Comparison of carbon balances between continuous-cover and clear-cut forestry in Sweden

An actively managed forest landscape that provides a large amount of sustained biomass yield while at the same time maintaining large standing forest carbon stocks, provides greater climate benefits in the long run compared to unmanaged forests (Nabuurs and Masera 2007; Lundmark et al. 2014). Different forest management strategies might however result in different climate benefits due to the complex interaction between forest management, carbon stored in the ecosystem and harvested wood products, and the amount of harvested products that leads to differences in substitution carbon benefit (Smyth et al. 2014).

At the stand level, the carbon balance at any given time of a managed forest is determined by the difference between the input flux of carbon (net primary production) and the output fluxes of carbon (heterotrophic respiration and leaching) together with biomass removals by harvests (Clarke et al. 2015). The long-term average annual change in carbon balance of a forest management system depends not only on the dynamics of carbon stock in the soil and standing biomass, but also on the carbon emissions related to silvicultural operations, and the level of sustained yield that can be attained. As a result of sustained forest production, some of society’s consumption will be based on renewable products reducing the net emissions of carbon to the atmosphere through the substitution of fossil-based materials. This substitution effect depends on what products that are consumed in society and substituted with forest products e.g., fossil fuels, steel, and concrete (Sathre and O’Connor 2010; Gustavsson and Sathre 2011; Poudel et al. 2011; Gustavsson et al. 2015). Several studies have shown the importance of a sustained or increased yield in actively managed forest to increase the climate benefit (Canadell and Raupach 2008; Malmsheimer et al. 2008; Poudel et al. 2012; Lundmark et al. 2014; Sievänen et al. 2014).

Intensively managed boreal forests in Finland and Sweden have high rates of productivity, low rates of natural disturbances, allowing for large transfers of forest raw material from forests to society. Forestry in these countries is largely based on clear-cut forestry (CF) with even-aged forest stands and an even age-class distribution on the landscape level (Yrjölä 2002). The carbon dynamics of CF on the stand level include a significant net biomass carbon removal from the forest stand at final harvest, rapid net carbon gain in young stands, and slower net carbon gain, after canopy closure in more mature stands (Hyvönen et al. 2007; Diochon et al. 2009). Growth of managed even-aged forest stands follows a universal pattern where current annual increment increases after stand establishment, peaks when maximum leaf area is attained, and then declines (Assmann 1970). Most often CF stands are harvested around the time when mean annual increment culminates (Möller et al. 1954). An alternative to CF is continuous-cover forestry (CCF), a silvicultural system without a clear-cut phase. CCF typically has uneven-aged stand structure and a continuously maintained forest cover, which does not follow a cyclic harvest-and-regeneration pattern on the stand level as it occurs in CF (Troup 1928; Gadow 2001). Instead, multiple selective cuttings over regular time intervals characterize CCF system. In the classical CCF silvicultural practice single-tree selection, individual trees are harvested throughout the stand to maintain an uneven-aged (and uneven-sized) stand, achieve a desired diameter distribution, and to allow establishment and ingrowth of new naturally established seedlings (Ahlström and Lundqvist 2015).

There has been a discussion for centuries on the comparison of production levels, yield, economic return, and silvicultural measures to be used in CF and CCF systems (Wallmo 1897; Holmgren 1914; Möller 1922; Troup 1928; Lundqvist 1989). The long-term trends in forest management in the Nordic countries have resulted in a completely dominant practice of the CF system (Anon. 2014a, b). Recently however, CCF has been put forward as an alternative to CF because of the provision of more ecosystem services related to a continuous forest cover (Tahvonen 2009; Kuuluvainen et al. 2012; Pukkala et al. 2012). Among those, some positive effects on the carbon dynamics has been suggested (Lindroth et al. 2012; Pukkala 2014). Despite the increasing interest, it is still not well described how CCF would affect the carbon balance and the resulting climate benefit from the forest in relation to CF.

In this paper we compare the climate benefit of CF and CCF applied as different silvicultural programs on the stand level under Nordic conditions. When doing such comparison it is important to consider time and scale perspectives (Kuuluvainen et al. 2012; Lundmark et al. 2014). A comprehensive life-cycle analysis integrating biological and technological features can help identify appropriate long-term approaches to carbon management through land use. In order to evaluate silvicultural programs in terms of climate change mitigation effectiveness, this study aims to compare the dynamics of carbon balances of CF and CCF as two alternatives for future management of an existing mature forest stand in the boreal zone in Sweden. We track carbon dynamics in standing biomass, litter on the forest floor, harvested products, and the contribution of forest biomass to substituting fossil fuels and carbon-intensive materials. Carbon balance is expressed as the combined effect of carbon sink/source effects in the forest ecosystem and harvested wood products and substitution effects.

