Seventy-six white spruce (Picea glauca (Moench.) Voss) trees were systematically sampled by 30, 40 and 50 cm diameter-at-breast height (DBH) classes from two natural stands in Saskatchewan located near Big River and Candle Lake. Mean ages of the tree samples were 120 and 110 years respectively. Based on sample trees, site indices at breast-height age 50 were 18.5 and 18.1 respectively. Wood basic relative density at breast height was determined for each sample tree by X-ray densitometry and mean values for each tree sample were 0.372 and 0.369 respectively. ANOVA of basic relative density on DBH class (R2= .31) and stand revealed that differences in mean density between stands were not significant. Differences in mean wood density between DBH classes (rate-of-growth) were significant (p = 0.001). Consequently, mean relative density values were determined for the 30, 40 and 50 cm DBH classes for the two tree samples combined. These were 0.387, 0.373 and 0.350 respectively compared to the species average of 0.354. Pith-to-bark density trends were inversely related to ring-width trends, consistent with expectations for white spruce.
Density trends observed in Saskatchewan coincided with those obtained from white spruce trees sampled similarly from three stands in northeastern British Columbia and three stands in north central Alberta. In the BC study, trends in breast-height wood density were reflected in similar and more significant trends in bending modulus of rupture (MOR), bending modulus of elasticity (MOE), MOE in compression, and ultimate compression strength (UCS) of small clear specimens. The combined results of the two previous studies demonstrated robustly that for stands of similar age and site index, wood density and related structural wood properties of white spruce are influenced primarily by rate-of-growth. Pooled results for the two Saskatchewan stands provide further confirmation of this wood density/growth-rate relationship.
Considered within each stand, wood density declined significantly (a = 0.05) as diameter class increased. An exception occurred at Candle Lake. In that stand, lower than expected mean wood density in the 30 cm DBH class resulted in no significant difference between it and the 40 cm class. On review, a similar lack of significant difference in mean density occurred between the 30 and 40 cm DBH classes in two previous samples, one in BC and one in Alberta, but in those stands the density hierarchy remained as expected. One plausible explanation could lie in the fact that these lower than expected density values for small diameter trees coincide with the three lowest site indices of the eight samples. This bears further investigation.
Wood density of Saskatchewan white spruce was higher than that observed in BC and Alberta with even the 50 cm class showing no significant difference from the species average. This suggests that faster growth can be pursued in Saskatchewan before encountering a detrimental reduction in average wood density. Pronounced increases in annual growth rate that occurred after cambial age twenty in the 40 cm and 50 cm trees at the Big River stand corresponded to pronounced declines in breast-height wood density. This was consistent with results observed in two of the previous six samples studied, and strengthens evidence that natural events or silvicultural interventions that result in release will reduce white spruce wood density.