Annotated Bibliography of the Global Literature on the Secondary Transportation of Raw and Comminuted Forest Products ( 2000 – 2015 )

Secondary transportation of raw and comminuted forest products is a major component in forest harvesting operations in terms of economics, public perception, and safety. Consequently, there is a substantial amount of literature on this topic. The existing literature has dealt with many of the technical aspects of transportation with a majority of them focusing on improving supply chain issues; however, there are only few specific to secondary transportation issues in general. This annotated bibliography will help practitioners, researchers, and stakeholders gain a better understanding of the existing literature from 2000 to 2015. To this end, we began by classifying the selected literature into six themes: cost, roads and routes, trucking, efficiency and safety, other modes of transportation, and supply chain and optimization. Woody biomass for bioenergy production was the most researched forest product with respect to transportation. About one-third of the articles were presented in the context of supply chain modeling and optimization. More than half of the studies originated from Europe while the United States had the most publications for any given country. Most articles (16) were published in 2013. Biomass and Bioenergy published the highest number of articles (29) during the timeframe.


Introduction
Transportation in forest operations can be broadly divided into two phases.The first phase involves moving wood from the stump to the roadside/landing sites, referred to as primary transportation.The second phase involves the hauling of the processed forest products (sawlog, pulpwood, or energy wood biomass) from the roadside/landing sites to the processing facilities, referred to as secondary transportation [1].Secondary transportation is considered to be one of the most expensive elements in the harvesting operation, generally accounting for 30-50% of the total cost, depending on the distance travelled and compared to the cost of the primary transportation [2][3][4].Therefore, improvements in secondary transportation may yield significant overall cost reductions.
Secondary transportation is predominantly road-based.Various factors influence the cost of secondary transportation including -but not limited to -the road network (road types) and conditions (infrastructure), cost of operating the truck, weight limitations, and hauling distance.
Research generally focuses on the transportation problem addressing one or some of these factors, but rarely all at the same time.It cannot be expected that one research activity can look into all of these factors because addressing each topic requires expertise in different domains.Nonetheless, having all informational aspects on transportation integrated will be of great value to stakeholders.
The purpose of this study is to address this gap, by proposing a classification of a collection of scientific literature and addressing several relevant topics in forest products secondary transportation.It provides an overview of the current state of the art, and helps in identifying knowledge gaps that require further attention.To this end, the objective of this article is to list the major findings from these studies and assess the chronological development of forest products secondary transportation research in the past 15 years.

Literature Review
The literature was searched using major online databases and library catalogs: CrossRef, Scopus, Google Scholar, and Web of Science.The initial search started in November 2016 with three Others (Journals with single publication) 1  23 Total 131 1 Publication details can be accessed from reference section.

Literature Categorization and Classification
Based on the scope and objectives of articles identified, six major research themes emerged: The classification is intended to facilitate compilation and reporting.Understandably, some of the themes overlapped.For example, there were several software and models which generated results that could be included in Theme II-Roads and route planning and VI -Supply chain and optimizations.Several articles dealt with more than one theme.Additionally, for minimizing ambiguity, no articles in one theme have been repeated in another.Theme I (Cost) primarily dealt with articles focusing on financial aspects of trucking operations.
The theme also included articles related to detailed time studies; strategies to minimize the overall transportation costs; assessing the impacts of transportation distance on final cost of delivered forest products; and evaluating the performance of transportation cost estimating software and models.
Theme II (Roads and route planning), focused on every aspect of forest roads including engineering, planning, design, construction, maintenance, spatial modeling, and computer software.Theme III (Trucking characteristics) was specific to road transportation: truck size and configuration, speed at various road conditions, weight limits, payload enhancement measures, trucking performance, and features of trailers.Theme IV (Efficiency and safety) dealt with fuel efficiency, log truck accident analysis, social surveys with related stakeholders, evaluation of fuel consumption capacity, and potential effects of forest road erosion on the supply chain.Theme V (Other modes of transportation) focused on articles dealing with railways and water transportation.Theme VI (Supply chain and optimizations) included modelling supply chain in different regions, geospatial evaluations, linear programming, strategic and tactical planning, optimization of supply chain, decision support tools for wood procurement and management, and simulation of logistics models.
Each article was evaluated for country of study, objectives researched and major findings, resulting in Table 2.

