Open access publications in the SLU publication database

If the stems diameters at stump/cutting height (dsh) in salix plantations exceeds 5-7 cm conventional agricultural harvesters based on direct-chipping suffers from decreased efficiency. Currently in Sweden there are areas of salix plantations of such over-grown character. A cost-efficient alternative in such plantations/stands can be to use forest harvesting machines.
Therefore, the objective of this study was to measure the productivity of forest thinning machine systems, adopted for small diameter trees, in the harvesting of over-grown willow plantations. The purpose was also to compare the cost-efficiency of these systems with willow harvesting systems based on agricultural machines.
A harvester-forwarder system and a one-machine system (harwarder) were studied in salix plantations of both normal and over-grown type. The normal stand contained 36 ODt/ha and had an average and maximum dsh of 2.7 cm and 6.1 cm, respectively. The over-grown stand contained 56 ODt/ha and had an average and maximum dsh of 3.1 cm and 10.0 cm, respectively. Comparative analyses to both conventional and hypothetical agricultural machine harvesting systems were performed in normal, over-gown and more-over-grown (75 ODt/ha) stands.
The field study show that the forest harvester productivity (ODt/hour) was mostly affected by the type of stand (harvested biomass/ha) and was 56% higher in the over-grown stand when compared to the normal stand. The forwarders productivity was mainly affected by the forwarding distance. The harwarders productivity was extremely low and it was also affected by the forwarding distance, however it was 36% higher in the over-grown stand when compared to the normal stand.
In the system analyses it was found that the harvester-forwarder system gave a positive net income in all willow stands, while the harwarder system gave a negative result in all the stands. It was found that the agricultural systems, in general, were more cost-efficient in all stands compared to the forest machine systems. The agricultural harvesting system with direct chipping gave the highest net income in the normal and over-grown stand, while a hypothetical bundle-harvesting system gave the highest net income in the more-over-grown stand. The trends show that the harvesting systems with direct-chipping and a hypothetical system with whole-stem-harvesting and direct-loading decrease their cost-efficiency when the biomass removal per hectare increases; while the forest machine systems and the bundle-harvesting system increase their cost-efficiency when the biomass per hectare increases.
Currently, conventional forest machine harvesting systems are not a cost-efficient alternative compared to the conventional agricultural direct-chipping system in over-grown salix stands. However, forest machines are a robust and reliable alternative that will always function regardless of thickness of stems and amount of biomass to be harvested.