The litter carbon stock in the forest varied considerably over time and between the scenarios, depending on whether residues and stumps where harvested or not (Fig. 1). The magnitude of change was higher for the CF scenarios than for the CCF scenarios (Fig. 1). The long-term average annual litter carbon stock change did not, however, vary much between scenarios, ranging 0.01, 0.02, 0.04, and 0.05 Mg C ha⁻¹ year⁻¹ for CF+, CF, CCF80, and CCF100, respectively (Table 4). A sensitivity analysis where the rate of decomposition in Table 1 was increased or decreased by 20 % gave only minor effects (<2 %) on the absolute values of the long-term climate benefit in Table 4 (results not shown).

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There was a considerable short-term variation in carbon stock in the living biomass of trees due to annual growth rate dynamics and periodic harvest operations (Fig. 2a, b). There were small differences between scenarios in long-term carbon stock in harvested wood products.

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Over the long-term, the substitution effect accounted for the largest component of the average net annual carbon balance for the different scenarios. With the assumption that forest biomass was used to maximize the substitution effect (substitution effect of 0.90 Mg CO₂-eqv for each cubic meter of harvested biomass), at the end of the study period it accounted for 82 % of the total climate benefit for the CF+ scenario and 91 % for the CCF100 scenario. Adding all components together and assuming a substitution effect of 0.90 Mg CO₂-eqv for each cubic meter of harvested biomass, the accumulated carbon balance values for the different scenarios CF+, CF, CCF100, and CCF80 during the whole study period were estimated to be 784, 642, 597, and 531 Mg C ha⁻¹, respectively (Fig. 2; Table 4). With these assumptions, the average net annual carbon balance values for CF+, CF, CCF100, and CCF80 were estimated to be 2.75, 2.25, 2.06, and 1.83 Mg C ha⁻¹ year⁻¹, respectively (Table 4).

When assuming a lower substitution effect (0.47 Mg CO₂-eqv for each cubic meter of harvested biomass), the average annual net carbon balance dropped to 1.81, 1.29, 1.02, and 0.91 Mg C ha⁻¹ year⁻¹ for CF+, CF, CCF100, and CCF80 scenarios corresponding to 50–60 % of the values obtained when assuming the higher substitution effect (Fig. 3a, b). Also with the lower substitution effect it remained the single most important component of the long-term climate benefit.

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The results show that biomass growth is more important than the choice of silvicultural system per se. When comparing the two scenarios CF and CCF100, representing two principally different silvicultural systems, but with similar growth, extraction, and product use, only minor differences were found (Table 4). The importance of forest growth and long-term sustainable yields have been shown important for climate benefit in a number of other studies (Poudel et al. 2012; Lundmark et al. 2014; Kilpeläinen et al. 2015; Torssonen et al. 2015) supporting this conclusion. Also in this study, the effect of forest growth and yield was obvious especially when comparing the two CCF scenarios where the climate benefit was lower for the scenario with a lower average growth rate (Table 4).

We have compared the climate benefit of CF and CCF systems where the spatial boundary has been limited to the stand level. As a starting point for the analyses, we assumed the existence of a relatively heterogeneous spruce stand with two management alternatives for future land use. We restricted the period of analyses to 1 year before the third clear-cut in CF, after 285 years, and 1 year before the last selective cutting in CCF, after 290 years. By doing this, a large share of the growth from the third rotation period in CF is not harvested with the consequence that the average carbon stock change in the CF scenarios becomes higher than it should have been if the third clear-cut would have also been included. This explains why annual average change in standing forest carbon stock was higher for CF than for CCF in this analysis. On the other hand, the substitution effect became smaller than if the third clear-cut would have also been included. For the CCF100 scenario, with comparable growth and extraction as the CF scenario, a larger share of the annual growth has been harvested giving lower average carbon stock change figures but corresponding greater impact of substitution (Table 4).

