Mekaniserad ungskogsbehandling för röjning och skörd :
Sammanfattning: Earlier attempts at mechanising pre-commercial thinning have been met with varying degrees of success. Most attempts have been based on techniques where residual stems are straddled. Early treatment is essential to limit damage on residual trees. One part of the present study is an evaluation of mechanised pre-commercial thinning using the new Vimek 404R. Vimek 404R is a fairly small machine that permits selective removal of stems, making it potentially suitable also for areas overdue for pre-commercial thinning. The study established the level of performance for the machine, as well as the improvement needed to make it an economically viable option. The method to do this was a time study in an area where stand data were collected prior to and after treatment. Hourly costs for the machine were estimated at 356 SEK. At present Vimek 404R cannot compete with pre-commercial thinning using brush saw, the dominant technique in Swedish forestry. There is however most likely a considerable scope for improvement, especially if pre-commercial thinning is done late, i.e., when stems are becoming thicker. Stand structure and merchantable volumes in a number of stands overdue for pre-commercial thinning were analysed, and six stands were selected for closer study. These six stands were selected among those with comparatively large biomass. Potential biomass extraction was calculated for a number of stand densities using biomass functions. Results indicate that it is not unusual for extraction levels to exceed 10 tonnes per ha, even if 2500 stems per ha are retained in the residual stand. Productivity required was specified for a Vimek 404R equipped with a harvester unit. Productivity was specified in relation to stand density, mean diameter at breast height, capital invested and annual usage. Vimek 606TT, having the same total width as Vimek 404R, was selected as the forwarder in such a harvesting system. Performance of the machine used for pre-commercial thinning was used as the basis when calculating the productivity required for a harvester version. Profitability of harvesting in young stands depends primarily on stand structure and performance of the harvesting system, as well as on chipping costs and energy prices. It was finally found that a management model with extraction of biomass for bioenergy from young stands showed promise. In such a model areas to compose future strip roads are left untreated at pre-commercial thinning, and biomass is thus allowed to accumulate in these areas until first commercial thinning/harvest is carried out.
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