Criteria for Assessing the Sustainability of Logging Operations—A Systematic Review

The comprehensive assessment of timber and fuelwood harvesting operations through the consideration of the three pillars of sustainability: Economic, social, and environmental has not received much attention. The use of criteria can significantly improve impact assessment. Therefore, the objective of this review paper is to compile and analyze the most commonly used criteria and indicators for each dimension of sustainability in logging operations over the last 6 years. This review provides an overview of these criteria for different harvesting machines, geographical areas, slope classes, time periods, types of research, and silvicultural treatments. The environmental pillar was the most studied (46%), followed by the economic pillar (38%). Productivity was the most investigated criterion (15%). On the one hand, productivity is linked to the environmental and social pillars, as it is related to the level of greenhouse gas emissions, energy consumption, and the employment rate. However, productivity is mainly used as a criterion of financial interest, as it is most often studied in combination with costs. In addition to productivity, the other most frequently examined criteria were costs (10%), soil nutrients (9.5%), and soil compaction (9%). The social dimension was the least studied pillar (16%). This may be due to a lack of knowledge of social sustainability issues in this area. Sustainability is achieved when all three dimensions are balanced. The results of this review show an imbalance, with economic and environmental aspects being weighted more heavily than social aspects. Balancing all three dimensions typically requires an assessment of trade-offs. This review provides a comprehensive summary of the criteria that have been studied to date and can be used as a checklist and guideline for future sustainability assessments of harvesting operations.


Introduction
Hardly anything else has gained as much importance in the last few centuries as the concept of sustainability.However, the term sustainable has a long history.Its roots lie in forestry and are attributed to Johann "Hannß" Carl von Carlowitz (1645-1714) [1].Since then, in forestry, the idea of sustainability has changed from a stable forest area and sustained timber yield to a wider variety of goods and B Julian Grünberg julian.gruenberg@boku.ac.atB Martin Kühmaier martin.kuehmaier@boku.ac.atExtended author information available on the last page of the article ecosystem values demanded by society [2,3].Since the "Earth Summit" in Rio de Janeiro in 1992, the concept of Sustainable Development has been the guiding principle of International Environment and Development Policy [4].The three-pillar model (economic, social, and environmental)as an anthropocentric approach to sustainability-became known worldwide.It represents the consensus of various scientific definitions of sustainability [4].The United Nations Conference on Sustainable Development (Rio+20), held in Rio de Janeiro in 2012, defined 17 overarching goals that go beyond the three dimensions of sustainable development.Today's Sustainable Development Goals (SDGs) add peace and international cooperation to the list [5].Often used interchangeably, the concept of sustainability and the concepts of sustainable development are distinct [6].While sustainability is concerned with the long-term existence of an ecosystem in use [7], sustainable development is concerned with the societal process of achieving an equilibrium between environmental, economic, and social aspects.Therefore, sustainable development is an attempt to achieve sustainability as a long-term goal [6].At the Second Ministerial Conference on the Protection of Forests in Europe (MCPFE), held in Helsinki in 1993, this trend towards sustainable forest management (SFM) was defined and adopted at the political level [8].The United Nations Forum on Forests (2007) defines SFM "as a dynamic and evolving concept, aims to maintain and enhance the economic, social and environmental values of all types of forests, for the benefit of present and future generations" [9].An important outcome of the MCPFE was the creation of six criteria that allow measuring the achievement of SFM goals in Europe [10]: • Maintenance and appropriate enhancement of forest resources and their contribution to global carbon cycles (Criterion 1) • Maintenance of forest ecosystem health and vitality (Criterion 2) • Maintenance and encouragement of productive functions of forests (wood and non-wood) (Criterion 3) • Maintenance, conservation, and appropriate enhancement of biological diversity in forest ecosystems (Criterion 4) • Maintenance and appropriate enhancement of protective functions in forest management (notably soil and water) (Criterion 5) • Maintenance of other socio-economic functions and conditions (Criterion 6) The use of criteria and indicators (C&I) arose from the need to assess specific benefits and sustainability of forest management systems [11,12].C&I belong to a hierarchical framework and represent the levels below the principles, which are the result of a political process [12].Criteria have the function of compliance with defined principles [12] and are used to assess their distinguishing characteristics [13].The level below the criteria is represented by indicators.Their function is two-fold: (i) to provide criteria with quantitative or qualitative parameters [12] and (ii) to derive the state of a particular criterion [14].Timber harvesting is a substantial part of the SFM and has multiple impacts on biodiversity conservation, renewable energy production, climate change, and carbon sequestration [15][16][17][18].It also plays an important role in the wood-supply chain, where wood material changes location or appearance and is transformed into services and products [2].It includes the technical processes of felling, delimbing, bucking, extraction, and debarking [19].Forest harvesting is one of the most sensitive activities compared to other sectors [20].For this reason, the social pillar examines the impact of the working environment on the forest worker [21][22][23][24][25][26].The environmental pillar evaluates the high impact on the environment, such as environmental impact [27][28][29][30][31], biodiversity [32,33], and soil impacts [34][35][36][37][38].The economic pillar is important for forests managed for the primary purpose of wood production, as logging operations represent the largest cost center [39].Aspects such as performance [40][41][42][43][44] and product quality [45,46] have been studied in this context.The concept of sustainable development is rather easy to comprehend and widely accepted.The difficulties normally arise when trying to operationalize the concept, for example, by trying to measure the level of sustainability [47].Sustainability of a single pillar, such as sustainable economic growth, might not lead to the desired results of balancing all three pillars.In fact, sustainability can only occur when all three dimensions are in balance.However, there may be conflicting objectives between the dimensions.In these cases, trade-offs must be made, which may lead to a weighting of the dimensions [48].In their paper, Blagojevic et al. (2019) discuss how trade-offs between different criteria can be managed in a structured way with respect to multi-criteria decision analysis [49].Little is known about a comprehensive assessment of logging operations that consider all three dimensions.As already used in the context of SFM, C&I can have a major impact on assessing the sustainability of harvesting operations.Therefore, the objective of this article is to compile and analyze the most used criteria and indicators for each dimension of sustainability in logging operations over the last 6 years (2017 to 2022).It is intended to provide an overview of the criteria used in logging operations and how they are weighted.It will also show the extent to which they have been studied in relation to the machinery used and in different regions of the world.We expect that this review will allow us to assess which sustainability dimensions are best addressed and which need further improvement.It will identify trends and show where there is currently room for improvement in assessing the sustainability of harvesting.In addition, as part of the review process, we have created a database that makes it possible to find specific studies that address a particular criterion or indicator.

