Introduction. GCB Bioenergy (2013) 5, , doi: /gcbb.12010

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1 GCB Bioenergy (2013) 5, , doi: /gcbb Effects of stump extraction on the carbon sequestration in Norway spruce forest ecosystems under varying thinning regimes with implications for fossil fuel substitution ASHRAFUL ALAM*, SEPPO KELLOMÄKI*, ANTTI KILPELÄINEN and HARRI STRANDMAN* *School of Forest Sciences, Faculty of Science and Forestry, University of Eastern Finland, PO Box 111, FI-80101, Joensuu, Finland, Centre for Sustainable Consumption and Production, Environmental Performance Unit, Finnish Environment Institute, PO Box 111, FI-80101, Joensuu, Finland Abstract The overall aim of this work was to assess the effects of stump and root extraction on the long-term carbon sequestration and average carbon storage in the integrated production of energy biomass and stemwood (pulpwood and sawlogs) under different thinning options (unthinned, current thinning and 30% increased thinning thresholds from current thresholds). The growth and development of Norway spruce (Picea abies L. Karst.) stands on a fertile site (Oxalis-myrtillus) in central Finland (Joensuu region: N, E) was simulated for two consecutive rotation periods ( years/160 years). Stemwood and energy biomass production, carbon sequestration, and average storage and emission dynamics related to the entire production process of biomass were assessed. The assessment was done by employing a life cycle assessment tool, which combines simulation outputs from an ecosystem model and the related technosystem emissions. It was found that stump and root harvesting constituted 21 36% of the total biomass production (energy biomass and stemwood) depending on the thinning regimes and rotation period. No considerable effect was found in stemwood production when stump and root extraction was compared to the regime in which stumps and roots were left at the site. Stump and root extraction did not affect carbon sequestration on the following rotation and, in fact, an increase in forest growth was found for the unthinned and 30% increased thresholds compared to the first rotation. The results also showed that if current thinning threshold is increased, win-win situations are possible, especially when climate change mitigation is the main concern. The substitution of coal with energy biomass is possible without reducing carbon storage in the forest ecosystem. The utilization of energy biomass, including stumps and roots, instead of coal could reduce up to 33% of emissions over two rotation periods depending on the thinning regimes. Even if stumps and roots were excluded, a maximum of 19% carbon emissions could be reduced by using only logging residues. Keywords: carbon sequestration, carbon stock, climatic impact, life cycle assessment, stump harvest, substitution, thinning regime Received 13 June 2012 and accepted 30 July 2012 Introduction The climate protective function of forests and forest ecosystems has been emphasized in EU policy regulations (EC, 2008). One of the realized mitigation strategies focused on increasing the share of renewable energy sources, in particular forest biomass for energy (energy biomass), in total primary energy consumption. That means that forest management and forest biomass can play a significant role in mitigating atmospheric carbon Correspondence: Ashraful Alam, tel , fax , ashraful.alam@uef.fi dioxide build up and can increase ecosystem carbon sequestration. In line with the EU commitment, the Finnish Climate and Energy Policy (Anon, 2008, 2009) also aimed to increase the production and harvesting of energy biomass to substitute the use of fossil energy (e.g. coal). In many countries, including Finland, a large part of current energy biomass harvesting represents logging residues, i.e. the top part of stems, branches, foliage, but there is a need to increase the production by extracting stumps and even a part of the coarse roots (Melin et al., 2010; Routa et al., 2011a; Alam et al., 2012). In Finland, the harvesting of stumps and coarse roots has been increasing since 2000 and the recent increase 2012 Blackwell Publishing Ltd 445

2 446 A. ALAM et al. was about 20% in 2010 compared to that of previous year (Ylitalo, 2011). The amount of stumps and coarse roots available after final felling is dependent on the management regime applied during the rotation period. Forest management changes (e.g. intensity and timing of thinning) have a substantial effect on the growth rate, growing stocks and further development of the forest stands (Renshaw et al., 2009; Alam et al., 2010). In the case of energy biomass production, the time difference seen between the immediate release of carbon in the energy use of biomass and carbon sequestration during the rotation is challenging from the climate change mitigation point of view, as well as when choosing an appropriate management option with multiple objectives. Earlier, the simultaneous production of major ecosystem products (such as stemwood (pulpwood and sawlogs)) and services (such as carbon storage) within a defined rotation period has been a key research question (Briceño-Elizondo et al., 2006; Garcia-Gonzalo et al., 2007; Heikkilä et al., 2009; Profft et al., 2009; Alam et al., 2012). Some previous studies suggested that maintaining higher tree stocking level over the rotation (up to 30%) compared to current recommendation could be a possible future forest management option, which may increase carbon stocks in the ecosystem without reducing production potential of stemwood and energy biomass (Thornley & Cannell, 2000; Briceño-Elizondo et al., 2006; Alam et al., 2008, 2010). However, finding a fitting solution in this respect is still an open question, but the future of forest management seems to be shifting from traditional stemwood-oriented management to the integrated production of different ecosystem services, such as energy biomass and stemwood and carbon sequestration in forest ecosystems (Lindner et al., 2008; Alam et al., 2012). In a mature boreal forest stand, stumps and coarse roots represent around 25 30% of the total tree biomass (Hakkila & Aarniala, 2004) and the carbon in the stumps and roots is an essential part of the total ecosystem carbon reservoir. When biomass in stumps and roots remain or are left in the forest they decompose slowly, which makes their role even more important with regard to carbon storage in the soil and the longterm sources of nutrients for forests (Palviainen et al., 2010; Helmisaari et al., 2011; Saana et al., 2011), and has implications when defining the overall climate impact of various management regimes. Until now, very little is known regarding how the extraction of stumps and coarse roots may affect long-term sequestration and the balance of carbon in managed forest