Forest road networks are managed to provide economic access to forest resources while minimising the environmental impacts [1
]. Stampfer et al. [2
] pointed out that the efficiency of a particular timber harvesting system is based on the existing network of forest roads. The primary forest transport infrastructure consists of all categories of forest roads, public roads that can be used in forest management and unclassified roads as well [3
]. A sufficiently dense and properly distributed network of forest roads is a basic prerequisite for intensive forest management, and therefore proper silviculture and harvesting operations, but also for fire-fighting activities [4
]. The location and operation of harvesting machinery, along with the design and construction of access roads, are important problems faced by forestry planners, making up about 55% of total production costs [5
]. The model of the lowest cumulative costs of timber extraction and building forest roads dictates that the denser the network of forest roads, the higher the costs of road construction, and the cheaper the transport of wood and vice versa [6
]. The optimal density of forest roads is determined by the road distance where the transport costs and costs related to the forest roads are the lowest. However, it should be emphasised that the accessibility of forests is not exclusively focused on timber extraction and long-distance transport of timber, but also other forest management activities.
Given the numerous challenges facing the forestry of the EU, as an area of the highly specialised niche [7
], the production of timber assortments in the wood supply chain, is one of the most critical operations during which forestry experts and practitioners must decide how and where the timber will be used. Paluš and Paroblek [8
] state that from the technical point of view, an only certain quality of timber can be used for specific purposes (veneer logs, sawlogs, cellulose and pulpwood etc.). In the past decades, climatic extremes (droughts, ice storms and similar) in Croatia had brought major problems in sustainable management and the production of high-quality timber assortments which resulted in differences between the planned and produced assortment structure of processed timber and between planned and realised production revenues. The mentioned differences go to the extent that the silvicultural operations are carried out at the cost-effectiveness limit, that is, they become unprofitable [9
]. South East Europe is faced with the highest frequency of drought events in Europe; after the year 2000, significant droughts and heatwaves were observed in 2002, 2003, 2007–2008, 2011 and 2012 [10
In Croatia felled timber volume from even-aged forests comes from: (1) the main felling which consists of: (1.1) shelterwood system regeneration, (1.2) salvage felling, (1.3) clearcut (due to the land conversion), and (2) the intermediate felling which consists of: (2.1) thinning, (2.2) intermediate salvage felling. Timber felled in shelterwood system regeneration is the volume prescribed by the existing management plan (MP) and consists of: (1) preparatory felling, (2) seedling felling and (3) final felling. Timber volume is mostly gained from thinnings and during main felling i.e., as the MP regulates, but also to a minor extent it can come from salvage felling or from a clearcut because of the land conversion. The removal is determined according to the production possibilities of the stand for each management class, by sub-compartments and tree species, and is developed by groups of assortments at the level of the management unit. The continuity of the removal is planned following the management classes of the main economic tree species. Prka and Krpan [11
] state that the quality of the stand is the result of different abiotic and biotic factors, and that the total stand volume cannot serve as the basis for planning of harvesting operations (cutting, timber extraction and transport) nor for calculating the financial inflow. In even-aged forests, felled timber volume comes from: (1) main felling which consists of shelterwood system regeneration as its main part and possible clearcut due to the land conversion, (2) thinning and (3) salvage felling. Timber felled in shelterwood system regeneration is the volume prescribed by the MP and consists of: (1) preparatory felling, (2) seedling felling and (3) final felling. Clearcut due to the land conversion applies to those areas that will be permanently be used for other purposes i.e., areas separated from the forest management area, roads, power line routes, canals, etc. Thinning volume is also prescribed by the MP. Timber volume gained from salvage felling consists of all timber that needed to be felled, but was not prescribed (expected) in the MP together with eventual illegal felling of timber. If such unplanned volume exists then the amount of prescribed (planned) thinning timber volume must be subtracted for the volume of salvage felled timber in each compartment.
The goal of this research is to show how increased salvage felling can affect the planning of the removal prescribed by the management plan. Also, indirectly it affects extraction distance which could be calculated by using factors which consider not only forest infrastructure network, but also the spatial position of the felling areas/sites and the amount of felled timber.
In most sub-compartments, the prescribed removal was fully realised through salvage felling, and in some sub-compartments, the volume of the removal was higher than the prescribed by the MP. As a result, the implementation of management regulations was realised (volume), but not for the prescribed area, which will lead to increased competition among trees and eventually dieback of weaker individuals.
