A viable indicator approach for assessing sustainable forest management in terms of carbon emissions and removals

Ecological Indicators 111 (2020) 106057

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A viable indicator approach for assessing sustainable forest management in terms of carbon emissions and removals

T

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Michael Köhla, , Hans-Peter Ehrhartb, Marcus Knaufc, Prem R. Neupanea,d a

Center for Earth System Research and Sustainability, World Forestry, University of Hamburg, Leuschner Strasse 91, D-21031 Hamburg, Germany Forschungsanstalt für Waldökologie und Forstwirtschaft, Hauptstraße 16, 67705 Trippstadt, Germany c Knauf Consulting, Dorotheenstraße 7, 33615 Bielefeld, Germany d Friends of Nature (FON), P. O. Box 23491, Kathmandu, Nepal b

A R T I C LE I N FO

A B S T R A C T

Keywords: Sustainable forest management Timber utilization Carbon offsets Timber harvesting C-Balance Displacement factor

In forest management, the sustainability of a multitude of economic, ecological and socio-economic impacts and services is assessed. The role of forests in the global carbon (C) cycle is almost exclusively assessed through an ecosystem approach that relates emissions from timber harvesting and removals through biomass growth and C sequestration. While harvested wood leads to a reduction in the C-stock in forest C-pools, the use of wood for energy and production of wood-based materials results in significant emission reductions by substituting in place of greenhouse gas (GHG) intensive fuels and energy-intensive non-wood materials respectively. We present a new indicator approach for the assessment of sustainable forest management that includes the entire emissions and removals associated with the harvested wood along the wood product value chain and thus represents represents C-sustainability with respect to emissions and removals. The indicator is implemented in two variants. If information available on wood use and the carbon offsets associated with it is sufficient, the indicator can be derived by presenting emissions from wood harvesting and removals from wood use in the form of a carbon balance (C-balance indicator). If the balance is even or if removals predominate, forest management fulfills the requirement of C-sustainability. If the information is insufficient, the emissions from the wood harvest and possible wood processing losses are summed up, and the necessary displacement factor (DF) is calculated, which is necessary to compensate the corresponding emissions by carbon offsets of the wood use (DF-indicator). A comparison of the necessary displacement factor with common carbon offsets of typical wood uses allows an assessment of C-sustainability. The effectiveness of the C-indicator for assessing sustainable forest management is illustrated by two case studies. We found that in order to achieve carbon neutrality, substitution factors between 1.9 (lignite) and 2.5 (gas) are necessary, depending on the fossil fuel substituted. If no energetic substitution is assumed, the DF increases to a value of 3.3. In situations with high harvest losses, the necessary DFs well exceed values that can be achieved even under very positive assumptions; C-sustainability is therefore not met. Both approaches allow an assessment of C-sustainability beyond forest boundaries by giving appropriate weight to the importance of wood use in the carbon cycle. In addition to the implication of the indicator to evaluate SFM, we claim that this indicator is responsive to the emerging global forest related international processes and their reporting requirements such as Reducing Emissions from Deforestation and forest Degradation in Developing Countries (REDD+) as well as nationally appropriate mitigation actions (NDCs, Low Emission Development Strategy).

1. Introduction Forests satisfy a variety of ecosystem services and functions. In managed forests, these forest functions include social, economic and ecological aspects. To assess the sustainability of forest management,

various indicator sets have been developed that cover different spatial and thematic domains. In the context of climate change, long-term global temperature goals in the light of Paris Agreement, and mitigation efforts to achieve emission reduction targets to fill the emissions gap, the carbon storage capacity of forests plays an important role. Due to

⁎

Corresponding author. E-mail addresses: [email protected] (M. Köhl), [email protected] (H.-P. Ehrhart), [email protected] (M. Knauf), [email protected] (P.R. Neupane). https://doi.org/10.1016/j.ecolind.2019.106057 Received 6 June 2019; Received in revised form 30 September 2019; Accepted 29 December 2019 1470-160X/ © 2020 Elsevier Ltd. All rights reserved.

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long-term multiple economic benefits), and 4. Institutional and legal criteria (indicators on institutions, infrastructure, legislation).

the sequestration of atmospheric carbon dioxide (CO2) by photosynthesis resulting in tree growth and the release of CO2 during biological degradation processes, the forest carbon (C) pools are subject to a temporal dynamic. Changes in forest C-pools are thus an important indicator for the climate-relevant assessment of forest management. Timber harvesting and the associated reduction of the forest carbon stock are often equated with an emission of CO2. However, the overall climate-relevant significance of forest management can only be assessed if the utilization of harvested timber is taken into account. On the one hand, the use of wood as an energy source replaces the use of fossil fuels and thus leads to lower emissions from non-renewable energy sources. On the other hand, wood products replace similar materials, which can only be produced with significantly higher energy consumption and emissions. The energetic and material substitutions by timber means that, in contrast to the use of fossil energies and non-renewable resources, overall emissions can be reduced and no additional carbon is fed into the global carbon cycle. Instead carbon is shifted in the atmosphere-vegetation system. Whether the use of wood is carbon neutral depends on a number of factors. In addition to the specific use of timber, losses during timber harvesting and wood processing also play a decisive role. Here we present two easy-to-use indicators that combine the losses of forest carbon storage through wood harvesting with the substitution gains from wood use. The proposed indicators differ in terms of the information necessary to derive them, but both allow for an improved assessment of the sustainability of timber harvesting and wood use with regard to the CO2 emissions, carbon balance, and climate change mitigation potentials of forest management.