ecosystems. Such an understanding is critical because a substantial increase in the use of stumps and coarse roots has been practised in recent years (Ylitalo, 2011). Moreover, human interference in natural biogeochemical cycles through the increased utilization of forest resources ultimately reinforces the importance of such information (Palviainen et al., 2010; Vanguelova et al., 2010; Walmsley & Godbold, 2010; Saana et al., 2011). It is generally assumed that energy biomass can be burnt with a consequence that the need to use fossil fuel is reduced and that the substitution of fossil fuel with energy biomass would compensate for the reduction of carbon storage in the forest ecosystem (European Environment Agency, 2011). However, this has been recently questioned (Searchinger et al., 2008; Kilpeläinen et al., 2012; Pingoud et al., 2012) and stump harvesting with a large share taken from the total biomass leads to radical changes in the ecosystem composition and the carbon cycle. Still, forest growth is a key question regarding future site productivity, sustainability of production (e.g. Morris & Miller, 1994; Merino et al., 2005; Palviainen et al., 2010; Helmisaari et al., 2011; Saana et al., 2011), and also management implications. Management may not only affect the potential production of the forest ecosystem but may also explicitly control the carbon dynamics of the whole ecosystem process. A holistic ecosystem scale evaluation of alternative management options is therefore needed to obtain an in-depth knowledge of this issue and also to increase the current understanding further. In this perspective, life cycle assessment (LCA) could be a useful tool to identify the contribution of different factors affecting sink and source dynamics of forest ecosystems and the role of energy biomass, with or without stumps and roots, in climate change mitigation (Kilpeläinen et al., 2011, 2012; McKone et al., 2011). There have been few studies to date that have evaluated the implications of stump harvesting on the net carbon balance in forest ecosystems (Repo et al., 2010; Sathre & Gustavsson, 2011; Zanchi et al., 2011; Kilpeläinen et al., 2012). None of them analysed the distinct or isolated effect of stump and root extraction on the integrated production of energy biomass, stemwood and carbon sequestration, and the associated long-term effect on net carbon balance, which represents the substantial gap in understanding that was clearly highlighted in a recent study by Walmsley et al. (2009). In this context, this study aimed to investigate how the extraction of stumps and coarse roots for biomass energy would affect the long-term sequestration and storage of carbon in the Norway spruce (Picea abies L. Karst.) stand in boreal forest ecosystem and its climatic impact under selected thinning regimes in total over a 160-year period (two rotations) from a life cycle perspective. We analysed Norway spruce stand because of its potential for stump harvest due to shallow root system, and also the possibility to improve the health of

3 EFFECT OF STUMP EXTRACTION ON CARBON SEQUESTRATION 447 spruce stand with removing stumps infected by Heterobasidion. Materials and methods Life cycle assessment (LCA) tool General outline. In general, the LCA tool utilized in this study calculated all of the flows of carbon from the atmosphere into the forest ecosystem (sequestration of carbon) and from the biosphere back to the atmosphere (emission of carbon) (Kilpeläinen et al., 2011). The LCA tool was able to integrate the simulation results obtained from an ecosystem model and further utilize them together with emission parameters, as an input for the emission calculation tool. The approach enabled the calculation of potential sources and sinks of carbon in the forest ecosystem and forest products for the entire production system. The system boundary within which the calculation for whole production chain was performed is shown in Fig. 1. In this study, the life cycle of stemwood (sawlogs and pulpwood) and energy biomass was assumed to start from seedling production in the nursery and proceed through management and harvesting, finally ending up at the yard of a pulp mill (pulpwood), sawmill (sawlogs) or power plant (energy biomass). The ecosystem carbon balance and net carbon balance of the forest production system were calculated on an annual basis and the unit was set as gco 2 m 2 a 1. Details of how the calculations of ecosystems carbon balance (C eco bal ) and net carbon balance (C net bal ) were done, and which outputs were used from the ecosystem model in the calculations is briefly explained in the following sections of the LCA tool. Calculation of C eco bal and C net bal. The net carbon balance (C net bal ) is the sum of carbon sequestered by the forest biomass and roots (C seq ), and the carbon emitted (C emi ) from all of the sources. In the production system, the sources of carbon include emissions from the soil decomposition of litter and the humus layer (C decomp ), from the combustion and degradation of harvested energy biomass and stemwood (C harv ), and from management and logistic operations (C man ) (Eqn 1). However, the ecosystem carbon balance (C eco bal ) was computed by adding C seq and C decomp as shown in Eqn (2). In the calculations, the value of C seq is negative, as carbon flows from the atmosphere into the ecosystem, whereas the value of emissions (C man,c decomp,c harv ) is positive, as carbon flows back into the atmosphere. C net bal ¼ C seq þ C decomp þ C harv þ C man C eco bal ¼ C seq þ C decomp Calculation of C seq,c decomp. As mentioned above, the simulation outputs of the ecosystem model (Sima) were used as an input for the LCA tool. The utilized outputs were (i) the annual growth of stem, branches, foliage, coarse and fine roots (C seq ), ð1þ ð2þ Carbon sequestration and emissions Carbon sinks: Carbon sequestration by forest biomass and ecosystem Carbon sources: Technosystem and ecosystem emissions All the necessary commuter traffic Transportation of scarifier to the forest Site preparation Seedlings production in nursery Transportation of seedlings to the forest Transportation of harvester and excavator to the forest Planting of seedlings Energy biomass thinning/ Commercial thinnings/ Final felling Amount of harvested biomass Transportation of forwarder to the forest Forwarding harvested biomass from forest to road side Transportation of truck to the forest Transporting harvested biomass from roadside to mill gate Ecosystem emissions Decomposition of soil organic matter Decomposition of humus Chipping Combustion of energy biomass Pulpwood and sawlogs in mills Usable wood-based products Net carbon balance (see equation 1) Ecosystem carbon balance (see equation 2) Fig 1 Diagram of forest production system boundary outlined by the broken line.

