenergian tuotannossa) and by the European Social Fund of European Union, North-Karelia ELY-Centre and Josek Ltd., which are both gratefully acknowledged.

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