The United Nations Forum on Forests (UNFF) in January 2017, and subsequently adopted by the United Nations General Assembly in April 2017 developed “The United Nations Strategic Plan for Forests 2030”, which provides a global framework for action at all levels to sustainably manage all types of forests by defining six Global Forest Goals and 26 associated targets to be achieved by 2030 (United Nations, 2019). 2.2. Emissions from forest degradation and deforestation As a result of climate change and the rising CO2 concentration in the atmosphere, forests play an increasingly important role in the global carbon cycle as sinks and sources of carbon. For the period of 1990 to 2007 Pan et al. (2011) estimate an annual global forest sink of 2.4 ± 0.4 Pg C and emissions of 2.9 ± 0.5 Pg C year−1 caused by deforestation of tropical forests. Pearson et al. (2017) studied emissions from forest degradation for a forest area of 2.2 billion hectares. Of the total emissions from deforestation and forest degradation, one quarter was due to forest degradation. In 28 of the 74 countries surveyed, emissions from forest degradation exceeded emissions from deforestation. 2.3. Carbon offsets by timber use In the first commitment period (CP) of the Kyoto Protocol (KP) (2008–2012), countries were encouraged to report changes in forest Cpool. Wood products were excluded from the reporting obligations. Therefore, any wood removed from forests was considered a direct emission of CO2. At the UNFCCC Conference of the Parties (COP) in Durban in 2011 it was decided that in the second CP of the KP (2013–2020) the carbon storage in wood products can also be taken into account. The different treatment of wood products in the first and second CP of the KP led to two approaches, which today dominate the discussions on forests and climate protection: 1. The “ecosystem approach” focuses above all on the climate-positive effects of forests through the maintenance and enhancement of their C-pools. 2. The “sector approach” takes into account not only the carbon storage in the forests but also emissions reductions and storage effects due to the use of wood. Under the ecosystem approach climate benefits are achieved by abandoning the use of timber and thus increasing C-stock in forest Cpools. The renunciation of timber harvest is raised to the primacy of mitigating climate change through forests. Removals by forest growth occur in young forest stands, but removals and emissions change into a balance between CO2 accumulation through biomass growth and CO2 emissions through biomass decomposition in their climax phase (Otto, 1994; Luyssaert et al., 2008; Lippke et al., 2011). The sector approach allows the transfer of carbon accumulated in wood from the forest Cpool to the product C-pool and takes into account the fact that fellings cause no direct emissions of CO2. In addition to the storage function, wood products lead to an emission reduction effect through the substitution of fossil fuels:

2. Background 2.1. Sustainability concepts and indicators The idea of sustainable forest management was born in Germany more than 300 years ago in view of degraded forests and shortage of timber supply (Carlowitz, 1713). Forestry today continues to face a multitude of challenges for sustainability which are partly in conflict of interests – provision of timber and ecosystem services, adaptation to climate change, or maintaining forest C-stocks (Fares et al., 2015). At the same time, there is an increasing demand for timber as renewable energy (Fares et al., 2015; Pelkonen et al., 2014; Searchinger et al., 2018). With regard to the role of forests in mitigating climate change, there is a tension between the preservation of the C-stocks of forests by not harvesting trees and the energetic and material use of harvested timber to reduce emissions from fossil fuels (Luyssaert et al., 2008; Bellassen and Luyssaert, 2014; Knauf et al., 2015). When assessing the C-sustainability of forests, enhancing forest C-stocks must be weighed against the most carbon-efficient use of harvested timber. Following the United Nations Conference on Environment and Sustainable Development (UNCED), held in Rio de Janeiro in 1992, criteria and indicators (C&I) for assessing sustainable forest management (SFM) were established in several regional processes (Baycheva et al., 2013; Caswell et al., 2014; Linser et al., 2018a). A criterion describes the different aspects of sustainability on a conceptual level and facilitates the validation of forest management by a set of conditions or processes (Linser and ÓHara, 2019). Each criterion is supported by a set of indicators that show changes over time and how well each criterion achieves the objective set for it. Criteria for SFM can be grouped into four categories:

- CO2 emissions from fossil fuels are avoided through the energetic use of wood (energetic substitution). - The use of wood in the manufacturing process generally requires less energy than the production of functionally equivalent non-wood materials and avoids emissions from fossil fuels and CO2 intensive products (material substitution).