Forest management is based on two paradigms: (1) perpetuity of the forest stand based on the equilibrium between standing volume and increment and removal and (2) constrained optimisation of commodities (marketable or not). The latter, output-oriented, paradigm, has favoured simplifications of forest ecosystem structure and composition [17
]. In this research, the differences between the areas on which the removal was realised and the areas on which the removal was prescribed are a consequence of salvage felling in both main felling and intermediate felling operations in sub-compartments where appropriate and expected thinning operations could revitalise broad-leafed forests if the cost balance of the management is positive [18
]. Salvage fellings in the seven-year period were realized in the area of 4588.77 ha, which is 74% of the total area of all realised felling operations (6238.35 ha). Of this area, 350.08 ha (7.6%) refers to the main felling, and 4238.69 ha (92.4%) to the intermediate felling operations.
In the seven-year period from 2004 to 2010, 65,890 m3 of timber volume was felled in the main felling operations, with an annual average of 9413 m3, which is 550 m3/year more than the prescribed average annual removal (8863 m3). By 2010, 73.4% of the prescribed 10-year timber volume (88,629 m3) had been felled. In the structure of realised main felling the shelterwood system regeneration participated with 54,249 m3 (82.3%) of timber volume, salvage felling with 4865 m3 (7.4%) of timber volume, and clearcut due to the land conversion with 6776 m3 (10.3%) of timber volume.
In the summer of 2005, which was an extremely dry year, dieback of trees occurred in almost all compartments in which the operations of the main felling were expected and prescribed. That year, 1818 m3
of timber volume from salvage felling was gained in 13 compartments, which is 37.4% of the total realised volume from the salvage felling in the monitored seven-year period. The total area (223.15 ha) on which the main felling was realised in 2005, 149.98 ha (67.2%) led to salvage felling. During 2006 in the sub-compartment 181a there was a mass drying of pedunculate oak because of the pathogenic fungus Armillaria mellea
Vahl. Ex Fr. Armillaria root disease is a contributing factor to oak decline [19
] which resulted in 1505 m3
of timber volume cut in salvage felling of 1882 m3
felled timber in that year. In 2006, 38.7% of the total salvage felling was realised in the seven-year period. During 2005 and 2006, a total amount of 3700 m3
of dead trees was cut, which is 76% of the total realised salvage felled timber volume gained during main felling operations.
Based on the MP, the prescribed volume of intermediate felling from 2004 to 2010 amounts to 84,433 m3, which is the intensity of 9.4% of the total thinning fund, i.e., 35.1% of the ten-year increment. Prescribed removal in thinnings in sub-compartments of the II age-class (area of 536.45 ha) was 11,751 m3 (22 m3/ha) and is obliged to be executed by the area while the specified prescribed timber volume serves only as an orientation. In the period from 2004 to 2010, a total of 78,168 m3 of timber volume from intermediate felling was gained, which is 92.6% of the prescribed removal (88,433 m3) from the intermediate felling. The total annual removal in intermediate felling ranged from 9219 m3 (2007) to 13,921 m3 (2008) with an average annual value of 11,167 m3. The realised average annual removal in the seven-year period was 2724 m3 higher than the average annual removal prescribed by the MP. In the seven-year period from the total amount of felled timber volume from intermediate felling (78,168 m³), 64.4% was generated through intermediate sanitary felling during July and August of the years of research and 35.6% was felled in thinnings.
During 2008 and 2009, due to the construction of the section of the highway, there was a permanent land conversion of one forest part. An area of 20.48 ha was excluded from the FO Velika Gorica and the clearcut of the stand led to 6776 m3
increase in the main felling volume. Figure 1
shows that the share of the timber volume from the clearcut in 2008 was 50.5% of the total main felling volume, and in 2009 17.6% of the main felling volume. This is just one of the situations how road network can negatively affect the forest stand by reducing the habitat available, by affecting movement patterns, and by extending the edge conditions into the forest [20
In the seven-year monitoring period there was the increase in road density from 13.16 m/ha to 14.08 m/ha i.e., the reduction of the distance between roads from 759.9 m to 710.2 m, and the average theoretical forwarding distance from 190 m to 177.6 m (Figure 5
). This happened as the result of the construction of multiple forest roads in 2006, 2007 and 2010, as well as the reduction of the MU’s area due to the construction of the highway. Meeting management and environmental goals are often complex because of the many aspects involved in forest road management, including the natural environment and socio-economic context in which the road network is located [21
The value of the forest road network factor related to the lowland forest is higher compared to previous publications from the literature [6
]. The reasons for the higher values are: (1) irregularity of the shape of the MU (4238.48 ha) with a circumference of 89.248 km, which is 3.43 times larger than the circumference of a square of the same area (26.04 km), and (2) the irregularity of the existing network of primary forest infrastructure with a length of 59,101 km, of which forest roads participate with 68.6% and public roads with 31.4%.