Europe 73
North America 43 Asia 8 Australia & New Zealand 3 Africa 2 South America 2

Results
Out of the 131 articles reviewed, 127 were published in peer-reviewed scientific journals, three in conference proceedings and one was a cooperative extension article.With more than 22% publications, Biomass and Bioenergy published the highest number of articles related to the field followed by Croatian Journal of Forest Engineering (Table 1).On a regional basis, about 56% of the research articles were published by authors based in Europe followed by 33% from North America (Table 2).However, the United States had the highest number of publications on a per country basis.
Several articles were authored by authors from multiple countries.
For the given period, the highest number of articles was published in 2013 (16 articles) followed by 2005 (15) (Figure 1).Hence the interest seems to be growing.On average, eight articles related to forest products transportation and supply chain were published per year from 2000 to 2015.Nearly 33% of the reviews were related to supply chain logistics and optimization (Figure 2).

Theme I -Cost
There were 19 articles in this category, accounting for approximately 15% of the total articles reviewed.An average of a little more than one article per year was published in this category.A majority of the articles (more than 80%) were based on comminuted forest products, mainly wood chips, logging and industrial residues for bioenergy and biofuel production (Table 3).The greatest number of publications (6 articles) were based in the USA, followed by Sweden, Finland and Austria.

Theme II -Roads and route planning
A total of 40 articles (about 31%) were categorized under this theme, averaging 2.5 articles published per year.Similar to the Theme I, the greatest number of publications were from the United States (12 articles) followed by Turkey, Croatia, Slovenia, Sweden, Iran, and Japan (Table 4).Articles related to GIS modeling and linear & mixed integer programming to solve forest road planning problems were included in this theme instead of Theme VI (Supply chain and optimization).
Similarly, for articles analyzing costs related to certain aspects of forest roads (i.e.construction) were included in Theme II instead of Theme I (cost).LiDAR datasets showed that the average difference in road width was 1.1 m while the slope differences of cut/fill was minimum of 4%.In addition, the difference in slope across road was only 2%.
Boston et al.As the spacing of cross-section increased, the capability of the model to capture differences in terrain decreased.Thus, the correctness of earthwork volume estimation was low.As a result, short cross-section spacing was favorable to improve accuracy in earthwork volume estimation in case of hilly and rugged terrain.The findings showed that the real GPS routes were not as same as shortest route generated by Network Analyst Tool (NAT).
Nevertheless, after manipulation of NAT the similarity increased to more than 90%.

Ghaffarian and
Sobhani [35] Iran Determining the best-fit forest road network that could minimize the total cost of road maintenance.
The data analysis in Network 2000 based on pre-existing forest road network showed that the best solution can be achieved in the cost of 27.19 €/m 3 .
Ghajar et al. [36] Iran Demonstration of a procedure that incorporated rock proportion for embankment construction in forest roads.
This approach was useful to show the variability of rock proportion and model excavation costs.
Greulich [37] (review paper) USA Analyzed the evolution of transportation network in forest harvesting Theoretical basis for transportation networks in forest harvesting was mainly developed by early European academics.From twentieth century, this theory sustained its development in America.In the last fifty years, there has been swift development in Europe and America, with Asia also putting significant contributions.
Gumus [38] Turkey Assessed the future use of forest road networks for sustainable forestry The findings showed that most of the roads in the study area was in the standard to fulfill sustainable forestry target.
However, the 20% of the roads were in worse condition.The study suggested that a road ecosystem approach that incorporates environmental gradient analysis should be used for planning and design of forest roads.

Murphy and
Stander [44] USA Developed a two-stage robust optimization model to deal with the actual transportation problems in forestry.
The results showed that the deterministic solution was unstable and dependent on some degree of uncertainty, that the robust solution was dependent on the variables used.