1. Forest resource criteria (indicators on changes in the forest area, characteristics of forests, wood production), 2. Environmental criteria (indicators on biodiversity, ecosystem productivity, soil conservation, water conservation, forest ecosystem health and vitality, contribution to global ecological cycles), 3. Socio-economic criteria (indicators on long-term social benefits,

In 2011, more than 1.3 billion m3 of firewood were used worldwide (Köhl et al., 2015). The International Energy Agency (IEA) estimates 2

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can be reduced to around 30% of the input material. Mill losses can be fed to further processing, for example, to produce thermal energy needed in wood processing (Sathre and O'Connor, 2010). Splinters and chips are often used in the pulp and paper industry, in chipboard production or for pellets, and thus make a contribution to material or energetic substitution. Burning mill losses as waste, leads to direct emissions without making use of substitution effects.

that the number of people dependent on biomass as their primary energy source will increase from 2.5 billion in 2004 to 2.7 billion in 2030 (IEA, 2006). Currently 60 to 80% of EU’s renewable energy consumption consists of forest biomass (Fares et al., 2015) and demand is expected to increase in the future (Searchinger et al., 2018). The potential material substitution effects of wood products depend essentially on the considered alternative non-wood products as well as on the considered functional units and the selected system boundaries (Sathre and O'Connor, 2010). Functional units can be individual products, buildings or services. The entire life cycle of a product is usually selected as the system boundary and includes the provision of raw materials, product manufacture, the duration of carbon storage in the product, and the recovery of bioenergy from processing losses. The fate of the product at the end of its life cycle is also crucial. Recycling, cascade use and final energetic use reduce CO2 emissions, while landfill disposal or uncontrolled decay worsen the carbon balance. Life cycle analyses (LCA) are used to derive displacement factors (DF) as the basis for calculating substitution effects. A displacement factor quantifies the amount of emission reduction achieved per unit of wood use (Sathre and O'Connor, 2010). Positive DFs indicate that the consumption of wood leads to the avoidance of emissions, negative DFs lead to higher emissions compared to other materials or energy sources due to the use of wood (Schlamadinger and Marland, 1996a; Sathre and O'Connor, 2010). Sathre and O'Connor (2010) found substitution factors in the range of −2.3 to 15 in extensive LCAs of wood products. Negative DFs are usually caused by improper disposal at the end of the life cycle. An average DF of 2.1 was described for the use of wood under European and North American conditions. This means that for every ton of carbon contained in wood products, the emission of 2.1 tons of carbon or 7.7 tons of CO2 is avoided by replacing alternative non-wood products (Sathre and O'Connor, 2010).

2.5. Removals by regrowth Each harvesting operation causes an increase in the diameter growth of the remaining trees, which is compensated for after a few years by the aging trend of decreasing growth. The stronger the intervention and the earlier it takes place, the greater the effect will be for light demanding tree species and deciduous trees (Assmann, 1961; Kramer, 1988; Nyland, 2002). This regrowth effect is exploited by thinning sustainably managed forest stands. The extent to which the remaining growing stock increases is largely determined by the intensity of thinning (Pretzsch, 2009). In the event of over-exploitation, the growth of the remaining stand collapses, resulting in forest degradation. The regrowth after timber harvesting plays an important role when looking at the C-balance at the level of individual forest stands. At the level of sustainably managed forest enterprises or regions, harvesting interventions are generally offset by the growth in non-harvested forest stands. This effect is captured by indicators, which compare fellings with increment on larger forest areas. 2.6. Holistic view of removals, emissions and avoided emissions For an overall assessment of the C-balance after logging interventions, the losses of the forest C-pool must be compared with the storage effects in wood products as well as the carbon offsets through energetic and material substitutions. Fig. 1 shows the different paths of use. Material removed from the forest C-stock by harvesting leads to emissions as harvest losses and processing losses or as a result of energetic use. The processing of wood products leads to a reduction of emissions compared to similar products made of alternative energy-intensive and non-renewable materials. This material substitution effect is reinforced in circular economy when wood products are reused at the end of their life cycle. For example, a roof beam can be recycled for the production of chipboard. Energetic substitution effects are achieved when wood products are used energetically at the end of their life cycle.

2.4. Logging and mill losses During timber harvesting, biomass is removed from the standing growing stock. A part of this biomass is extracted from the forest and utilized for energy or manufacturing of wood products, while another part remains in the forest as harvest loss and decomposes. In the decomposition process the C stored in the wood is oxidized and emitted into the atmosphere as CO2. Therefore, harvest losses are to be considered as emissions. In sustainably managed forests in the temperate zone, the proportion of wood harvest losses amounts to between 20 and 30 percent of the wood biomass removed from the growing stock (Altwegg et al., 2010; Riedel et al., 2017). In tropical forests, according to a rule of thumb established by Enters (2001), for every cubic meter of timber removed, an additional cubic meter of timber remains as harvest losses in the forest. Dykstra and Heinrich (1992), Dykstra (1992), and Noack (1995) report harvest losses of 46 percent for Africa and Malaysia, 54 percent for Asia and the Pacific and 44 percent for South America. For Belize, Bolivia, Brazil, Indonesia, Guyana and Congo, harvest losses were described that were 2 to 5 times higher than the volume of wood used (Pearson et al., 2014). The introduction of reduced impact logging techniques can significantly reduce timber harvest losses (Putz et al., 2008). Harvesting concessions are generally based on timber volume extracted from the forest but not on the timber volume removed from the growing stock. This practice often results in overexploitation of forest and high logging losses. Wood harvest losses can therefore cause far more emissions than the amount of wood used. In a study in Surinam, Rüters (2016) found that almost half of the logging losses could be used as saw timber. The unsatisfactory resource efficiency further continues in timber processing. High processing losses result from outdated or rudimentary processing technologies with yield rates between 30 and 60% in sawmills and 43 to 50% in plywood production (Steffen, 1995; Enters, 2001; Schönfeld, 2016). By using efficient processing methods, losses