Methodology
A systematic literature review was conducted to provide an overview of sustainability criteria in forest harvesting.The literature database scopus (www.scopus.com) was used to find suitable studies in the years 2017-2022.The search criteria used are listed in Table 1.Scopus returned more than 3900 studies for the period considered (Fig. 1).The following restrictions were used for study selection: • The studies examined the impact of harvesting machinery on economic, social, or environmental aspects.• The impacts had to be related to the logging technology used.• Articles and reports published in English language were considered.
In addition to harvesting machines, other parameters that helped analyze C&I such as slope class or silvicultural treatments were extracted from the studies (Table 2).The slope classification was developed from the classifications used in the reviewed studies [50][51][52][53].The selected studies were assigned an ID and compiled in an Excel file with the respective parameters, i.e., one study represents one entry in the data set.Descriptive analysis identified the most important criteria (also called key criteria) across all studies by summarizing studies that addressed the same criteria.Due to the large number of criteria in the environmental dimension, we decided to present the most important 70% of the criteria for each parameter.This was done by calculating the share of each environmental criterion of the environmental pillar and sorted in descending order.Finally, the most frequently studied criteria were selected by accumulating the shares from the top down to 70%.In the social and economic pillar, where not many criteria were identified, the three most studied criteria were selected unless the following criterion had the same number of studies as the third criterion.

Results
A total of 423 studies were identified for this review.Onedimensional sustainability was examined in 379 studies.Four of the studies examined all three dimensions of sustainability, whereas 40 studies considered two dimensions.Economic and environmental pillars were examined together in 31 studies, while social and economic pillars were examined in eleven studies.Environmental and social pillars were investigated in two studies.

The Relevance of Sustainability Criteria
The environmental pillar was the most frequently studied (n = 182), with 114 studies examining the impact of machines on forest soils.The economic pillar was investigated in 141 studies, and the social pillar in 55 studies.According to the three pillars of sustainability, 33 criteria with their respective indicators were identified from the studies (Table 3).For the sake of clarity, we have grouped the criteria into ten categories.For example, the category termed terrain was comprised of soil rutting, soil disturbance, machine trafficability, and water erosion.The environmental pillar contained the most criteria (n = 22) and was defined by soil, biodiversity, and environmental aspects.The economic pillar was defined by the two categories for quality and performance.The social pillar included measures such as hazards and ergonomic issues and was represented by the categories exposure (e.g., vibration) and working environment.Mental workloads, such as burnout, gaze fixation duration, repetitive workload, and job satisfaction rate, were grouped together under the working conditions criterion.
As shown in Table 3, the category performance had the highest number of reported studies (n = 174), followed by soil physics (n = 112) and environmental impact (n = 85).The number of studies is not necessarily the sum of their criteria since several studies examined multiple criteria.It is often the case that a study examines several criteria at the same time.The most studied criterion was productivity (n = 159), followed by costs (n = 103), soil nutrients (n = 98), and soil compaction (n = 97).It should be noted that the values of the individual indicators were not relevant for this study.However, they will be important for practical application.