Theme III -Trucking characteristics
This section comprised of 14 articles which was about 11% of the total (Table 5).About 60% of the studies were related to the transportation of sawlogs and pulpwood.A total of five studies were carried out in the USA followed by four in Finland.Apart from trucking features, Theme III also included topics such as GPS tracking system, options for backhauling empty trucks, and solution for truck scheduling problems in forest operations.The overall cost to collect and haul hand-piled slash was $34 ODT -1 .Roll-off trucking system was found to be better for short hauling distances because trucking costs significantly increased with small increase in distance.

Laitila and
Väätäinen [68] Finland 2 Evaluated the truck transportation productivity for whole trees and energy woods Whole-tree harvesting, chipping, and trucking near roadside landing was the most cost-efficient technique.

Theme IV -Efficiency and safety
There were 11 articles in this category contributing to 8% of total literature, with an average of less than 1.0 article was published per year (Table 6).There were two modules in the system: Cutting Pattern Generation (CPG) and the Global Bucking Optimization (GBO).
Apart from these the biometrics like tree height, taper and volume were also integrated in the system for optimal solution.The results showed that the focus for the wood suppliers and transporters was the mill service.The top ranked service focus was "maintaining suitable stock level", while the second ranked was "delivery precision".

Theme V -Other modes of transportation
Only five articles comprised this category which was the least of all categories (Table 7).Even though this theme is associated with modes of transportation other than trucking, certain articles involved trucking as internodal or intermediate transportation.A new push barge system was compared to the available powered barge system for wood transportation.Setting with three barges' together gave the lowest transportation costs when the distance was higher than 100 km.

Theme VI -Supply chain and optimization
The supply chain logistics and optimization was the most studied topic related to forest products transportation in the given timeline.There were 42 articles in this category with an average frequency of 2.6 articles per year (Table 8).More than 70% of the studies were based on biomass, energy woods and logging residues.Highest number of studies were carried out in Canada and US with eight articles each, followed by Sweden (6 articles), Finland, and Greece (5 articles each).The generated model can be used to minimize total cost of operation including fuel wood transportation.
Asikaine The results showed that the purposed MIP approach could achieve 9% profit compared to deterministic model that uses average parameter value.The sensitivity analysis showed that the accurate inventory of standing trees and market conditions were the most important variables.The review identified opportunities to improve the agility of WPS.The suggestion from the review was to focus on higher investments in agility section to gain higher profits in wood supply chain.
[108] Russia 4 Developed a GIS-based system to support planning of shortwood transport in Russia.
The system showed an increase of 40% in the efficiency of shortwood transportation.The system could be used for numerous purposes.The overall supply chain cost decreases with reduction in distance.The regional terminals were crucial for cost reduction.In order to get optimal supply network, costs of transporting to terminals and to plants should be considered.

Discussion
The collection and classification of scientific literature on secondary forest products transportation found that more than half of the studies addressed the transportation of woody biomass from forest, and industrial residues for bioenergy generation.Generally, woody biomass generated from forest operations and forest products industries are regarded as low value products compared to the primary forest products such as sawlogs and pulpwood.This raises a question as to why the frequency of the scientific studies was higher for transportation of low-value biomass.
Woody biomass constitutes forest residues with low bulk density that are not economically feasible to transport in the raw form.In regions without a demand for biomass, forest residues are generally left at the harvesting site [5].
Much of the reasoning for this comes from the funding sources for research.Empirical evidence tells us that most research funding comes from government agencies, as opposed to direct industry input, and government policies lately have been focused more upon biomass than high values forest products.Much of this is driven by interests in atmospheric carbon issues and in reduction of hazardous forest fire fuels On a regional basis, higher number of studies were published from Europe followed by North America and Asia.This high number of articles from Europe can be attributed to the policies favoring biomass utilization.While transportation cost is considered to be one of the major limiting factors for biomass utilization, the feasibility also depends heavily on favorable policies.Country wise, USA had the highest number of publications which could be due to large area of managed timberland, and a developed economy which sustains on high level of research.
Biomass and Bioenergy journal published highest number of articles on the subject.This is related to the higher number of studies focusing on the transportation of forest residues.Forest products transportation is an important component of forest engineering, thus there were significant number of articles in journals like Croatian Journal of Forest Engineering and International Journal of Forest Engineering.
Two of the most studied research topics on forest products transportation were supply chain models and forest roads.Many site and region-specific optimization and supply chain models have been tested and presented, which increased the number of publications.Similarly, forest roads are crucial for hauling wood materials from harvesting sites to the markets.Construction and maintenance of roads requires huge amount of investment.In this study, there were only 14 articles directly related to trucking characteristics, however, most of the other articles (dealt in the study) also discussed trucking in many different ways.
Overall, the results indicated a need for more research on increasing the efficiency of transportation systems, specifically trucking.Except for one study done in Finland, there was no research focusing on the overall challenges facing the forest trucking industry as a whole [6].The cost of transportation is also another major topic that needs to be addressed in the future.