3. Material and methods 3.1. Conceptual approach In order to assess carbon offsets, the GHG impact chains of the forestry and timber industry must be combined (Fig. 2). Here both, changes in the C –stock in forest and harvested wood products (HWP) pools and substitution effects from energy or material use of timber must be accounted for. Changes in C-stock of forest C-pools can be detected by forest monitoring (IPCC, 2003, 2006a; Baldauf et al., 2011). It is far more difficult to capture changes in the C-pool of HWPs and substitution effects of wood use (IPCC, 2006a). This requires extensive information on wood utilization and life cycle analyses of wood products to estimate changes of HWP C-pools and substitution effects (Schlamadinger and Marland, 1996b; Sathre and O'Connor, 2010; Gustavsson et al., 2015; Knauf, 2015; Knauf et al., 2015). This information is often not available with sufficient reliability. Therefore, the indicator for carbon offsets from wood harvesting and wood use is presented in two alternatives. (1) The first alternative is based on the assumption that sufficient information is available for the timber sector. In this context, statistics 3

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Fig. 1. Emissions from timber harvesting, energy use and manufacturing processes. The figure shows the carbon (C) transfer pathways between the atmosphere, forest carbon pool and harvested wood product (HWP) pool and cascade use of wood products. Furthermore, the figure illustrates the material and energetic substitution effects of wood use.

wood use, a negative emissions balance results, which proves the Csustainability. This approach is referred to as the C-balance indicator. (2) If there is insufficient information on the use of timber, a second alternative to the indicator shall be used. Here the limited information on emissions and removals of wood use is offset against the emissions from timber harvesting. For the assessment of

on the transformation of harvested wood in different applications of material and energy use as well as corresponding life cycle analyses allow the estimation of carbon offsets with sufficient reliability (Bird, 2013; Knauf et al., 2015; Wolf et al., 2016). The indicator represents an overall view of emissions, removals and avoided emissions in the form of a C-balance. If the emissions from harvesting are lower than the removals and avoided emissions from

Fig. 2. Allocation of timber harvest quantities to uses for the assessment of carbon offsets, after Friedrich and Knauf (2016). 4

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Fig. 3. Flow-chart depicting the implementation of the two alternative indicators.

are manufactured from timber with a specific service life (=material use) or the timber is used directly or indirectly in the form of mill losses as an energy source (=energetic use). For the primary use of timber, a distinction is made between 1) sawmill industry, 2) wood-based panel industry, 3) pulp and paper industry, and 4) energetic use. The harvest quantities are assigned to these types of use according to wood assortments and tree species (Table 1). These end products are the calculation basis for the evaluation of the carbon offset effects of material and energetic timber use. For specific uses, DFs are derived from life cycle analyses. Table 2 shows material and energetic DFs for selected products.

sustainability, a displacement factor is determined, which would be necessary to offset the remaining emissions by substitution or storage effects. A comparison of the determined displacement factor with known displacement factors from the typical use of wood allows a conclusion to be drawn about C-sustainability. This approach is referred to as displacement factor (DF) indicator. The two alternative indicators (see Fig. 3) are illustrated by case studies, (i) German Federal State of Rhineland-Palatinate, and (ii) a timber harvest concession in tropics, respectively

3.2. C-balance indicator 3.3. Displacement factor (DF)-indicator The first case study shows the use of the C-balance indicator for the regional level, where comprehensive information on timber harvesting and timber is available. An example of the German Federal State of Rhineland-Palatinate, which has a forest area of 840,000 ha, was selected to present the indicator. In 2012 the average growing stock was 303 m3/ha. The most common tree species are beech (Fagus sylvatica) (21.8%), oak (Oak spp.) (20.2%), spruce (Picea abies) (19.5%) and pine (Pinus sylvestris) (9.9%). Between 2002 and 2012, the annual growth was 10.7 m3/ha, compared with an average annual harvest of 6.1 m3/ ha. Thus, 57 percent of the timber growth is utilized. Dead wood amounts to approximately 23 m3/ha (MULEWF, 2015). The quantities of timber volume extracted from the forest must be corrected with bark percentages and timber harvest losses in order to estimate the quantity of wood volume removed from the growing stock. The biomass was calculated from the volume removed from the growing stock using biomass expansion factors (Burschel et al., 1993) and converted into weight units using the specific wood densities. The carbon content was derived from the biomass weight with a factor of 0.5 tC per ton of dry biomass (IPCC, 2003). The quantities harvested are evaluated in a wood utilization model with regard to their carbon effects. For this purpose, they are allocated to product groups (material or energy use) according to their use in the wood or energy industry. With regard to the end product of wood-based production, an unambiguous classification is possible: Either products

