Underväxtens påverkan på produktiviteten och gallringskvalitén hos två gallringsskördare

Sammanfattning: Before thinning, there may be a need for undergrowth clearing. This is because the undergrowth can create problems during the thinning operation. If an undergrowth clearing is performed before thinning, the harvester may achieve higher productivity, and the average volume per harvested stem may be a higher. However, undergrowth clearing is a costly activity, and if the undergrowth stems are few, they hardly affect the harvester’s performance. This means that if the extra cost of an undergrowth clearing is too high, it´s not economically justified to perform before thinning. The purpose of this study was to investigate how different densities of undergrowth affect the productivity and thinning quality of two thinning harvesters in SCA Skog’s spruce and pine forests in Medelpad. The study was carried out by placing sample plots in both uncleaned and cleaned parcels, and in these, the number of undergrowth stems was inventoried. Thereafter, a time study was then performed with two different thinning harvesters, and the results were then analyzed. The thinning quality was also assessed in each parcel. The results showed that productivity (m3fub/h(G0)) for harvester1 was higher than harvester2 in both the undergrowth-cleared parcels and the un-cleared parcels. Productivity increased by 3 m3fub for both harvesters in the undergrowth-cleared parcels, compared to the uncleaned parcels. However, this study was most likely affected by the observation effect (Hawthorne effect). One conclusion from this study is that harvester1 handled 93% fewer undergrowth stems per hectare than harvester2. This indicates that harvester2 used more time than harvester1 on work that did not belong to productive work. The second conclusion is that, under circumstances similar to this study, undergrowth clearing is not economically justified to perform before thinning.