Conclusions
Regardless of categorization into different research themes, the main aim of all the collected articles in this study was to address the challenges faced by the secondary transportation of forest products.Major details on each article including research location, forest products dealt, primary objectives and key findings related to transportation were presented in tabular format.This review is expected to help researchers' for summarizing prior studies on forest transportation.It will also provide insight on the specifics that are lacking in this sector and show the way ahead for future research and innovation.This article is based on literature collection and assortment and thus should not be regarded as critical literature synthesis article.
Author Contributions: Conceptualization of this article was done by A.K and A.R.K.; methodology, data analysis, and manuscript preparation for this article was primarily done by AK; All authors contributed to editing and partially to data collection; Project Administration and funding acquisition for the study was done by A.R.K.

I.
Cost of transportation II.Roads and route planning III.Trucking characteristics IV.Efficiency and safety V.Other modes of transportation VI.Supply chain and optimization.

Figure 1 .
Figure 1.Distribution of articles related to forest products secondary transportation over the years of publication (2000 to 2015).

Figure 2 .
Figure 2. Distribution of articles in forest products secondary transportation based on research themes.
economic gain in construction of surface layer of roads by improving subgrade Results from the study showed that a 34% saving in road aggregates cost was possible with improvement in road from ACO were equal to or inferior to the MIP solution.However, ACO algorithm took less computational time than latter.digital elevation model with high resolutions.
tons of wood consumed had increased steadily from 11 in 1991 to 19 in 2003.Hall et al. systems of logging residues to an energy plant and evaluate the associated costs The cheapest system identified ranged from NZ$22 -37 ODT -1for residues from the landing and NZ$29 -42 per ODT for those collected from the cutover.
from the forest to sawmill averaged 51 km.The average share on forest roads within a route to the sawmill was 14% with an average speed of 14 km per hour.Transport cost was € 11 m -³ for average load size of 25 m³. the average fuel consumption was 0.77 L/km of diesel.The framework was based on two phases.The tactical phase was supported by software called LogiLab.In the second phase, the user evaluated this policy at the operational/execution level on combination with execution policies, using a discrete events simulation supported by Simio software.that the fuel consumption of trucks decreased by 0.5 L m -3 by the use of new trucks and trailers during study period.that by considering different environmental effects in transportation plans, an alternative road networks could be made.This can help reduce the loss of large amount of sediments.Ranta andCorpinen[86] FinlandMaximized the forest fuel supply availability to power plants The total availability of forest fuel to the CHP (combined heat and power) plants was 7 TWh at a maximum transportation distance of 100 km.
of the key findings that can increase the potential of intermodal transportation were increased cooperation, sharing of transport resources and infrastructure, joint purchases, and others.the cheapest option for transporting densified biomass feedstock from mid-west to southeast USA.Unit trains were the cheapest mode for distance over 340 km, from mid-west to the west USA.For shorter distances, trucking was the cheapest option.Challenges associated with rail transportation of forest products were short length of trip; many point of origins with limited shipping volumes; difficulty to reach destination without rail to rail interchanges; and lack of rail access.*Wood products: Sawlogs (1); Pulpwood (2); Biomass (3).Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 12 June 2018 doi:10.20944/preprints201806.0170.v1