The second case study refers to the local level and deals with a timber harvest concession in a tropical forest. Conventional logging and reduced impact logging (RIL) with subsequent energetic use of logging losses and wood processing losses are compared. Compared to timber from boreal and temperate forests, tropical timber generally has higher wood densities and thus a higher volume-related calorific value. Merbau (Intsia spp.), Shorea (Shorea spp.) or Keruing (Dipterocarpus spp.) commonly distributed in South East Asia, for example, have wood Table 1 Use of wood in the 1st sales stage in German Federal State of RhinelandPalatinate (2006–2012). The table shows the primary timber use according to wood assortment and tree species. Tree species

Larch/ pine Spruce Douglas fir Oak Beech Other hardwood

5

Timber use Sawmill industry

Wood-based panel industry

Pulp and paper

Energy

Total

65% 75% 75% 27% 20% 20%

22% 16% 16% 8% 16% 16%

9% 7% 7% 3% 6% 6%

4% 2% 2% 62% 58% 58%

100% 100% 100% 100% 100% 100%

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in CO2-TR, CO2-LL, and CO2-total, respectively. The total loss of carbon in the forest C-pool results from harvest quantities and harvest losses. Intermediate storage of carbon in dead biomass or in soil is not taken into account due to the rapid decay processes in tropical forests and the associated release of CO2 into the atmosphere (Otto, 1994). Since mill losses are occurring during timber processing, only part of the removed timber s available for the production of wood products, the C content of which is

Table 2 Displacement factor (DF) for selected uses, after Sathre and O'Connor (2010) and (Mues et al., 2017). The table shows the material and energetic DFs for the selected wood products and energetic use of wood. Wood products and energetic use of wood

Displacement factor [tC/tC]

Material use Solid wood furniture Packaging Other solid Flooring Window frame Furniture engineered wood material Wood door vs. steel door Single family house Appartment buildings Construction engineered wood material Plywood Paper Poles

1.62 1.35 1.50 1.35 to 14 1.7 to 4.6 1.46 3.00 2.3 to 15 4.4 to 7.5 1.30 1.62 0.00 0.60 to 5.8

Energetic use Pellets Fuel wood Paper Energy industry Combined heat and power unit Bark fuel wood Bark industrial energy Bark Combined heat and power unit

0.67 0.54 0.52 0.67 0.67 0.48 0.48 0.60

CHWP = CTR ∗ (1 − pLMill )

where PLMill is the proportion of CTR lost during processing. From CTR energy, ETR, can be produced either from mill losses, EMill, or from harvested wood products at the end of their lifetime, EHWP.

ETR = EMill + EHWP = {CTR ∗ pLMill ∗ pEMill ∗ CVMill} + {CTR ∗ (1 − pLMill ) ∗ pEHWP

where pEMill = proportion of mill losses, CTR*pLMill, used for energy pHWP = proportion of harvested wood products used for energy CVMill = caloric value of mill losses CVHWP = caloric value of harvested wood products The quantities of CO2 released during the energetic use of energy sources depend largely on the technologies used to generate electricity or heat (Schlamadinger and Marland, 1996a; Bird, 2013). With a water content of 20%, the calorific value of 800 kg of timber is around 13.9 GJ (Kollmann and Coté, 1968; Bayrische Landesanstalt für Wald und Forstwirtschaft, 2015; Krajnc, 2015). Thus 106 kg CO2 are emitted for the production of 1 GJ of energy from wood. If the IPCC default values (IPCC, 2006b; UBA, 2016) are taken as a basis, 101 kg CO2/GJ, 74.1 kg CO2/GJ and 56.1 kg CO2/GJ respectively are emitted during energy generation by lignite (often referred as brown coal), heating oil and natural gas. This allows the CO2 emissions of various fossil fuels, CO2_fossil, to be calculated for a given amount of energy, ETR, e.g. CO2Lignite, CO2-Oil, or CO2_Gas. Thus, the carbon offsets resulting from the energetic substitution of fossil fuels can be calculated as

nTR

nTR

∑ VTRi ∗ρi = 0.5 ∗ ∑ WTRi i=1

(1)

i=1

where nTR = number of trees removed VTR_i = removed volume of tree i in m3 ρi = density of tree i WTR_i = weight of removed volume of tree i in kg

CO2_C _offset = CO2_total − CO2_fossil

A carbon factor of 0.5 is used to calculate the carbon content from the weight, WTR, of the removed tree volume. The carbon of logging losses, CLL, is given by n

CLL = 0.5 ∗

⎛ TR − WTR _i ) + (B ⎜ ∑ TR _i ⎝ i=1

nLL j=1

(2)

