Growth rate effects on correlations among ring width, wood density, and mean tracheid length in Norway spruce (Picea abies)

The main hypothesis tested in this paper is whether heavy thinnings affect the correlations among ring width, wood density, and mean tracheid length. Within-tree correlations were calculated between time series of yearly measurements. Among-tree correlations were computed (1) between averages over a growing period and (2) year by year. Correlations were analyzed on 20 fast-grown and 20 slow-grown Norway spruces (Picea abies (L.) Karst.) from an even-aged, plantation-grown stand near Rendeux, Belgian Ardennes. In the within-tree approach, fast-grown spruces showed a stronger negative correlation between ring width and fiber length. In among-tree approach 1, the widely held negative correlation between ring width and wood density vanished when the spruce growth rate was above 2.2 cm/year in circumference. Among-tree approach 2 demonstrated that the magnitude and sign of the correlations also depended on the year; a few years showed a significant correlation between ring width and wood density for the fast-grown Norway spruces, whereas the correlation was systematically negative and significant on many years for the slow-grown spruces. This study may explain part of the contradictory results reported in the literature concerning hypotheses similar to ours.