Finland
, and the test results indicated the possibilities of cost savings over the company's current practices.The company further customized the models.The approach resulted in real cost savings for

Forsberg
problems studied used mixed integer programming models.The main objectives of the reviewed articles were to minimize biomass supply chain cost and to some extent maximize profit.projects to improve supply chain were described.The models provided better decision support.The major benefit included objective based discussions and decision ''over the borders'' between stakeholders.Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 12 June 2018 doi:10.20944/preprints201806.0170.v1Peer-reviewed version available at Forests 2018, 9, 415; doi:10.3390/f9070415 of wood chips in EU was logging residue which in future could be replaced by stumps and round wood.With the development of novel technology, countries could improve the efficiency of high variation in productivity and costs of different systems.The system that utilized the self-loading truck was proved efficient.available in the study area was split into subsections of varying sizes.This system helped in determining and analyzing the cost of harvesting and transporting of forest biomass for articles were from Biomass and Bioenergy journal, whereas the year with most publications was 2007.The primary focus was on system design for

Table 2 .
Distribution of articles related to forest products secondary transportation based on the geographic location of the study.

Table 3 .
Published scientific articles handling the cost of transportation.The cost values mentioned 171 reflects the actual value presented in the article.

Table 4 .
Published scientific articles handling various aspects related to forest roads.The recommendation/ findings are specific to the research/ region and conditions described in the article.

) Major findings and/or suggestions
Algorithm, and their hybridization with traditional solution techniques into meta-heuristic algorithms can offer opportunities for future research.Aruga et al. [27] USA Optimized road alignments using the Dijkstra shortest path method and cubic spline function Solution with Spline function was inferior compared to solution without it.Additional investigations using given functions were recommended Beck and Sessions [28] USA Determining access of woods chip trailers in forest roads utilizing Ant Colony optimization and Breakeven analysis Forest transportation network that can accommodate larger trucks could lower hauling costs.Feasibility of biomass operations were depended on road modification cost, transport volume, and transportation costs.Beck et al. [29] USA Developing a novel algorithm using Aerial LiDAR for forest road extraction Comparing geometric variables from aerial and terrestrial

12 June 2018 doi:10.20944/preprints201806.0170.v1
Peer-reviewed version available at Forests 2018, 9, 415; doi:10.3390/f9070415 More than 90% of the planned roads were in minimum negative impact zone while only about 10% of the planned roads were in maximum risk zone.The purposed criteria for future road development was 20 m/ha.construction of 758 km of new roads was planned at the national level to access some parts of the inaccessible forests.The researcher believed that the model could be used for different scenarios and for other regions in the world.Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted:
CroatiaAnalysis of the quality of existing forest road network using GISThe analysis helped the forest managers to allocate resources efficiently to specific forest areas.The overall relative openness of the selected forest areas was 81.04%.The road network efficiency coefficient obtained from the analysis was 42.37%.Preprints (www.preprints.org)|NOT PEER-REVIEWED | Posted:The study showed forest road network in Slovenia was not considered optimal.It was getting increasingly worn out, and the new transportation technologies demand certain adaptations to be made in the existing technical elements.

Table 5 .
Published scientific articles handling various aspects related to forest trucking.The recommendation/ findings are specific to the research/ region and conditions described in the article.

Table 6 .
Published scientific articles handling various aspects related to efficiency in transportation.The recommendation/ findings are specific to the research/ region and conditions described in the article.

Table 7 .
Published scientific articles handling various aspects related to other modes of primary transportation.The recommendation/ findings are specific to the research/ region and conditions described in the article.

Table 8 .
Published scientific articles handling various aspects related to forest products supply chains.The recommendation/ findings are specific to the research/ region and conditions described in the article.