⎠

DF =

where

CO2_C _offset CHWP ∗ 3.67

=

CO2_C _offset CO2_HWP

(7)

and without energetic uses as

nLL = number of trees lost through logging BTR_i = biomass weight of removed tree i BLL_i = biomass weight of tree j lost through logging damage

DF = =

(3)

Ctotal = CTR + CLL

(8)

4.1. C-balance indicator

The reference basis for the current study is a harvested cubic meter (m3) of timber with a wood density of 800 kg/m3 and a carbon content of 400 kg/m3. Thus, Eqs. (1) and (2), can be simplified to

CTR = 1m3 ∗ 800

CO2_total CO2_HWP

4. Results

The total carbon from logging, Ctotal, are

CLL = VL ∗ 800

(6)

The carbon offset, CO2_C_offset, can be used to determine the displacement factor, DF, necessary to compensate for the total emissions caused by logging. With ernergetic use the DF results as

⎞

∑ BLL_j⎟

(5)

∗ CVHWP }

densities of around 800 kg/m3. The carbon of trees removed, CTR, is given by

CTR = 0.5 ∗

(4)

kg ∗ 0.5 = 400 kg m3

The C-balance indicator measures the C-sustainability of the forest over a defined period of time, which is achieved by storing additional carbon in the forest or in wood products and by substituting other materials for material or energetic use. Table 3 shows that the annual increase of the forest C-pool of 1 million tC (4.48 tCO2/ha) is slightly lower than the storage and substitution gains from the use of wood of around 1.35 million tC (5.9 tCO2/ha). The change in the forest C-pool includes both the timber growth of the remaining stand and the amount of timber extracted by harvesting. The observed removals are due to the fact that less wood was harvested than grown during the observation

(1a)

kg ∗ 0.5 ∗ 3.67 m3

(2a) 3

where VL is the volume of harvest losses in m . CTR, CLL, and Ctotal can be converted into CO2 by multiplication with the factor 3.67 resulting 6

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Solber (2004) compared wood flooring with alternative materials such as vinyl, linoleum and two types of carpet and found a maximum DF of 14. In comparison to other DFs, these two applications represent exceptions (Scharai-Rad and Welling, 2002; Sathre and O'Connor, 2010; Werner et al., 2010). Even when considering energetic substitution DFs are necessary, which cannot be achieved with the common material use of wood. Thus, the DF-indicator shows for the case of conventional logging that the forest C-pool loss due to harvesting cannot be fully compensated by substituting alternative materials or energy sources by timber. Sustainability with regard to a C-balance is therefore not given. At reduced impact logging the assumptions are made that harvested quantities and harvest losses are the same and together amount to 2936 kg CO2. DFs are calculated under the assumptions that 40% (587 kg CO2) of the input material is lost during processing and that 80% of mill losses and wood products at the end of their life are used energetically. Substantially lower DFs are required due to the lower forest C-pool losses. To achieve carbon neutrality, substitution factors between 2.1 (lignite) and 2.6 (natural gas) are necessary, depending on the fossil fuel substituted. The ratio of necessary DFs between lignite and gas (2.6/2.1 = 1.24) is lower than the ratio of emissions from lignite and natural gas (101 kg CO2 GJ−1/56.1 kg CO2 GJ−1 = 1.8). The difference can be explained by the harvest and unrecovered mill losses that have to be compensated for with both energy sources. If no energetic substitution is assumed, the DF increases to a value of 3.3. Thus, lower harvest losses and energetic substitution have a pronounced effect on the carbon balance of harvesting and utilizing timber. For compensating the forest C-stock losses due to timber harvesting DFs are required that are likely to be achieved by the material use of timber (Table 2). A merely energetic use of harvested timber is not sufficient to compensate the losses of the forests C-pool. Whether harvesting and subsequent use of timber leads to a sustainable C-balance therefore depends essentially on the material use of timber. A sustainable Cbalance from wood harvesting and the subsequent use of wood is generally possible, but depends essentially on the material use of wood.

Table 3 Annual removals and avoided carbon emissions in the German Federal State Rhineland-Palatinate for the period 2013 to 2015. The change in forest C-pool includes regrowth in forest after selective logging and the amount of timber harvest from the forest. Removals and avoided emissions [t C year−1]

[t CO2 year−1]

[t CO2 year-1ha−1]

Forest C-pool Harvested wood products Cpool Energetic substitution Material substitution