Harvesting Machines and Their Common Sustainability Criteria
Three machine types for felling and processing and seven types for extraction were examined.In some studies, more than one machine was examined at the same time.The most commonly used machine type was the forwarder (n = 144), which was closely followed by the skidder (n = 141).The most researched felling and processing machine was the harvester (n = 120), followed by the chainsaw (n = 114).Animal/manual (n = 21), tractor (trailer) extraction (n = 24), and helicopter logging (n = 3) were not evaluated further due to their relatively low frequency.The environmental pillar was most often studied in combination with the skidder (n = 95), followed by the forwarder (n = 90) and the harvester (n = 61) (Fig. 2).The economic pillar was studied most for the harvester (n = 61), followed by the forwarder (n = 59) and the chainsaw (n = 48).The social pillar had the highest number of studies for harvesting operations with the chainsaw (n = 41), followed by skidder (n = 18), whereas the harvester and the forwarder shared the same count of 16 studies each.Comparing the share of pillars among various machine types, the skidder had the highest share in the environmental pillar followed by forwarder and tractor.The economic pillar had the highest share with the cable yarder, followed by harvester and feller buncher.The social pillar had a clear peak with chainsaw accounting for a third of the data collected.

Felling and Processing Machines
With a small lead over the chainsaw, the harvester was the most studied felling and processing machine.The harvester is an articulated harvesting machine, designed to fell, delimb, and buck trees into products [54].The most common harvester is the wheeled harvester which is limited to operate on slopes <40 %.Rugged terrain limits its range.Tracked harvesters can work on slopes up to 55%, whereas winchassisted harvesters can be operated on slopes up to 80% [55].The harvester is mainly used in cut-to-length (CTL) operations [55].Compared to the chainsaw, the harvester has higher productivity [56,57], fewer work injuries and ergonomic problems, and improved occupational safety [58].However, harvester operation has shortcomings in terms of environmental impact (emissions and soil compaction) [59].The environmental pillar was the most studied (Fig. 2), of which soil nutrients had the highest share (9.9%1 ) followed by soil compaction (9.6%), soil moisture content (6.8%), and emissions (6.1%) (Table 4).However, the most studied criterion was productivity (19.1%), which belonged to the economic pillar, followed by costs (12.0%).The social pillar was the least studied, of which working conditions was the most studied criterion with a total share of 3.1%.The chainsaw is a machine with a wide range of applications (felling, delimbing, bucking).It is used on flat to very steep terrains.Its application is not limited by soil conditions.Especially in steep and rough terrain, there is little alternative to this method of felling [55].Compared to fully mechanized harvesting operations, the chainsaw has a lower productivity [56,57] and causes a higher rate of tree damage, but on the other hand produces less emissions and higher full-time equivalents2 [59].In combination with helicopter extraction, there is hardly any soil compaction.In a system with a cable yarder, soil disturbance is more likely in the form of loosening of the top-soil as compared to soil compaction [59].Most of the studies were carried out on the economic and environmen-tal pillars (Fig. 2), of which damage to stand had the highest share across all machines and criteria (24.4%), followed by soil nutrients (23.3%) and emissions (21.2%) (Table 4).In the economic pillar, productivity was the most studied criterion (18.1%).Physical workload (5.9%) followed by exposure to vibration (5.1%) and hazards (4.6%) accounted for the largest share of the social pillar compared to all harvesting machines.The feller buncher is a harvesting machine that is designed to fell trees, collect more than one trunk (if size permits), and place them on the ground [54].
The "drive-to-tree" feller buncher is a rubber-tired machine and should be restricted to a maximum slope of 25%.On tracked machines, the felling head is mounted on a swinging boom.Combined with a self-leveling cab, they can operate on slopes up to 50% [55].Most of the criteria were examined on the environmental pillar (Fig. 2), which is dominated by soil nutrients and soil compaction (10.5% each) (Table 4).The most studied criterion was productivity (16.1%).Exposure to noise was the dominant criterion in the social pillar with a total percentage of 2.8%.Productivity was the most studied criterion (mean = 41 studies per machine) for all three types of felling and processing machines, followed by costs (mean = 28 studies).Hazards and physical workload dominated the social pillar (mean = 7 studies per machine, respectively).It is noteworthy that the order of the criteria in the economic pillar was the same for the three felling and processing machines: productivity was the most common, followed by costs and energy consumption.The criteria of fiber loss and stem breakage were seldomly reported.

