Overstorey density influence on the height of Picea abies regeneration in northern Sweden

The study aimed specifically at investigating if canopy openness was a better predictor of the height growth of Norway spruce (Picea abies (L.) Karst.) advance regeneration than overstorey basal area or overstorey standing volume. In 1990, a field experiment with 3 x 2 factorial design and two replications (blocks) was established in an uneven-aged Norway spruce forest. Plots had a net plot area of 30 x 30 m, each with a 10-m-wide treated buffer zone. Three overstorey density levels retained approximately 15, 40 and 70 per cent of the pre-harvest overstorey standing volume and were allotted to the plots. Two types of thinning that harvested smaller trees or harvested larger trees were randomly allocated to each pair of overstorey density plots. In mid-June 2000, canopy openness was estimated from hemispherical photographs taken at five marked points in the centre of each of the plots at 0.9 m from ground to the top of the 'fish-eye' camera lens. Regression results showed that canopy openness was a better predictor of height increments of spruce seedlings (0.1 < height < 0.5 m), saplings (0.5 <= height < 2.0 m), and small trees (height >= 2.0 m, diameter at 1.3 m height < 5 cm) than with overstorey basal area (m(2) ha(-1)) or overstorey standing volume (m(3) ha(-1)). The height increment of the spruce advance regeneration was not significantly correlated to stand basal area or to standing volume. Overstorey basal area in the net plots was significantly negative (P <= 0.05) with mean canopy openness estimates, and the r(2) value was 0.40. Results indicated that basal area was not linearly related to canopy openness as it increased, which might explain the lack of predictive power of retained basal area on spruce regeneration height in dense stands in boreal Sweden.