−1,025,400 −127,200

−3,763,218 −466,824

−4.48 −0.56

−515,300 −706,800

−1,891,151 −2,593,956

−2.25 −3.09

Total

−2,374,700

−8,715,149

−10.38

period. The increase in the C-pool of HWPs (127,200 tC or 0.56 t CO2/ ha) provides the smallest contribution to the carbon balance. The combined contribution of energetic (-515,300 tC or −2.25 t CO2/ha) and material (-706,800 tC or −3.06 t CO2/ha) substitution alone exceeds the net-change of the forest C-stock. The C-balance indicator clearly shows the carbon offsets achieved by harvesting and using timber. The management of forests in Rhineland-Palatinate does not result in emissions but makes a significant contribution to removals (-2.37 Mio. tC year−1 or −10.4 tCO2 ha−1 year−1). 4.2. Displacement factor (DF)-indicator The DF-indicator is shown for two harvesting methods common in tropical forests: reduced impact logging and conventional logging (Table 4). With conventional logging, it is assumed that five times the amount of the wood used is to be incurred as harvest losses. When wood is used, processing losses of 60% are assumed which are not used for energy purposes. This compares the 8808 kg of CO2 released by logging with 587 kg of CO2 that can be used to produce HWPs. In case there is no energetic use of mill losses or wood products at the end of their life cycle, the 578 kg CO2 available for HWPs must therefore compensate for the emissions of 8808 kg CO2 from forest C-stock loss, which corresponds to a DF of 15. A DF of 15 is far above the values of DF found for common applications (Table 2). Buchanan and Levine (1999) report a DF of 15 for using wood for the construction of single family houses under ideal assumptions such as landfill being capped. Petersen and

5. Discussion and conclusions According to UNEP (2018) “the gap in 2030 between emission levels under full implementation of conditional nationally determined contributions (NDCs) and those consistent with least-cost pathways to the 2 °C target is 13 GtCO2e (and) in case of the 1.5 °C target is 29 GtCO2e”. Since forestry sector is the second largest contributor to the global GHG emissions

Table 4 CO2 balance and displacement factor for different logging scenarios: reduced impact logging with energetic use and conventional logging with and without energetic use. Reduced Impact Logging 1

[1] Timber removed from the forest , C=2-TR [2] Harvest losses1, CO2_LL [3] Total forest C-stock loss, CO2_total [4] Mill losses, CO2_TR*pLMill [5] Remaining for timber utilization, CO2_HWP [6] Energetic use from [4] and [5], ETR [7] Substitution effect for producing energy equivalent to ETR [6] from Lignite, CO2_Lignite Light fuel oil, CO2_Oil Gas, CO2_Gas [8] Carbon offsets, CO2_C_offset, with energetic substitution of Lignite Light fuel oil Gas [9] Necessary displacement factor without energetic substitution [10] Necessary displacement factor with energetic substitution of Lignite Light fuel oil Gas 1

3

Conventional logging

1 m = 1,468 kg CO2 1 m3 = 1,468 kg CO2 2,936 kg CO2 40% of 1,468 kg CO2 = 587 kg CO2 881 kg CO2 80% = 11.12 GJ

1 m3 = 1,468 kg CO2 5 m3 = 7,340 kg CO2 8,808 kg CO2 60% of 1,468 kg CO2 = 881 kg CO2 587 kg CO2 40% = 5.56 GJ

−1,123 kg CO2 −824 kg CO2 −624 kg CO2

− 562 kg CO2 − 412 kg CO2 − 312 kg CO2

1,813 kg CO2 2,112 kg CO2 2,312 kg CO2 3.3

8,246 kg CO2 8,396 kg CO2 8,496 kg CO2 15

2.1 2.4 2.6

14.0 14.3 14.5

Density: 800 kg/m3, equivalent to 1468 kg CO2/m3. 7

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products and substitution effects from wood use (Butarbutar et al., 2016). From the perspective of climate benefits, the renunciation of use is clearly the better alternative here. In sustainable managed commercial forests, different spatial and temporal system boundaries lead to apparently contradictory findings. If the emission effects of timber harvesting measures are considered in a single stand, the renunciation of use can lead to a more positive emission balance than timber use. This applies in particular if the climate impacts of wood use and the increased growth of the remaining stock are disregarded. Whether the wood use results in a negative, balanced or positive balance depends not only on the type of wood use but also on the period under consideration and the condition of the forest at its beginning. Analogous to investments, the capital growth achieved only becomes apparent over time. The view of larger forest areas or permanent forests is completely different. The aim of sustainable management can be to maintain sustained and increasing wood supply and thus carbon storage. Losses due to timber harvesting are offset by growth gains in forest stands without timber harvesting. In this situation, turnover play a decisive role. High turnover rates are the result of high utilization quantities (i.e. carbon outflows) and high increases (i.e. carbon sequestration). Carbon outflows from timber harvesting are investments in emission reduction. Indicators for sustainable forest management must not be viewed in isolation (Larrubia et al., 2017). In their entirety, indicators for sustainable forest management serve to assess inter alia the diverse ecosystem services and functions provided by forests. Forest management is often faced with conflicting decisions that require weighing different objectives and seeking compromises. A particular area of tension is the whereabouts of deadwood. If deadwood remains in the forest, it decomposes over time and releases CO2, which is negative from the point of view of climate mitigation. On the other hand, deadwood is an important element of biodiversity as it is a habitat for xytophageal organisms. In addition, the nutrients and carbon contained in the dead wood are transferred to the soil by the decomposition processes. In the long term, these processes lead to an accumulation of nutrients and C in the soil (Block and Gauer, 2012), which improves site quality, especially on soils with a low binding capacity. Site improvement promotes higher biomass production and carbon sequestration. The new indicator approach has implications for policy, practice and research. We claim that this indicator approach is responsive to forest related international and regional processes (Linser et al., 2018a; Linser et al., 2018b) as well as the emerging global forest related international processes and their reporting requirements such as Reducing Emissions from Deforestation and forest Degradation in Developing Countries (REDD+) as well as nationally appropriate mitigation actions (NDCs, Low Emission Development Strategy). By treating the carbon effects in the entire forest-wood chain, informed policy decisions can be taken to promote forest management and timber utilization for reducing GHG emissions. This applies in particular to measures to control the energetic and material use of timber. The indicator allows practice to see forest management in the light of CO2 removals and emissions. Forestry practitioners can align their management to produce assortments that allow greatest possible carbon offsets through the use of wood. Science can identify areas in material flows of wood and wood products where knowledge gaps on carbon offsets should be closed by life cycle analysis.