Extraction Machines
Forwarders are articulated extraction machines with usually six or eight wheels.They are designed to transport processed logs by carrying them on a log bunk [54,55].The operating characteristics in relation to the slope are the same as harvester.The forwarder is generally used in combination with harvesters, which are capable of producing CTL assortments [55].The forwarder was the most studied extraction machine (n = 144).The environmental dimension was studied more frequently as compared to the economical and social dimension (Fig. 2).The highest overall percentage was productivity (13.7%), followed by soil compaction (12.1%) and costs (10.5%) (Table 5).The dominant criterion in the social pillar was exposure to vibration (1.8%).
The skidder is an extraction machine that drags the felled trees by cable, grapple, or both to the side of the road or to a landing [54,55].They can be wheeled or tracked when large loads need to be hauled up steep slopes.Depending on the direction of skidding (preferably downhill), these machines can typically operate on slopes of up to 45% (wheeled) or 50% (tracked) [55].The skidder was researched in 141 studies.Compared to the other harvesting machines, the envi-  [1] Bulk density; [2] penetration resistance; [3] share of damaged trees and wound healing rate; [4] shear resistance; [5] C/N ratio, soil organic matter, total C/N, and nutrients content; [6] seedling quality index; [7] Shannon index, evenness index, Ellenberg index, and Simpson index; [8] soil biological quality index; [9] musculoskeletal disorders and repetitive strain injuries; [10] thermal overload (Wet-Bulb Globe Temperature Index); [11] full-time equivalent; [12] eyeball fixation duration ronmental pillar had the largest share for the skidder (Fig. 2).Dominant criteria on all pillars were soil compaction (12.0%) followed by soil nutrients (10.8%) (Table 5).In the economic pillar, productivity was the most studied criterion (8.5%).However, the skidder was the only extraction machine where productivity was not the most studied criterion.Work conditions had the highest share of the social criteria (1.5%).
The dominant criterion in the social pillar was exposure to vibration (2.0%).Tractors are often used for small-scale harvesting operations.They can be either equipped with a winch or a grapple and used as skidders.Downhill extraction with a tractor is possible on slopes up to about 25% [60].They can also be equipped with an attached cable winch to extract wood [61].The environmental pillar was the most studied followed by the economic pillar (Fig. 2).The criterion with the highest overall percentage was productivity (17.0%) followed by costs (10.2%) (Table 5).Soil compaction (9.5%) and soil nutrients (7.5%) had the highest share in the environmental pillar.Physical workload and hazards dominated the social pillar.Cable yarding is described as the transportation of hanging or partially hanging trees by means of a cable system [54].The basic components of a cable yarding system are the yarder, the carriage, and the operating crew [55].Cable yarders are used on steep slopes where ground-based timber extraction does not allow for safe or environmentally sound operations.Inadequate ground conditions for skidding or forwarding are another typical reason for using a cable yarder [55].The cable yarder is the only logging machine where economy is highest in all three dimensions (Fig. 2).The productivity of the cable yarder had the highest share (21.7%), followed by costs (18.5%) (Table 5).The environmental pillar was dominated by soil nutrients (8.7%) and emissions (7.8%).Hazards were the most researched criterion in the social pillar (2.6%).As with felling and processing machinery, productivity was the most studied criterion for four types of extraction machinery examined (average = 34 studies per criterion), followed by soil compaction (average = 28).The social pillar was examined with an average of at least three studies per criterion.In the social pillar, physical workload and hazards dominated with an average of four studies each.
In the economic pillar, the order of the criteria was the same for each extraction machine: productivity with the most studies, followed by costs and energy consumption.

