JOHANNS A-VALUE USED AS COMPETITION INDEX FOR DETERMINING THINNING EFFECTS

The study reports about a thinning experiment in mixed Norway spruce - Scots pine stands by applying the A-value of JOHANN (1982, 1983 and 1 990), a method for determining competitive trees within selective thin ning. The method is based on the assumption that the distance between a subject tree and his competitors can be expressed as the ratio of th eir breast height diameters weighted by the height of the subject tree and an assumed A-value, accounting for different thinning intensities . Competitors are removed when the distance to the subject tree is sho rter than the distance limit calculated. JOHANN (1982) concluded that this procedure is a precise measure for calculating the crown release within thinning experiments and in 1986 the German Association of Fore st Research Stations incorporated this measure into their official pro gramm for testing increment responses after thinning treatments. Resul ts of the study indicate that the A-value can also be used for determi ning the competition level of single trees within a stand. However an update of the A-value is suggested because previously assumed A-values are precise measure for determining the distance to the neighbouring competitors but do not represent the competition situation after crown release accurately enough. For Scots pine the crown release resulted in an increase of the basal area increment only in the treatment follo wing first 5 year growth period. For the more shade tolerant Norway sp ruce the effect of thinning was also evident in the second 5 year grow th period after thinning and increased even slightly.