Analyzing the various parts of the carbon balance in the different scenarios reveals that the most important component was the substitution effect (Fig. 2). When assuming a high substitution level, the substitution effect accounted for more than 70 % of the total annual average carbon balance in all scenarios, stressing the importance of product use strategy and the availability of forests biomass for the total climate benefit of forestry. Also with the lower substitution level used in this study, the substitution effect was the most important component of the carbon balance. As a consequence, the largest climate benefit was achieved with CF+ because of whole-tree use (higher extraction level) and smallest with CCF80 because of the lower production level (Figs. 2, 3). Assuming two different product use strategies, one representing a scenario where forest biomass is mainly used for energy and construction purposes (high substitution effect) and one which represents the current product use in Sweden, showed that the total future climate benefit may vary from 0.91 to 2.75 Mg C ha⁻¹ year⁻¹ (Fig. 3). If the CF scenario with a substitution effect of 0.47 Mg CO₂-eqv for each cubic meter of harvested biomass is considered as the business as usual scenario, it can be concluded that the future climate benefit of forestry can increase or decrease relative to the present climate benefit of 1.29 Mg C ha⁻¹ year⁻¹ depending on the future growth and extraction rate of the managed forests and the use of extracted biomass. In order to make additional climate benefits compared with today, the most efficient strategy for the Nordic forests is to increase growth and yield and to maximize the substitution benefit. This can serve as a policy insight for coming discussions about future forest management and product utilization strategies in the context of climate change mitigation.

The simulation approach included a different set of models for CF and CCF. The Heureka system has been shown to give reliable results in traditionally managed Norway spruce stands (Fahlvik et al. 2014). Representation of CCF in the data behind the growth models in the Heureka is however limited. To better simulate the transition of trees between diameter classes in CCF it was decided to use growth models specifically developed for CCF (Chrimes and Lundqvist 2004). The models were based on six experimental plots within a small geographical area, which limited the scope of the simulations. To ensure that model dependent differences of the estimated growth level were not introduced, the production level in the CCF scenarios was adjusted according to scenarios (see “Materials and methods” section).

In the present study, it was assumed that a reverse J-shaped diameter distribution could be retained over a long period of time according to the model approach used. A sustainable J-shaped distribution is dependent on sufficient ingrowth to compensate for mortality and harvest. The ingrowth used in the simulations was comparable with levels found in field experiments in northern and central Sweden (Lundqvist 1993; Lundqvist et al. 2007). Lähde et al. (2010) studied a spruce dominated stand in Finland and found that the initial reversed J-shape diameter distribution remained 15 growing seasons after selection cutting. In a study by Ahlström and Lundqvist (2015) on uneven-aged spruce-dominated stands, it was concluded that it is possible to both maintain and restore a reverse J-shaped diameter distribution after harvest in heterogeneous spruce stands. Hence, these studies support the assumptions made about the sustained diameter distribution used in the CCF scenarios.

Comparative studies on mortality in CCF and CF are missing, and mortality in CCF was adjusted to equal the mortality proportion out of MAI in CF. Probability of mortality decreased with increasing DBH in the simulations which is in accordance to Pukkala et al. (2009). However, mortality in terms of proportion of stem number was lower than that found within experiments on uneven-aged spruce in central Sweden (Lundqvist 1993; Lundqvist et al. 2007).

The ambiguous results from studies of soil carbon in boreal forests, made us choose to not make different assumptions about older soil carbon development in the different scenarios. This can be questioned since the decomposition and mineralization of different carbon pools can be affected by disturbances like harvesting and also by environmental changes due to silvicultural measures such as thinning and clear-cutting. In this respect, the soil carbon pool may have been somewhat overestimated in the clear-cut scenarios, especially in the CF+ scenario where residues and stumps were also harvested (Egnell et al. 2015). Since soil carbon stocks are largely affected by forest growth (i.e., carbon input) the soil carbon pool may also have been overestimated in the CCF80 scenario due to the assumed lower growth rates. We also used a constant decomposition rate for the estimate of litter carbon remaining in the forest for all scenarios (Table 3). Since litter production is influenced by forest growth and decomposition of litter by factors such as temperature and humidity this assumption might also have led to some differences between scenarios that have not been identified. The analyses of sensitivity to increased or decreased decomposition rates showed however that the effects were small in terms of annual average change in litter carbon stock and consequently did not influence the total carbon balance. These uncertainties regarding soil carbon together with the conflicting results from the various studies call for more studies on soil and litter carbon dynamics in relation to different silvicultural systems and within those different applied silvicultural programs. We recognize the uncertainties in our estimates of carbon stocks in the soil for the various scenarios, but we argue that differences that might not have been captured between the scenarios in this regard would not be decisive for the results.

The choice of a silvicultural system per se was not important for the climate benefit. Instead, forest growth and yield together with the product use strategy determined the long-term climate benefit of forestry when analyzed at the stand level. In the long run, carbon stock changes in standing biomass, litter, and products were very small in managed forest systems as the ones used in this study when growth were assumed to be equal between the systems. As a consequence, the long-term average growth and yield will be more critical when discussing the silvicultural systems CF versus CCF, than other factors affecting the climate benefit of forestry.