Geographical Regions
Based on the results, studies were conducted in Africa, East and South-East Asia, Europe, Latin America, the Middle East, North America, Oceania, and Russia (Fig. 3).Africa (n = 5 studies, 1.2%) is represented by South Africa and Ghana.Most of the machines used in Africa were chainsaws and forwarders (Fig. 4).Productivity was studied with 27.3%, followed by stand damage with 18.2% (Table 6).For East and South-East Asia, 36 studies (8.5%) investigated the impact of harvesting machinery.Among others, 15 studies Soil porosity 2.40 0.00 3.50 [1] The percentage refers to the sum of all criteria from the respective machine were conducted in Indonesia and six in South Korea.The chainsaw was the most studied machine (n = 19), followed by the skidder (n = 10) and the tractor (n = 9) (Fig. 4).In East and South-East Asia, the environmental criteria were the most commonly assessed at 45.2%.The largest deviation from the global trend was in the environmental impact category, which was characterized by stand damage (9.7%) (Table 6).Harvesting costs was also weighted more heavily at 15.1%.Soil compaction, on the other hand, received less attention (5.4%).The social dimension accounted for 11.9%.Exposure was not considered in these studies.However, working conditions (4.3%) and hazards (3.2%) were above the global trend.Half of the countries considered (n = 44) belong to the European region, where 193 (46%) studies were conducted.Representation of the main EU countries, 39 studies were conducted in Italy, followed by Poland (n = 24) and Germany (n = 17).In most studies, a forwarder was used (n = 80 studies), followed by a harvester (n = 65) and a chainsaw (n = 53) (Fig. 4).The percentage distribution of the examined criteria in Europe was relatively similar to that of the global distribution (Table 6).
The most noticeable differences were linked with energy Fig. 3 Relevance of global regions (number of studies per region) modified according to the regions sectioning of the World Bank and the SDG framework of the United Nations Fig. 4  consumption and soil biodiversity, where the percentages were higher in Europe.Europe was also more likely than the rest of the world to examine the social pillar criteria.However, the share of the physical workload was lower.
European studies showed significantly lower percentages for water erosion, cation exchange capacity, and hydraulic soil conductivity.The Latin American studies (n = 75 studies, 18%) were mainly conducted in Brazil (n = 68).Harvester and skidder were the most studied machines (30 studies each), followed by forwarder with 27 studies (Fig. 4).The environmental pillar was the most studied for Latin America (44%), which is similar to the global share.However, it had the highest share of emissions of all regions (5.3%).In turn, the other environmental criteria were less strongly represented.The economic criteria, other than costs and stem breakage, were below the global share.Within the social pillar, physical workload, exposure to vibration, and noise were strongly represented.Middle East (n = 53 studies, 13 %) is represented by Iran (n = 39) and Turkey (n = 14).Thirtyseven of the 67 machines surveyed were skidders, followed by twelve chainsaws and nine tractors (skidding/winching) (Fig. 4).The environmental dimension accounted for 65% of the criteria studied-with 49% related to soil physical and chemical properties.Compared to Europe or North America, the Middle East had higher percentages for soil compaction, soil moisture, soil porosity, water erosion, soil pH, soil cover (humus, litter), and root regeneration (Table 6).
Performance and social criteria received comparatively little or no attention.North American studies (n = 45 studies, 11%) were mainly conducted in the USA (n = 35) and ten in Canada.Priority was given to the skidder (n = 26), followed by the feller buncher (n = 23) and the forwarder (n = 18) (Fig. 4).The representation of sustainability pillars was highest for environmental (54%), followed by economic (40%), with only three studies considering social aspects.More than one-third of the studies conducted in North America (37%) included soil physics and chemistry in the environmental pillar, which is higher than the global share (28%).In the economic pillar, 90% of the studies were focused on productivity, which is again roughly in line with the global average of 88%.Eight studies (2%) were conducted in Oceania, seven of which were conducted in Australia.The most commonly studied machines were the harvester, forwarder, and skidder (n = 4 each).The studies considered performance aspects such as productivity (33.3%) and costs (27.8%).Results were well above the global average.It is worth noting that the wood quality criteria were examined at 5.6% each.This is higher than the global comparison.Very few environmental and social criteria were examined.Russia contributed to 3% of the total reported studies (n = 11).Ground-based harvesting equipment such as forwarders, skidders, tractors, and harvesters were prioritized (Fig. 4).Nearly a quarter of the studies focused on the trafficability of the machine (22.7%).Soil compaction ranked second with 18.2%, followed by soil rutting (13.6%) and productivity (9.1%).

Slope Class
The number of studies decreased with increasing slope where 53% of the studies were conducted on flat terrain and only 6% on very steep terrain.In all slope classes, productivity was the most studied criterion, ranging from 14% in flat terrain to 32% in very steep terrain (Table 7).In steep and very steep terrain, costs was the second most studied criterion (18.2, 20%).Energy consumption was also evaluated on all slopes, with the highest total percentage in very steep terrain (10%).In the environmental pillar, soil nutrients and soil compaction were studied most frequently, except on very steep terrain, where emissions had the highest share (6%).Soil compaction was studied most often on flat and moderate slopes, while it was second in the steeper slope classes.The percentage share decreased with an increasing slope (very steep = 4%).In the social pillar on steep and very steep terrain, the hazards criterion was predominant (1.4,4%).In flat and moderate terrain, exposure to vibration and noise and physical workload were considered in the range of 1.0%.

Silvicultural Treatment
Five different silvicultural treatments were used in the studies: clear-cutting (Cc), thinning (Th), selection cutting (Sc), shelterwood cutting, and salvage logging.Clear-cut was the most studied harvesting system (n = 98 studies) followed by thinning (n = 90), and 56 group or individual selection cuts were made in uneven-aged stands.The silvicultural treatments of shelterwood cutting and salvage logging were not further considered due to their low representation.Most of the environmental criteria were investigated in clear-cutting (Table 8), of which soil nutrients had the highest share (10.7%).Soil compaction was the most studied criterion in selective cuttings (12.8%), while stand damage was most studied in thinnings (11.8%).Productivity and costs were significantly higher for thinning (22.2, 16.5%) compared to clear-cutting or selection cutting.Working conditions was investigated in all harvesting treatments, but proportionally most often in thinnings (2.4%).

Time Spans
Most studies analyzed the immediate effects of logging, within a period of less than 1 year (n = 384, 88%).The fewest were medium-term studies (n = 33), looking at a period of time from 2 to 5 years.Long-term studies had a share of 8% (n = 33) and a time period of 6-30 years.Economic criteria such as productivity and costs were more often examined in studies with a short time period (S) (Table 9).Most social criteria were examined in short-term studies: the highest proportion had exposure to vibration (2.8%), followed by physical workload (2.7%).Medium-term (M) and longterm (L) studies mainly investigated environmental criteria concerning soil aspects (M, L), root regeneration (L), and biodiversity aspects (M).