4 448 A. ALAM et al. (ii) the annual litter fall and consequent emissions of carbon from the litter including the humus layer of the forest floor (C decomp ) and (iii) the amount of harvested energy biomass and stemwood (C harv ). The Sima model was parameterized for Scots pine (Pinus sylvestris L.), Norway spruce, silver birch (Betula pendula Roth.), downy birch (Betula pubescens Ehrh.), aspen (Populus tremula L.) and grey alder (Alnus incana Moench., Willd.) growing between the latitudes N 60 and N 70 and longitudes E 20 and E 32 within Finland (Kellomäki et al., 1992a,b, 2008; Kellomäki & Kolström, 1994; Talkkari & Hypén, 1996; Kolström, 1998). The model was run on an annual basis and the computations were applied to an area of 100 m 2. In the model, the simulation of the forest ecosystem processes was based on the Monte Carlo simulation technique, meaning that occurring events are stochastic. Whenever the possibility appeared for an event to occur, the algorithm selected whether or not the event would take place by comparing a random number with the probability of the occurrence of the event. The probability of an event occurring is a function of the state of the forest ecosystem at the time when it is possible. Each run of a Monte Carlo code constitutes one realization of all possible time courses of the development of the forest ecosystem. Repeated simulations were, therefore, needed to determine the convergence of the model outputs (Bugmann et al., 1996). In this study, 100 replications of each scenario were conducted. The reported outputs were the mean values of these replications. The variability in output among the repeated simulations within a scenario for different years of the rotation period was calculated and the estimated standard deviation was 1% from the mean value. The model has a remarkable history of utilization and validation, and has been previously discussed in detail by Kolström (1998) and Kellomäki et al. (2008). The model was found to be capable of predicting the development of boreal forest ecosystems, especially in Finnish conditions. In a recent study, Routa et al. (2011b) compared the growth performance of Norway spruce and Scots pine in 13 different site conditions by employing parallel simulations with the Sima model and the Motti model (Hynynen et al., 2002). The results from the comparison (Routa et al., 2011b) showed that the Sima model appeared to slightly (10 20%) underestimate the growth compared to the Motti model. Furthermore, the analysis in which the performance of the Sima model was analysed, using data from 10 Forest Centres in southern Finland based on National Forest Inventory measurements (Peltola, 2005), showed a close correlation (R 2 = ) between the measured and simulated growths (see Routa et al., 2011a). In the model, the growth of a single tree is based on stem diameter growth (1.3 m above ground level), based on which growth of tree biomass components (foliage, branches, stem and roots) were calculated via allometric relations. The diameter was also used to calculate the tree height by applying the height model of Näslund (1936), modified to include the current temperature sum for specific sites (Kellomäki et al., 2008). The actual diameter growth of an established tree is the product of the potential diameter growth and growth multipliers. Growth multipliers were assumed to be determined by four environmental subroutines. These subroutines (temperature, light, soil moisture and decomposition) determined the site conditions regarding the temperature sum (degree-days, +5 C threshold), within-stand light conditions, and soil moisture and soil nitrogen levels. These factors directly affect the growth of trees and indirectly influence the death of trees in tree population. In this model, the death of trees is determined by the crowding with a consequent reduction in growth, which determines the risk of a tree dying at a given moment. Furthermore, the random mortality was included, with a small fraction of the trees dying each year. As mentioned above, temperature controls the geographical thresholds and annual growth responses of each species and their ecotypes. Simultaneously, competition for light controls tree growth and is dependent on tree species and their height distributions. The effect of soil moisture is described through the number of dry days, i.e. the number of days per growing season with soil moisture equal to or less than that of the wilting point specific for soil types and tree species. Soil moisture indicates the balance between precipitation, evaporation and runoff (Kellomäki et al., 1992a,b, 2008). The total available nitrogen for the trees and the ground vegetation was the difference between the total mineralized nitrogen and the immobilized nitrogen in decomposition (Kellomäki et al., 1992a,b, 2008). The availability of nitrogen is controlled by the decomposition of litter and humus. Decomposition of litter and humus determines the weight loss and carbon dioxide emission from the decomposing litter and humus. The litter cohort refers to the annual amount of dead material originating from trees. Litter is divided into foliage, twig, root and woody litter. Decomposition of litter is initiated by calculating the ash-free weight of cohort, and the amount of carbon and nitrogen contents in cohort. The weight loss of a litter (WLoss,%) is a function of the current ratio between lignin and nitrogen (L/N) contents and the actual evapotranspiration (Meentemeyer, 1978; Pastor & Post, 1985; Meentemeyer & Berg, 1986): WLoss ¼ A B ðl=nþ where A and B are variables dependent on the actual evapotranspiration (AET, cm); i.e. A = (AET) and B = (AET). Whenever the nitrogen concentration of decaying litter in a particular cohort exceeds the critical concentration, the organic matter and nitrogen in the cohort are transferred to organic matter and nitrogen in humus, which refers to the organic matter with no clear origin any more. Decomposition of humus is dependent on AET and the ratio between carbon and nitrogen contents in humus (C/N). Weight loss of litter and humus is converted to carbon oxide, which is emitted to the atmosphere. The values of the main parameter used in the calculations are listed in Table 1. In the model, it is possible to manage the forests in terms of planting, energy biomass thinning, commercial thinning and final felling (Kellomäki et al., 2008; Alam et al., 2010). Thinning can be performed with a specified thinning pattern e.g. thinning from below or thinning from above. In thinning from below, trees from among the suppressed trees were removed first, and then from among the dominant trees, until the ð3þ