(Rakatama et al., 2017) that cause climate change, halting global forest loss is a top priority. The contribution of forests to mitigating climate change also extends to the utilization of harvested timber, which is often neglected in a strictly ecosystem approach within forest boundaries. Through the energetic or material use of harvested wood, on the one hand emissions from fossil energy sources are avoided and on the other hand less energy and thus emissions are consumed in comparison to producing functionally identical materials. Properly applied, the use of wood is therefore a suitable means of achieving carbon offsets. According to the Cambridge Dictionary (2019) carbon offsetting is “the process of trying to reduce the damage caused by releasing carbon dioxide into the environment by doing other things that remove carbon dioxide”. A decisive question in the assessment is whether the losses of the C-stock in forest C-pool due to timber harvesting are compensated by the carbon offsets from timber use or whether a positive net contribution remains as a climate benefit. The current systems of criteria and indicators used to assess sustainable forest management (Forest Europe, 2015; ITTO, 2016; Larrubia et al., 2017; Linser et al., 2018a; Linser and ÓHara, 2019) do not include the full potential of timber harvesting and subsequent wood use to produce carbon offsets. The revised pan-European indicator now also includes the changes in the C stock of the HWP pool and thus reflects IPCC's accounting rules, but does not take into account the energy and material substitution effects (Forest Europe, 2015). The Montreal Process C&I contains three indicators relating to the contribution of forests to the global carbon cycle: (1) total forest ecosystem carbon pools and fluxes, (2) total forest product carbon pools and fluxes, and (3) avoided fossil fuel carbon emissions by using forest biomass for energy. Although these indicators are broader than the pan-European indicators, the substitution potential of material wood use is not fully captured. We are therefore presenting a new indicator approach that compares carbon offsets from the energetic and material use of wood with emissions from the harvest. To broaden the generality of the application of the indicator two alternatives are proposed. If meaningful information on the substitution effects from wood use is available, which on the one hand requires information on the specific material and/or energetic areas of use and on the other hand requires an estimation of the substitution effects via life cycle analyses, a C-balance can be drawn up which relates changes in the C-pool of forests and wood products to the carbon offsets of wood use. If this information is not available, a displacement factor can be calculated from the harvested timber extracted from the forests and harvesting losses, which the wood use has to provide in order to compensate for the C-stock losses in forest C-pools. The resulting DFs can then be compared with known DFs to make a decision on the C-sustainability of the forest timber chain. With this new approach, it is possible for the first time to consider the sustainability of emissions and removals beyond forest borders. In a recent essay, Körner (2017) compares the longevity of carbon storage in forests with the economic analogy of turnover and capital. The carbon stock corresponds to a bank balance. Whether the capital increases does not depend primarily on the turnover, but on the amount of the debits. From this analogy it can be deduced that the growth of forests should exceed the quantities used and, as a result, the age and dimensions of the remaining stock (i.e. the capital) increase. However, economic action also knows the blessings of investments; the total capital can be increased in the medium term by cleverly used capital outflows. The fact that investments can also fail and lead to the destruction of capital belongs to the bitter wealth of experience of every economically acting human being. The interpretation of the C-indicator depends on the system under consideration and the system boundaries. Particularly in tropical natural forests, timber harvesting reduces the carbon storage of the remaining stock mainly through timber harvest losses, which can exceed the used timber volume by several times (Noack, 1995; Enters, 2001). As a rule, these losses cannot be offset by carbon storage in wood

Acknowledgements This work was partially supported by the German Ministry for Food and Agriculture (BMEL) through the project “Sustainable Forest Management Approaches to foster Forest Law Enforcement, Governance and Trade and Reduced Emissions from Deforestation and Forest Degradation Interactions (SAFARI)”, by the Ministry of Environment, Energy, Food and Forestry Rhineland-Palatinate through the project “Scenario analysis of the potential climate protection 8

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performance of the Rhineland-Palatinate Forestry and Timber Cluster through the simulation of alternative forest management measures and wood use options” and by the cluster of excellence “Climate, Climate Change and Society (CLICSS)”, University of Hamburg, Germany.

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