Research Types
Of the 423 included studies, 346 (82%) were case studies in which harvesting was observed in the field.Theoretical studies (6%) involved mathematical modeling without direct use of harvesting machinery.LCA studies (4%) examined the environmental impact of a harvesting system or an entire wood chain.Some experimental studies (3%) investigated harvesting processes under laboratory conditions, e.g., in workshops.Literature reviews (2%) and meta-analyses (<1%) were also included.Interviews (<1%) dealt exclusively with ergonomic and occupational safety issues.In the case studies, productivity was the most studied criterion with a total share of 15.4%.The dominant criteria in the environmental pillar were soil compaction (10.7%) and soil nutrients (9.6%).Physical workload and exposure to vibration (2.40% each) were the most studied criteria in the social pillar.In the theoretical studies, productivity and hazards were the most frequently investigated with 16.3%, while machine trafficability was the second most frequently investigated with a total of 14%.Physical workload and working conditions had the highest share (4.7% each) in the social pillar.

Discussion
This paper provides an overview of the criteria used to assess the sustainability of logging operations.The criteria of the social pillar have been less studied than those of the environmental and economic pillars.A more complex understanding of social sustainability may be one reason.The social sustainability aspects of forest operations may also be less emphasized by the wood industry.There may also be a lack of knowledge about social sustainability issues among those involved in logging operations.Social criteria were most often examined for the chainsaw.This may be due to the fact that the motor-manual work is more dangerous and causes more accidents and more exposure to various hazards [62].Mechanized systems tend to have lower accident rates, cause less damage to the remaining stand, and result in higher efficiency than semi or non-mechanized systems [63].Approximately 70% of the world's annual roundwood is harvested using fully mechanized harvesting systems [64,65].This proportion is expected to increase.Its potential, particularly in developing regions such as Latin America, South-East Asia, and the Middle East, has yet to be reached.Newer technologies such as bogie tracks and traction winches allow expanding operations on steeper slopes [66].This may explain why skidders, forwarders, and harvesters were the most commonly studied machines to test technological innovations.However, the proportion of studies investigating fully mechanized systems (harvester, feller buncher, forwarder) is lower (40%).Thirty-eight percent of all studies focused on the productivity of harvesting machines.On the one hand, productivity is linked to the environmental and social pillars, as it relates to the level of greenhouse gas emissions, energy consumption, and the employment rate [67,68].For example, less damage to the residual stand could be achieved by working more carefully at lower productivity.However, productivity is mainly used as a criterion of financial interest, as it is most often studied in combination with costs.Motor-manual harvesting systems have the lowest efficiency [63], which may explain why productivity is most studied in systems where the chainsaw is used for felling and processing.The studies that addressed the emissions criterion were mainly related to harvesting systems using a chainsaw and skidder or tractor.This may explain the relatively high number of studies dealing with chainsaw emissions.The cable yarder had the highest relative share of productivity.However, the economic criteria could have been studied even more often than the environmental criteria, as the cable yarder has little impact on the soil criteria.
The skidder is the only extraction machine where productivity has not been studied the most.This is because the skidder has the highest proportion of research in the Middle East, where its impact on the soil has been a prioritized.
Looking at the distribution of criteria across slope classes, soil criteria are more relevant in trafficable areas due to the machine-soil interaction as compared to steep areas where cable yarders are used.Few studies have been carried out in very steep terrain (6%).However, they have mainly focused on efficiency and economic criteria.This can be explained by the challenging accessibility of difficult terrain.Harvesting systems with manual and demanding tasks are mainly used in steep to very steep terrain, where hazards and physical workload become the main concerns.In this context, a higher proportion of studies in the working environment category would be expected compared to studies describing machine-soil interaction.Thinning is the most expensive and challenging treatment in terms of impact on residual trees.This may explain the higher scores for productivity, cost, and stand damage.Because stand density is very high and stem diameters are small, thinning must be done very carefully to minimize damage to the remaining trees.This results in lower productivity as compared to clear cut operations.
It is important to carefully balance the benefits and costs of thinning to ensure that it is done in a sustainable and economically viable manner.The use of costly fully mechanized harvesting systems should be proven [69].We assume that more heavy machinery was used for clear-cutting and selection cutting, and therefore, more emphasis was placed on soil criteria.Although not considered in this review, salvage logging may become more important in the coming years, especially in the EU, due to the high proportion of climate change-related forest disturbances.Productivity, cost, energy consumption, soil compaction, and stand damage are the criteria where the effects can be observed immediately and are therefore conducted in short-term studies.The social criteria, exposure to vibration, and physical workload can only be measured when the machine is running to analyze the effects of machine-worker interaction.For the soil criteria, there are some studies that analyze the effects immediately after harvesting, but more importantly, how long it takes for the soil to recover.The same is true, to a lesser extent, for the soil microbial biomass and root regeneration.