5 EFFECT OF STUMP EXTRACTION ON CARBON SEQUESTRATION 449 Table 1 Values of the main parameter used in the calculations for the litter of Norway spruce. Here N is used for nitrogen Litter type Ash correction Initial N, % Initial lignin, % Foliage litter Twigs Wood Roots Critical N for converting litter to humus, % number of trees removed was equal to the target number. The order of treatment was opposite in the case of thinning from above. In addition to reducing the stem number, thinning can also be specified in terms of removal of a fraction of the basal area (m 2 ha 1 ). The upper limit for the basal area required for thinning is specified as a function of the dominant height of the stand. A stand was thinned whenever the upper limit value was attained during the rotation. Trees were thinned until the basal area of the remaining trees was equal to the minimum allowed basal area for a given dominant height. The final felling was done as for thinning, but all of the trees were removed at the end of the specified rotation period. The amount of biomass harvested in each thinning and final felling can be assorted into energy biomass and stemwood. Calculation of C harv. For the purpose of this study, the harvested biomass (C harv ) was converted into usable energy biomass (in this case energy biomass to produce energy) or woodbased products (pulpwood and sawlogs). The release of carbon by combustion of energy biomass was assumed to take place immediately after the harvesting and the emissions from wood-based harvested biomass were calculated by applying Eqn (4) (Karjalainen et al., 1994). a PU ¼ d 1 þ be ct ð4þ where PU is the proportion (0 100) of products in use; a (120), b (5), d (120) are fixed parameters; c (year 1 ) is lifespan of a product (0.15 for medium-short and for medium-long) and t (year) is time. In this work, pulpwood represented the items with a medium-short lifespan, whereas the sawlogs indicated items with a medium-long lifespan. Accordingly, the half-lives for the pulpwood and sawlogs were 15 and 36 years, respectively. The carbon released from products that were no longer in use was assumed to completely convert into carbon dioxide. Calculation of C man. The carbon emitted in seedling production, management operations, harvesting and all necessary logistical operations, within the system boundary of the production system (Fig. 1), was included in the calculations in terms of consumption of fuel (diesel) or electricity. The parameters for productivity of operations and fuel consumption of machines, with corresponding units and references, were collected from the available literature (see Kilpeläinen et al., 2011). The values for container seedlings were used to define the values for seedling production. Site preparation was assumed to be performed with an excavator or scarifier. The parameter values for logging (cut-to-length method) and forest haulage were for a harvester and forwarder with different values for thinning and final felling. The values for the transportation of machines (e.g. harvester, scarifier) from site to site were based either on productivity per area or per solid m 3. The long-distance transportation of energy biomass and stemwood was assumed to be performed with a truck. The return journey for the truck (empty) was assumed to consume 70% of the fuel needed for a full load. In the case of energy biomass, chipping was assumed to be done at the yard of the power plant with a large-scale drum chipper. In addition, commuter traffic in various phases of production was included in the calculation by assuming working hours and travel using a personal car. The manufacturing and maintenance of working machines were excluded from the calculation. Layout of simulations and thinning regimes In general, three thinning regimes were utilized in this study. The thinning regimes were defined as (i) Current thinning regime (CuT): where basal area thresholds before and after commercial thinning as well as final felling were similar to those currently recommended by Tapio (2006); (ii) Increased thinning thresholds (30T): where currently recommended basal area thresholds both before and after commercial thinning were modified by increasing them 30% compared to the current recommendations (see Fig. 2); and (iii) Unthinned regime (UnT): where the final felling was done at the end of the rotation period without employing any thinning during the rotation period. As the aim of this study was to investigate the effect of stump and coarse root extraction on the long-term carbon sequestration in the forest ecosystem, the three thinning regimes were further distinguished in a way that they either extracted the stumps and coarse roots for use as an energy biomass or left them in the forest as litter to be decomposed slowly over time. At the beginning of the simulation, as an initial condition of the stand, it was assumed that stumps and roots were collected from the forest floor at the end of previous rotation for the thinning regime that extracted stumps and roots, and they were retained for the thinning regimes that left stumps and roots in the forest floor. The selected thinning regimes were simulated for a Norway spruce stand growing at Oxalis-myrtillus site type in eastern Finland (Joensuu region: N, E, degreedays). According to the Finnish classification of forest types (Cajander, 1949), the Oxalis-myrtillus site type is highly productive and the most fertile type of site, and Norway spruce is often the common tree species found at this site. The simulations were run for two rotation periods, one after another, corresponding to a total simulation period of 160 years. At the end of each 80-year period, final felling was executed where all of the trees were removed from the stand. In the simulations, thinning from below was employed. An initial stand density of 2500 seedlings per ha was used, which was similar for all of the thinning regimes applied in this study. At the inception

