Risken för spridning av röta vid förröjning i granskog i södra Sverige

Abstract: Root rot causes large economical losses for Swedish forestry. The spread of root rot can partly be prevented by treating the spruce stumps in thinnings. Late pre-commercial thinning (PCT) is one of the forestry measures where stump treatment is not used. Late PCT can be defined as: The act of cleaning the stand with the aim of improving accessibility and sight within the stand, and so to increase the mean stem´s volume during the first conventional thinning. The aim of this study was to examine how late PCT is performed, its spatial extent and if it aids the spreading of root rot. The study was limited to encompass late PCT before first conventional thinnings in spruce stands in southern Sweden. Samples of spruce stumps were taken from four stands where late PTC had been carried out. The samples were then analysed for the existence of root rot. The stumps from the cleaned trees were divided into four classes according to diameter over bark; 5.0-6.9, 7.0-8.9, 9.0-10.9 och ≥11.0 cm. In eight other stands, sample plots were placed objectively to determine the distribution of stump sizes within a stand. At each of these plots, stumps with a diameter greater than 5 cm were counted and measured. In addition to the field measurements, a survey was distributed to 22 representatives for three forest companies in southern Sweden. The main questions asked in the survey were to which extent late PCT is done, which instructions are included, and if the risk of Heterobasidion spore infection is taken into consideration. The spruce stump samples showed a high frequency of root rot infection. In the eight stands where stump diameter distribution was measured, the number of spruce stumps per ha varied between 169 to 512 with the stumps in the smallest diameter classes being most numerous. The survey indicated that vast areas in southern Sweden yearly are treated with late PTC. Based on these results and the results from earlier studies, it can be concluded that there is a risk of spreading root rot when performing late PTC in the manner which it is presently done today.