How Sustainable Timber and Fuelwood Harvesting Differs by Geographic Region
Technology adaptation to locally diverse and complex conditions and timber and fuelwood harvesting practices varies widely around the world.In this way, technological innovations are also created in response to a wide range of different requirements [70].The situation in East and South-East Asia with respect to sustainable forest management is diverse and complex [71,72].Japan and South Korea have longstanding efforts to promote sustainable forest management [73,74], while Indonesia and Malaysia have a history of unsustainable logging practices that have led to widespread deforestation and degradation of forest ecosystems [75,76].Despite regulations and policies related to forest management and certification schemes, sustainability in forest management remains a challenge [77,78].The framework commonly used in this region is called reduced-impact logging (RIL) and is considered a step towards sustainable forest management.RIL is a harvesting technique (selection cutting) that aims to minimize the negative impact of logging operations on the forest environment [79], while still meeting the economic needs of the forest industry [80].However, implementation varies from country to country.For example, Indonesia and Malaysia have adopted RIL extensively by integrating it into government policies and regulations [81].The most common method used in the region is the tree-length (TL) or whole-tree (WT) method, where trees are felled (TL and WT) and processed (only TL) with chainsaws and extracted using various systems.The relatively high share of tractors and especially skidders can be explained by the fact that many countries are currently experiencing labor shortages, leading to greater reliance on mechanization [82].In countries with high labor costs (e.g., Japan), fully mechanized CTL systems are commonly used [83].The implementation of RIL could be a reason for the high proportion of the environmental dimension studied.In addition, this region has unique characteristics.The high rainfall and humid conditions can lead to soil sensitivity and environmental stress.Looking at the situation in Europe, mechanized harvesting systems have been the norm for several decades, especially in Northern Europe (Sweden, Finland, Norway) [84].In some Eastern European countries, particularly those with the most affluent popula-tions, an increase of the CTL method can be observed [85].This may explain the higher number of studies investigating the harvester and forwarder.Combined with the predominant use of the chainsaw in small-scale forestry and the use of the chainsaws in steep terrain, e.g., in the Alpine region, a high proportion of studies dealing with chainsaws can be explained.Europe has the highest share in the global number of studies.This explains why the percentage deviations from the global distribution of criteria are small.The forestry sector in Latin America is well represented by extensive plantations (Pinus spp.and Eucalyptus spp.).As in East and South-East Asia, RIL is used in natural tropical forests, where chainsaws are most commonly used to fell and process trees.
Extraction is usually done with skidders.Until recently, the skidders used in RIL operations in Brazil, Peru, and Bolivia were showing their age.This could explain the higher share of the fuel and energy consumption criterion.In 2021, 45% of Brazil's wood came from CTL operations [65], which is roughly in line with the study count for Latin American CTL systems in this review (39%).However, changes in average stand age and an increase in harvesting on steeper slopes could result in changes in the percentage distribution of harvesting systems.Due to pressure on land-use for agriculture and livestock, there is currently a trend in Latin America to move forest plantations to hillside locations.In addition, smaller logs are being used to reduce the rotation time of forest plantations.At the same time, new technologies are being used to produce wood products such as glued laminated timber and cross laminated timber.The differences observed in the environmental and economic pillars may be related to the high number of harvesting operations in forest plantations (68%).This requires information on their economic efficiency [86,87] as well as their impact on the soil (soil compaction) in order to adapt silvicultural treatments against soil compaction (soil conditioning) [88,89] for future forest conversions or forest stand reforms.The aspects of the social pillar are related to the strict certifications and labor laws that require compliance with health and safety regulations.
In the Hyrcanian forests of Iran and Turkey (Middle East), the combination of large tree diameters, steep terrain, unevenaged stands, and lack of initial investment to purchase fully mechanized harvesting systems has resulted in motor-manual tree felling being the only common system.Extraction is done with wheeled and tracked skidders, which cause the most environmental problems-namely soil compaction and degradation of forest soils, surface runoff, soil erosion, and the risk of flooding.In recent years, mechanized harvesters with advanced CTL technologies have been used only by a few private forestry companies [90].Cable yarding techniques are not yet present in the Hyrcanian forest area, but to some extent in the mountainous regions of Turkey.Until 2010, the focus in the Middle East was on economic aspects such as productivity and costs.Subsequently, according to the sixth "Five-Year Development Plan" (FYDP), the contribution of environmental criteria to the evaluation of forestry activities should be expanded.However, attention to environmental criteria in logging operations, especially in Iran, has not resulted in minimizing environmental impacts.This is due to land-use changes, forest grazing, pest and disease invasions likely triggered by climate change, and increasing public concern [91].The result has been a ban on timber harvesting in the Hyrcanian forests of northern Iran.
Only windthrown areas and damaged trees are allowed to be harvested [91].Hydrological problems caused by logging operations have received little attention in watershed studies because the water resources in the rivers are not very large compared to other regions.However, in recent years, the study of runoff and water erosion at plot and watershed levels has been developed [92][93][94].A positive trend is an increase of studies on work conditions and physical workload [95][96][97][98].Despite being only represented by two countries (Canada and the USA), North America offers broad topographic and climatic conditions that, in addition to entrepreneurial traditions and market demands, contribute to the use of multiple harvesting systems.Even within relatively short distances (≤ 150 km), considerable differences in harvesting systems can be detected within federal boundaries.This is particularly the case for eastern Canada, where about 70% of the wood harvested in the province of QuÃ©bec is with harvesters and forwarders as compared to only about 25% in the adjacent State of Maine.The main harvesting system used in Oceania is the harvester/forwarder (CTL).In New Zealand, manual felling is still used, particularly in steep terrain, with cable or cable-assisted machines.In Australia, manual felling and processing have been limited due to concerns about workplace accidents.In some areas, such as the Green Triangle (South Australia) and Tasmania, the industry is concerned about the amount of residue left on sites after harvesting.Where the soil is rich, whole tree harvesting is more commonly used than CTL due to lower operating costs.Its roadside residues are then recovered for bioenergy purposes [99].This may explain the skidder operations.Economic criteria such as productivity, cost, and value recovery are highly desired by foresters and logging contractors, which is the main reason why these criteria are receiving more attention in decision-making.