6 450 A. ALAM et al. (a) Current thinning regime (b) Changed thinning regime Basal area (m 2 ha 1 ) Energy biomass thinning Basal area just before thinning Basal area just after thinning Basal area (m 2 ha 1 ) Dominant height (m) Dominant height (m) Fig 2 Principles of the thinning regimes based on the development of basal areas and dominant heights, as used in this study. The current recommendation was considered as the current thinning regime (a). Commercial thinning in the current recommendation was changed by increasing both the thinning thresholds when thinning was done and the remaining basal area threshold after thinning (b). of the simulation, the seedling diameter of 2 cm at breast height was used. As recommended by Tapio (2006), energy biomass thinning was performed earlier than the first commercial thinning when the dominant tree height of between 8 and 14 m was reached. The remaining basal area threshold after energy biomass thinning was also determined by following the siteand species-specific recommended number of trees. The first commercial thinning was executed when the tree dominant height of between 12 and 15 m was reached. Computation In all the management regimes, energy biomass and stemwood were harvested and collected. Stemwood was collected during commercial thinning and final felling, whereas the energy biomass was sourced only from energy biomass thinning and final felling. Stemwood refers to the pulpwood and sawlogs. Based on its diameter, stemwood was assorted into sawlogs (stem part above 17 cm diameter) and pulpwood (stem part of cm diameter). Bucking was conducted based on the length of the stem, and the stemwood was expressed as Mg ha 1. Energy biomass, also expressed as Mg ha 1, corresponds to the small-sized trees (regardless of diameter) harvested from energy biomass thinning, and branches, needles (30% loss assumed in harvesting), tops of stem, and/or stumps and large roots extracted during final felling. However, carbon storage in forest ecosystems (Mg ha 1 ) refers to the carbon in the stem, branches, leaves and roots in the growing stock and in the ground vegetation and forest floor, including standing dead trees, and is calculated as the mean annual accumulation over the rotation period. In the calculation, a wood density of 400 kg m 3 was utilized and the carbon content was assumed to be 50% of the dry biomass. The energy content of 3.2 MWh Mg 1 (dry biomass) was used. By utilizing the carbon content and the energy content of the wood biomass, the net carbon balance (C net bal ) and the ecosystem carbon balance (C eco bal ) were computed using Eqn (1) and (2), respectively. The C eco bal indicates the differences between carbon sequestration in the biomass and soil, and carbon emissions only from soil decomposition, and the C net bal is the difference between carbon sequestration and all sources of carbon dioxide, including management activities, combustion of energy biomass, degradation of wood biomass and soil decomposition. The C eco bal and C net bal were expressed as gco 2 m 2 a 1. For calculating emissions for energy biomass production, the unit kg CO 2 MWh 1 was used. The allocation of carbon exchange for energy biomass was performed according to the produced energy biomass proportion over the first and second rotation separately. The emissions were calculated from the C eco bal,c man and C harv (combustion of energy biomass). The substitution effect of energy biomass was calculated by utilizing Eqn (5) and the value for the emission of coal used 396 kg CO 2 MWh 1 (Gustavsson et al., 1995), which is the sum of end-use emissions, and the emissions from energy use for extraction, conversion, and distribution. The value is slightly higher than the reported combustion-related emission of coal (341 kg CO 2 MWh 1 ) for Finnish conditions in Statistics Finland (2011). emission ðcoalþ emission ðenergy biomassþ Substitution ¼ emission ðcoalþ ð5þ For each of the thinning regimes, the stemwood and energy biomass production, carbon storage in the forest ecosystem and in the wood product during use, carbon sequestration (C seq ), carbon emission from all sources (C deccomp, C harv, C man ), C eco bal, and C net bal were calculated annually and also over the entire rotation period. Afterwards, the carbon emissions per unit of energy biomass production and how much of energy biomass from Norway spruce forests growing at the most fertile site in Finland could be utilized to substitute coal were analysed for a period of 160 years, split into two rotation periods. The analysis for all of the above-mentioned factors were performed via a comparison of CuT and other regimes, between the first and second rotations, and between regimes that extract stumps and roots and those that left them in the forest. In the calculation of net climatic impact (kg CO 2 MWh 1 ), the thinning regime, which leaves stumps and roots in the forest stand was considered as the reference management. The net climatic impact of harvesting stump and root biomass was the difference in C net bal between regimes that leave stumps and roots in the forest and those regimes that extract them for the

7 EFFECT OF STUMP EXTRACTION ON CARBON SEQUESTRATION 451 utilization of energy biomass. This was done by applying Eqn (6): Net climatic impact ¼ C net bal ðtreatment-referenceþ ð6þ Total MWh ðtreatment-referenceþ Results Carbon source and sink dynamics of forest products and ecosystems Figure 3 shows the annual carbon flows, i.e. carbon sequestration (C seq ) and carbon emissions (C emi ), of forest products and ecosystem processes together with the ecosystem carbon balance (C eco bal, see Eqn 2) and the net carbon balance (C net bal, see Eqn 1) for two consecutive rotation periods under the CuT, as an example. In Fig. 3, carbon dynamics are shown without (panel a) and with (panel b) the extraction of stumps and roots. Clearly, forests were the carbon source at the beginning of the stand development for each of the rotation periods. Afterwards, during the intermediate to mature stage, as a fact of a faster growth rate, forests started to sequester more carbon, which turned the forests into a carbon sink for the rest of the rotation periods. As expected, for both rotations, the period of being a source and also the values of emissions from soil decomposition were higher for the stands that left stumps and roots in the forest compared to those that extracted them (Fig. 3a and b, ecosystem carbon balance box). However, combustion-related emissions, especially at the end of each rotation were considerably higher in the stand with the extraction of stumps and roots compared to the stand without extraction (Fig. 3a and b, net carbon balance box). Over the first rotation period, with or without extraction, the forest was a net carbon sink, but turned into a net carbon source during the second rotation period. This was mainly due to the increasing share of combustion and degradation of the biomass harvested from thinning and final felling during the first rotation as well as during the second rotation period. Yield of stemwood and energy biomass Effect of thinning regime. Compared to the CuT, the UnT increased stemwood