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Abstract

This thesis is based on analyses of permanent sample plot data gathered over periods of 10-34 years from an experiment in which a wide range of thinning regimes, and thinnings combined with N-fertilisation, were applied to 25 even-aged Norway spruce (Picea abies (L.) Karst.) stands in southern Sweden (56-63 No). At the start of the experiment, before the first thinning, the dominant height was 12-18 m. The overall objective was to evaluate the extent to which growth allocation along the bole and the stand structure of Picea abies stands can be controlled by different silvicultural regimes. To do this the data were used in four studies to evaluate the impact of: thinning and N-fertilisation on stem form and taper (Study I); different thinning regimes on the removal and growth in the diameter at breast height (DBH) of individual stems (Studies II and III); and the thinning regimes on the growth in mean DBH of four classes of the largest stems by DBH ha 1 (Study IV). The studies (ii) and (iii) form a growth model. In stands subjected to different thinning regimes, one model predicts which individual trees will remain at future points in time and an associated model predicts the future DBH of the remaining stems. Separate models were developed for stands thinned from below, stands thinned from above and unthinned stands. In Study IV the actual and genuine increases in the arithmetic mean DBH of the 100, 200, 300 and 400 largest stems by DBH ha 1 associated with six different thinning regimes in periods up to 35 years were compared to the corresponding stems in unthinned stands. The goals of achieving rapid diameter growth and low stem tapering cannot be attained simultaneously as heavy thinnings cause increased tapering, and thus silvicultural regimes must reflect a compromise between these and other production objectives. Trees in thinned and N-fertilised stands had the same taper as trees in equally thinned, unfertilised stands. Heavy thinnings from below promote high frequencies of thick stems and extra heavy thinnings promote high frequencies of extra thick stems. Thinning from above (or no thinnngs) may be an alternative to thinning from below in situations where a main crop consisting of moderately thick stems would be regarded as a satisfactory outcome. The actual mean DBH of larger stems can be increased, compared to the corresponding stems in unthinned stands, by up to 2.6 mm per year if extra heavy thinnings are carried out. The biological response to thinning of thick stems is influenced by the thinning intensity but not by the thinning method. The variation in DBH increases over time but increases more in stands thinned from above and unthinned stands than in stands thinned from below.