LAUSR

Abstract The efficiency of young stand management can be improved by preventing stump sprouting of unwanted hardwoods in conifer plantations allowing reducing the number of repeated pre-commercial thinning operations. A biological sprouting control using a decay fungus, Chondrostereum purpureum, is an environmentally-friendly option due to its ability to decay stumps and thus prevent sprouting when applied on stump surfaces. The aim of this study was to investigate the biological sprout control efficacies of four different clearing saw methods, a lightweight mini-harvester Tehojatka and a separately done manual treatment, and to compare their efficacies to control (cutting only). In addition, the possible effect of the hot blade of a clearing saw on the viability of C. purpureum was studied, and time and inoculum consumption were determined. The study was conducted in eight forest regeneration areas of Norway spruce (Picea abies [L.] H. Karst) with naturally grown birch (Betula pendula [Roth] and B. pubescens [Ehrh.]) seedlings. After two growing seasons, mortality was highest in the manual fungal treatment (43.3%) and lowest with the Tehojatka (16.0%) whereas in the control it was only 4.7%. The manual fungal treatment and the clearing saw methods that sprayed the inoculum directly on stump surfaces were more effective (nozzle at the side of blade, stump mortality 36.0%; nozzle at the stem of a clearing saw, 26.9%) than those methods that sprayed the inoculum on the blade (nozzle under the blade, 22.9%). The automatically functioning clearing saw (fungal inoculum is spread only when touched on the stem of a sapling) resulted in the poorest mortality rate (16.0%). The hot blade of a clearing saw slightly decreased the number of colony forming units of C. purpureum in fungal inoculum, but in practice there were still a sufficient number of colony forming units for biological sprout control. The manual fungal treatment consumed the least amount of inoculum and was thus the most accurate. However, it was also the most time-consuming option due to the double work (separate cutting of saplings and the application of inoculum), and therefore would hardly be selected for a practical sprout control. Tehojatka showed the highest consumption of inoculum, and that in the clearing saw methods was also at an unacceptable level. Thus, improvement in the accuracy will be needed to reduce the consumption before the clearing saw or mechanized fungal treatment methods can be used as commercially sustainable solutions for the biological control of hardwood sprouting.