production under both situations, whether stumps and roots were left or extracted. On the contrary, the UnT decreased the energy biomass production under the regime that left stumps and roots in the forest (Fig. 4a and b). Depending on the rotation period, the changes were from 4 to 9% in stemwood production, and from 20 to 8% in energy biomass production. In the 30T regime (thinning with increased basal area thresholds), stemwood production was mostly decreased (by up to 7%), but the energy biomass production was increased up to 21% compared to the CuT regime for both rotation periods and for both conditions that include and those that exclude stumps and roots for energy biomass estimation (Fig. 4a and b). Effect of rotation period. When stumps and roots were left in the forest, the second rotation increased (1 2%) stemwood and energy biomass production compared to the first rotation for the UnT and CuT regimes, although the production of both were reduced for the 30T regime by 3% (Fig. 4a). When stumps and roots were extracted, a 4 5% decrease in stemwood and energy biomass production was found for the CuT in the second rotation compared to the CuT in the first rotation (Fig. 4b). No substantial effect was found in energy biomass and stemwood production for the UnT and 30T regimes during the second rotation compared to the first in the case of extracting stumps and roots (Fig. 4b). Effect of extraction of stumps and roots. As expected, stump and root extraction increased the yield of energy biomass compared to that when they were not extracted, and, depending on the thinning regime and rotation period, the increase in relative terms was found to be between 80% and 146% (Fig. 4a and b). However, there was no considerable effect of stump and root extraction found for stemwood production. It was found that stump and root extraction slightly decreased stemwood production by 1% for the 30T regime during the first rotation and by 3% for the CuT regime during the second rotation, and was otherwise increased by up to 5%, depending on the thinning regime and the rotation period. Mean values for C seq,c emi,c eco bal,c net bal Effect of thinning regime. In general, carbon sequestration (C seq ) was found to be higher in the UnT and 30T management regimes, compared to the CuT regime for both the first and second rotations, in regimes that either extracted or left stumps and roots (Tables 2 and 3). As such, carbon emissions (C emi ) from soil decomposition were also higher in the UnT and 30T regimes compared to the CuT regime. In relative terms, the increase was from 6 to 25% for C seq and from 4 to 30% for C emi. In contrast, the carbon exchange value for ecosystem carbon balance (C eco bal ) was found to be higher for the UnT and 30T regimes, compared to that of the CuT regime, with or without the extraction of stumps and roots in both the first and second rotation. In both cases, whether stumps and roots were extracted or left, the carbon exchange value for net

8 452 A. ALAM et al. (a) Current thinning, CuT (stump and roots left in the forest) Ecosystem carbon balance (C eco-bal ) C seq C emi (soil decom) C eco-bal g CO 2 m 2 Net carbon balance (C net-bal ) C net-bal (eco+techno) C emi (soil+man+com+degra) Years g CO 2 m Years (b) Current thinning, CuT (stump and roots extracted) Ecosystem carbon balance (C eco-bal ) C seq 2000 C emi (soil decom) C eco-bal g CO 2 m Net carbon balance (C net-bal ) C net-bal (eco+techno) C emi (soil+man+com+degra) Years 1000 g CO 2 m Years Fig 3 Annual carbon sequestration (C seq ), carbon emissions (C emi ), ecosystem carbon balance (C eco bal ) and net carbon balance (C net bal ) under current thinning (CuT) for the first (1 80 years) and second ( years) rotations when stumps and roots were left in the forest (panel a) and when they were extracted (panel b). In the simulations, stumps and roots were also collected before starting the simulation for the stand that collected stumps and roots and vice versa.

9 EFFECT OF STUMP EXTRACTION ON CARBON SEQUESTRATION 453 Energy biomass (Mg ha 1 ) (a) Stumps and roots left in the forest UnT(R1) 30T(R1) CuT(R1) 120 UnT(R2) 30T(R2) CuT(R2) Stem wood (Mg ha 1 ) Energy biomass (Mg ha 1 ) (b) Stumps and roots extracted Stem wood (Mg ha 1 ) Fig 4 The relationship between energy biomass and stemwood production under three thinning regimes (unthinned: UnT, current thinning: CuT and thinning with increased basal area thresholds: 30T) for the first and second rotations when excluding (panel a) and including (panel b) stumps and roots as an energy biomass. Symbols filled with solid black represent the first rotation (R1) and unfilled symbols correspond to the second rotation (R2). Table 2 Mean values for carbon sequestration (C seq ), carbon emissions (C emi = C decomp + C harv + C man ), ecosystem carbon balance (C eco bal ) and net carbon balance (C net bal ) under different thinning regimes over the first (1 80 years) and second ( years) rotation periods when excluding stumps and roots as energy biomass. Unit is g CO 2 m 2 a 1. (E.Bio is for energy biomass) Stumps and roots left in the forest C emi C seq C decomp C harv Trees Roots Litter Humus E.Bio Pulp Saw C man C eco-bal C net-bal Current thinning, CuT First rotation Second rotation % increase in thinning thresholds, 30T First rotation Second rotation Unthinned regime, UnT First rotation Second rotation carbon balance (C net bal ) was found to be highest in the 30T regime during the first rotation and to be the lowest in the CuT regime during the second rotation (Tables 2 and 3). Effect of rotation period. For all of the thinning regimes applied in this study, the second rotation gave higher values for C emi (total), C eco bal and C net bal compared to the first rotation, with or without the extraction of stumps and roots (Tables 2 and 3). C seq (negative values) were also higher during the second rotation, largely for the regime that left the stumps and roots, compared to the first rotation. Depending on the thinning regime, the increase, in relative terms, was 0 2% for C seq,27 34% for C emi and 10 34% for C eco bal. In the case of C net bal, as expected, the relative changes were considerably higher (Tables 2 and 3). Effect of extraction of stumps and roots. There was no considerable effect found for C seq and C emi values when the thinning regimes aimed to extract the stumps and roots were compared to those that left them in the forest (Tables 2 and 3). The ranges of relative changes were from 1.1 to 0.8% and from 0.8 to 2.5% for C seq and C emi, respectively, for both the first and second rotation periods. In the case of C eco bal and C net bal, the effects of stump and root harvesting were found to be larger. The value for C eco bal was always found to be lower for all of the thinning regimes that extracted stumps and roots compared to the regimes that left them in the forest, in both the first and second rotation (Tables 2 and 3). The decrease, in relative terms, was as low as 48% in the CuT regime at the first rotation, and up to as high as 400% for the UnT regime in the second rotation. Conversely, the value for C net bal was found to be constantly