Even though forest roads play a dual role in providing both access to the forest and transportation mobility and, together with extraction routes normally form a relatively even network in the forest [22
] the values of geometric forwarding distances varied. As this is an area of lowland forests with level slope class and very even roughness class, the significance of geometrical extraction distances on both MU and compartment level would is great [23
]. However geometric forwarding distance was under the influence of the size of the felling areas and their spatial distribution and position to the primary forest road network.
The reasons for the deviation of the realised average geometric forwarding distances and the weighted realised forwarding distance (Figure 7
) was because: (1) the structure of felling areas by different types of felling, (2) the structure of felled timber by different types of felling, (3) the spatial distribution of felling areas by different types of felling to the primary forest road network. For two years in which the most significant deviations of accessibility factors were recorded, it can be concluded that in addition to the spatial position of felling areas to forest infrastructure, the structure of different types of felling expressed by the volume of felled timber and felling areas size showed their effect. The calculated weighted arithmetic mean forwarding distance by years of research (Figure 6
), allows the analysis of the MU’s accessibility with respect to the quantities (volume) of felled timber in space and time.
Relative openness as a quality parameter is still used in grading forest openness but with variations in buffer width [24
]. Several examples of accessibility evaluation have been developed in other studies, considering different distances in relation to the type of machines and techniques applied [25
]. GIS technology could be used to analyse the topographic, ecological and morphological characteristics of the study area and the application of precision forest harvesting may contribute significantly to the enhancement of efficient cut-to-length technology, i.e., a harvesting system in which trees are delimbed and bucked into assortments prior to subsequent transport to the landing site [26
] as in this study area. Unfortunately, in Croatia GIS is infrequently used in planning harvesting operations and is not in use when calculating average extraction distances (either forwarding or skidding) in compartments and sub-compartments. The occurrence of diebacks beyond the shelterwood regeneration, but within the regeneration period of stands of the last age-class represents a special problem in rational forest management. A large amount of tree dieback brings unfavourable effects of low felling densities and planning of time-space distribution of felling areas. Future management and development of oak forests due to oak dieback and decreasing stand structure quality and their spatial heterogeneity depend on actual age-class structure, intensity and spatial-temporal dynamics of forest regeneration [14
]. The most probable reasons for such a large share of salvage felled timber are: (1) unfavourable climatic conditions (dry summers) where climate change affects forest ecosystems and timber production [17
], (2) falling groundwater levels due to the construction of Sava—Odra canal where river regulation disrupts flood patterns of lowland forests [27
] and increases stress to ecosystems [28
], (3) low removal values in past MP’s, and (4) non-execution of the prescribed regular fellings due to their replacement with salvage fellings. The planning and execution of timber harvesting should be sustainable with emphasis of so-called 5-E standards: economic, environmental, energy-efficient, ergonomic and esthetical [29
]. Development of region-specific, practically relevant performance criteria are highly desirable that meet local needs and maintain flexibility to evolve and be capable of incorporating ever-changing work environments and challenges [30
Negative effects of the piece-volume law and low harvesting densities on forest operations are highlighted in this research due to high amount of salvage felling particularly in the intermediate felling by replacing timber volume that should have come from thinnings. The negative effect on the performance of harvesting operation and planning due to the lower value of felled timber also, should not be ignored.
Various variants of the average forwarding distance (theoretical, average geometric, average realised geometric, weighted arithmetic mean) can show forest accessibility to a certain extent depending on other terrain characteristics such as ground roughness and obstacles, which here was not the case. However, for planning harvesting operations in space and time cutblock position factor and removal position factor give much more information to forestry experts. Cutblock position factor takes into account the spatial position of the felling areas/sites, while removal position factor besides the spatial reference takes into account the amount of felled timber (i.e., volume). The analysed relative forest openness by using geo-processing workflows in GIS environment showed four types of opening areas in the studied management unit: single-opened, multiple-opened, unopened and opened areas outside MU. The model can be used in other situations and forest areas only with minimal changes in buffer width adopted to local conditions and harvesting systems i.e., skidder winch length, forest skyline lateral rope length etc. while determining the quality of designed primary infrastructure network, but also when including secondary infrastructure network for more detailed information. The use of GIS and computer software should be encouraged for use in practical forestry. Unfortunately, in Croatia improvised methods for calculating average extraction distances are still often in use.
In the future, a multi-entry system for assessing the quality of primary forest openness by relief areas should be developed, which would be based on some other quality parameters of forest accessibility (e.g., road network factor or the road structure accessible forest areas) in addition to other parameters such as road density, average forwarding/extraction distance and relative forest openness. The building of new forest roads is complex and expensive; however, in the changing climatic conditions with high amount of oak dieback it is clear that optimal planning, road alignment and design are mandatory to face challenges of the upcoming change.