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CTL technology (Forst-Platte-Papier, Institut fur Alpine Naturgefahren und Forstliche Ingenieurwesen, 1991). Density of forest skid roads is mainly adapted to prevailing tractor skidding, but regarding sufficient for CTL technology. In addition, there is also the ownership structure which is dominated by very small private forest properties (Medved et al. 2008, Pezdevšek Malovrh et al. 2010).
Background – Prethodna istraživanja
Despite the huge diversity of working conditions, CTL technology is in an expansion and consequently in many places effectively competes with currently dominant MM technology. We are facing a period when the new and old technologies are used close to each other – sometimes in nearby and similar working conditions. The advantages of both technologies within the various stages of production processes vary. Economic advantages of each technology are not the only reason for its choice. The process of modernization is therefore affected by many aspects of forest operation (safety, disposal of technical and human resources, human capacity building and tradition in conducting the forest operation, storm damaged forest), which have an important role in the implementation and dynamics of technological modernization.
The need for the introduction and use of CTL technologies is also expressed through trends, volume and structure of allowable cuttings. Total felling volume recorded in Slovenia in 2012, according to the Slovenian Forest Service (SFS, 2012), was 3,910,807 m3 with 2,152,467 m3 of conifers and 1,758,348 m3 deciduous trees. The average tree volume which has been cut in the last 14 years at the state level was 0.75 m3 (SE 0.09 m3) and was lower in broad-leaved trees 0.59 m3 (SE 0.04 m3) than in conifers 0.94 m3 (SE 0.15 m3). The SFS report indicates that the amount of allowable cuttings is increasing in the last period.
The recorded cuttingis significantly lower than the maximum allowed annual cut (SFS, 2012). SFS recorded the largest deficit in the realization of cutting in private forests, even though analysis including the data from permanent sample plots shows a different picture (Medved et al. 2008). There are several reasons of unrealized cuttings in private forests. The cost exceeds revenues, particularly in younger stands. The forest owner alone is independent in that respect and often very selectively, i.e. from tree to tree, decides what will be cut and what kind of technology will be used. The use of CTL technology makes speculation of the rising amount of early thinning more certain. Solving the problem of economically proven early thinning of conifer and mixed stands will therefore have an important influence on future market opportunities – not only in this country but in other countries of Central Europe as well.
Modern cutting technology has been developed in northern countries basically for conifers stands. Cutting broadleaves with CTL technology in those countries has always had small importance. The productivity of mechanized cutting together with cost evaluation in those stand and terrain conditions was often the research subject – mainly in the Scandinavian countries (Lageson 1997, Eliasson 1998, Glöde 1999, Kärhä etal. 2004, Ovaskainen 2005, Nurminen et al. 2006). Having in mind the prevailing work condition in Scandinavia it is understandable that researchers have primarily examined the impact factors like tree species, dimensions and the cutting pattern and intensity on productivity of CTL technology. These approaches have been also followed in Central European countries with some peculiarities concerning terrain slope (Hittenbeck 2013, Visser and Berkett 2015) and thinning intensity (Stampfer 2001). Many authors have focused on original studies or compilation of different studies from abroad relating to CTL operation under different terrain conditions or machines but ignored the problem of cutting different tree species (i.e.: Bültemeier et al. 1998, Neruda, Valenta, 2003).
The impact of broadleaved tree species and associated morphological characteristics of trees on the mechanized cutting productivity has been studied less.Thus, it was established in Sweden that the productivity of the mechanized cutting was the most affected by the average size of trees and number of cutting trees per hectare (Lageson 1997). They also recorded greater productivity in the final felling in comparison to the selective cutting (thinning operation).
Productivity depends on stand density and tree species, too. We have anticipated that the main reasons for different productivity in broadleaved stands are tree species morphological characteristics and wood hardness. Some authors do not distinguish working times between softwood and hardwood species in places where hardwoods are not dominant and do not exceed volume more than 0.4 m3 / tree (Pausch, 2002). There is more interest paid in studies of CTL techniques in broadleaves in southern countries where mixed stands are more frequent (Poršinsky, Krpan, 2004). In situation like this the demand for developing different models which can evaluate the impact of unmeasured combinations on calculated productivity and cost calculations are frequent and are not discussed here.
We assume that mechanized cutting in hardwood stands compared with those of conifer stands is more pretentious, the productivity is lower and the proportion of productive time is smaller compared to the work in conifer stands. The advantages of modern technologies are therefore less obvious and are shown only on the specific sites, working fields and stand conditions. The main influential factors on the productivity of mechanized cutting of hardwood stands are tree species, their dimension, cutting intensity, size and shape of crowns, thickness of branches and morphology of stem form. Tree and terrain characteristics also affect the