10 454 A. ALAM et al. Table 3 Mean values for carbon sequestration (C seq ), carbon emissions (C emi = C decomp + C harv + C man ), ecosystem carbon balance (C eco bal ) and net carbon balance (C net bal ) under different thinning regimes over the first (1 80 years) and second ( years) rotation periods when including stumps and roots as energy biomass. Unit is g CO 2 m 2 a 1. (E.Bio is for energy biomass) Stumps and roots extracted from the forest C emi C seq C decomp C harv Trees Roots Litter Humus E.Bio Pulp Saw C man C eco-bal C net-bal Current thinning, CuT First rotation Second rotation % increase in thinning thresholds, 30T First rotation Second rotation Unthinned regime, UnT First rotation Second rotation higher for the thinning regime that extracted stumps and roots compared to the regime that did not extract. The increase, in relative terms, was % for the first rotation period and 0.4 2% for the second rotation period. Emissions, carbon storage and substitution of coal Effect of thinning regime. In general, the CuT regime had the lowest amount of emissions produced per unit of energy biomass for corresponding rotation periods with or without the extraction of stumps and roots (Fig. 5a and b). Compared to the CuT regime, the UnT and 30T regimes had 5 55% higher emissions for producing an energy unit of energy biomass depending on the thinning regime, rotation period and the extraction of stumps and roots or not. Regardless of whether the regime extracted stumps and roots or not, the CuT regime, in addition to having lower emitted carbon per MWh of energy biomass, had the highest effect of substituting coal for both rotation periods. However, carbon storage in the CuT was lowest among the thinning regimes (Fig. 6a and b). On the other hand, the UnT regime was found to store the highest amount of carbon during the first and second rotation periods and also for both cases whether stumps and roots were left or extracted, but it was found to have the least effect on coal substitution (Fig. 6a and b). Effect of rotation period. Depending on the thinning regimes, the second rotation increased the emissions per unit of energy biomass (10 33%) and carbon storage in the forest ecosystem (1 2%), but decreased the effect of coal substitution with or without stumps and roots, except for the CuT regime, which extracted stumps and roots (Fig. 5a and b & Fig. 6a and b). Emissions, kg CO 2 MW h (a) Stumps and roots left in the forest UnT(R1) 30T(R1) CuT(R1) UnT(R2) 30T(R2) CuT(R2) Emissions, kg CO 2 MW h Energy biomass (Mg ha 1 ) Energy biomass (Mg ha 1 ) (b) Stumps and roots extracted Fig 5 Emissions (kg CO 2 MWh 1 ) against energy biomass production under three thinning regimes (unthinned: UnT, current thinning: CuT and thinning with increased basal area thresholds: 30T) for the first and second rotation (R1 and R2 respectively) when excluding (panel a) and including (panel b) stumps and roots as energy biomass. The emissions were calculated from the C eco-bal, C man and C harv (combustion of energy biomass).

11 EFFECT OF STUMP EXTRACTION ON CARBON SEQUESTRATION 455 (a) Stumps and roots left in the forest (b) Stumps and roots extracted Substitution ratio UnT(R1) 30T(R1) CuT(R1) UnT(R2) 30T(R2) CuT(R2) Substitution ratio Carbon storage (Mg ha 1 ) Carbon storage (Mg ha 1 ) Fig 6 Substitution ratio (energy biomass and coal) and carbon storage (mean) under three thinning regimes, unthinned (UnT), current thinning (CuT) and thinning with increased basal area thresholds (30T), for the first and second rotation when excluding (panel a) and including (panel b) stumps and roots as energy biomass. Symbols filled with solid black represent the first rotation (R1) and with white colour correspond to the second rotation (R2). Effect of extraction of stumps and roots. Depending on the thinning regimes, the extraction of stumps and roots decreased emissions per energy unit of energy biomass production by 13 18% during the first rotation, and by 22 26% during the second rotation, compared to when they were not extracted (Fig. 5a and b). However, the carbon storage and substitution ratio between energy biomass and coal were slightly increased for all of the thinning regimes and in both the rotation periods (Fig. 6a and b). Net climatic impact of harvesting and utilization of stumps and roots Over the two rotation periods, the net climatic impact of harvesting and utilization of stumps and roots for three thinning regimes were found to be between 50 and 59 kg CO 2 MWh 1. During the first rotation, differences in C net bal between the management (with and without stump and root extraction) were higher compared to the second rotation, which had a clear effect on the net climatic impact calculation (Table 4). Discussion The overall aim of this work was to assess the effects of stump and root extraction on the long-term carbon sequestration and average carbon storage in integrated production of energy biomass and stemwood in the forest ecosystem under different thinning options. One hectare of Norway spruce stand growing in a fertile site in central Finland (Joensuu region: N, E) was managed for two 80-year consecutive rotations altogether over a 160-year period to assess stemwood and energy biomass production, carbon sequestration, average carbon storages and sink and source dynamics over the entire production and utilization chain of biomass. In the analysis, modelling approach was adopted for the stumps and roots harvest data and potential soil humus horizon disturbance were not evaluated. The system boundary, including temporal and spatial aspects is a key factor in analysing the carbon balance of a dynamic production chain like a forest ecosystem (Lindner et al., 2008; Zanchi et al., 2011). A little change in these aspects could alter the output substantially. In this study, the time frame for the assessment was fixed because the aim was to compare different thinning options with or without extracting stumps and roots. This enables the whole production and utilization chain of energy biomass to be assessed, as well as the subsequent consequences on carbon when utilizing stumps and roots to substitute fossil fuels and mitigate climate change. The analysis presented here is subject to some uncertainties as the simulations did not include the effect of plausible climate change (e.g. increase in temperature, precipitation, carbon dioxide concentration of the atmosphere) on forest growth and development, but instead assumed that current climatic conditions would Table 4 Net climatic impact of harvesting stumps and roots for the three thinning regimes (current thinning CuT, unthinned UnT and thinning with increased basal area thresholds 30T) over the first, second and both rotations First rotation Second rotation Over two rotations CuT UnT 30T CuT UnT 30T CuT UnT 30T C net-bal difference (kg CO 2 ha 1 yr 1 ) Net climatic impact (kg CO 2 MWh 1 )