Study Limitations
Harvesting systems are usually a combination of different felling, processing, and extraction machines.When a criterion was studied for a specific machine, it was automatically applied to all machines used in the harvesting operation.For example, if a chainsaw was used in combination with a skidder, the impact on the ground was studied only for the skidder, but for our analysis, the criterion was also applied to the chainsaw.This approach was taken because it was not always easy to link a particular impact to a respective machine.To maintain consistency, the criteria studied in a publication were assigned to all the machines studied in that publication.As a result, the number of studies that address the actual environmental impact of chainsaws is slightly less than the 90 studies assigned.Criteria may vary by region because specific working groups may have chosen to focus on certain criteria.This is particularly the case in regions where only a small number of research institutions are involved in forest technology research.For example, in the Middle East, working groups have focused on soil criteria (51% of the studies) [92][93][94].In Russia, a working group focused on the topic of machine trafficability (46% of the studies) [100][101][102].The decision as to whether a study falls within the scope of the research question depends, among other things, on the individual judgment of the people involved.Therefore, it was very important that all researchers involved in data collection were instructed to use the same methodology.This was ensured by cross-checking the identified papers by all involved.As mentioned in the methods section, one of the criteria for selecting studies for this review was that they had to be published in English.This may have reduced the number of studies from Asia, Europe, Latin America, the Middle East, and Russia.Due to the choice of a medium-term review period, the results may differ from a long-term trend.

Conclusion
Sustainability is achieved when all three dimensions are balanced.Our study highlights the need for a holistic and interdisciplinary approach to assessing the sustainability of timber and fuelwood harvesting practices.In this review, we compiled criteria and indicators used in recent literature that address the economic, social, and environmental impacts of logging operations.We found 33 criteria in the literature that quantify these impacts.Our results show an imbalance where economic and environmental aspects are weighted more heavily than social aspects, at least when considering the frequency of published material.Trade-offs have to be accepted when trying to balance all three dimensions.This review provides a comprehensive summary of the criteria examined within a 6-year period.Building on the findings of this paper, the criteria that have the greatest impact on sustainability issues need to be identified to ensure effective evaluation of a harvesting system with the least negative impact.Coordination of the parameters studied will be of great importance: which machines to use in which slope class and in which silvicultural method.Establishing thresholds for individual indicators to classify and label criteria will be one of the most important priorities in conducting sustainability assessments of timber and fuelwood harvesting practices.For example, the effects of different harvesting and tillage practices on soil organic carbon dynamics (CO 2 , CH 4 , and δ 13 C-CO 2 fluxes-soil respiration) were not often studied during the last 6 years.This criterion may become increasingly important as climate change progresses.Setting a threshold of 80 mg m -2 h -1 (efflux) would limit the negative impact on climate change caused by harvesting operations.The results could be of use in planning future researches and practices in various regions of the globe to assess the sustainability of forest operations.Care needs to be taken that preferences made by local communities, industries, and governments may vary highly among different regions which may result in different levels of sustainability in forest harvesting operations.

Fig. 1
Fig. 1 Screening Process of Literature Searching

Fig. 2
Fig. 2 Number of studies and percentage of pillars per machine type

Table 1
Search criteria used in scopus

Table 2
Key parameter of the studies typeLive cycle assessment (LCA), Meta-analysis, review, theoretical

Table 6
Key criteria in different geographical regions (AF Africa, ESA