12 456 A. ALAM et al. prevail throughout the modelling period. Moreover, natural disturbances, such as wind throw, insect attack and forest fire have been excluded from the simulation as their affects appear to be less in managed forests, as reported by Seidl et al. (2008). It was found that stump and root harvesting may increase the total biomass production (energy biomass and stemwood) by approximately 21 36% depending on the thinning regimes and the rotation length. Notably, the increase was mainly from the additional stump biomass harvested for energy purposes, and, as such, no considerable effect was found in stemwood (pulpwood and sawlogs) production between stumps extracted and stumps left. Among the thinning regimes, the UnT yielded the highest amount of sawlogs, and also the highest amount of stemwood, jointly with pulpwood. This was partly because of the number of stems remaining at the end of the rotation before final felling and the fact that this study also utilized the bucking approach commonly practised in Finland. However, the stemwood production in the UnT would not be economically feasible as the mean diameter of the stem was less than that of other regimes. This is because of the fact that the thinning operations conducted, for example, in CuT allow more growing space for the remaining trees, thereby shifting the distribution of growth to larger and more valued trees (e.g. Petritsch et al., 2007; Alam et al., 2012). When current thinning practices were obeyed in this study simulation, pulpwood production was found to be highest. This is because all of the stemwood produced in the commercial thinning was pulpwood. The diameter development of standing trees that seldom went above 17 cm until commercial thinning restricted the trees to be felled to the sawlogs category. Diameter development is also the reason behind the higher total pulpwood production in CuT compared to other thinning regimes, and the higher or fairly similar stemwood production compared to 30% increased thinning thresholds regime. In this study, only the CuT had one commercial thinning in between the final felling and the energy biomass thinning during each rotation. The inconsistent number of thinning regimes between the current and 30% increased thresholds (30T) regimes were the outcome of the utilized basal area and dominant height-based thinning threshold systems implemented in this study. The approach was reflected quite well in the results of this study in the analysis of ecosystem carbon storage for all thinning regimes. The results showed that unthinned and 30% increased thinning threshold regimes had higher ecosystem carbon storage compared to CuT, which is in agreement with other studies, such as those by Briceño-Elizondo et al. (2006), Garcia-Gonzalo et al. (2007), and Alam et al. (2008). With the extraction of stumps and roots, the second rotation did not affect or even increase the carbon sequestration (forest growth) in trees and roots compared to the first rotation. This could be mainly because of the utilized site fertility type, which may not be so sensitive as to effect the growth to increased nutrition removal (Raulund-Rasmussen et al., 2008; Egnell, 2011; Mason et al., 2012). Comparative studies with other types of site fertility in Finland could be useful as Mason et al. (2012) found in their study that growth response to whole tree harvesting was affected by site type and soil fertility. Some previous studies reported growth reduction due to removal of whole tree biomass (Proe et al., 1996; Mann et al., 1998; Jacobson et al., 2000; Walmsley et al., 2009; Mason et al., 2012). In those studies, the whole tree biomass was defined as all of the above ground biomass including needles, which is the most nutrient concentrated part of trees. In this study, 30% of the needle biomass was retained in the forest after final felling and neither the stumps and roots nor needles, branches and tops of stems were collected during commercial thinning, which might have affected the results in comparison with the above-mentioned studies. In addition, nitrogen deposition could also be an important factor that could have affected the nutrient balance and subsequent growth of forests in the studied site. A holistic analysis of nutrient balance affected by different thinning regimes is needed to increase the understanding further, although future scenarios for nitrogen deposition are inherently uncertain (Reay et al., 2008). The results of this study showed that there was a considerable effect on the net carbon exchange between the stands whether stumps and roots were retained or extracted. Clearly, emissions from soil decomposition were higher if stumps and roots were left in the forest. However, if they were extracted and combusted, the emissions from harvested energy biomass were much higher (see also Repo et al., 2010; Zanchi et al., 2011). In the former case, emissions are held slowly and go through the decaying process over a longer time period, whereas during the latter situation the carbon is emitted all at once. No matter whether the stumps and roots were left or extracted, it was found that the ecosystem carbon balance (C eco bal ) stayed always as a net sink of carbon in both the first and second rotations (Tables 2 and 3). However, the inclusion of emissions from harvested pulpwood and sawlogs in the calculation of net carbon balance (C net bal ) turned the stand into a net source of carbon, especially during the second rotation. This may again not always be true as a significant fraction of carbon is locked away, not only in the fossil fuel that would have otherwise been emitted if energy biomass was not used but also in the harvested stemwood that could store a maximum of 312 Mg CO 2 ha 1 at the