Black spruce trees from uneven-aged, old-growth stands produce more dimensionally stable wood than trees from fire-origin even-aged stands

In the eastern Canadian boreal forest, long fire return intervals lead to over 60% of stands having an irregular, uneven-aged structure, which is associated with slower growth. The aim of this study was to evaluate the effect of stand structure on the dimensional stability of black spruce wood [Picea mariana (Mill.) B.S.P.]. Sixty trees were sampled from stands with regular and irregular structures from the North Shore region of Quebec, Canada. Nonlinear mixed-effect models were developed for each stand structure to describe the variation in two indicators of dimensional stability, the differential volumetric shrinkage ratio (G_V) and the dimensional hygroexpansion coefficient (R-ratio), in small defect-free wood samples as functions of cambial age and annual ring width. G_V and R-ratio were strongly related to cambial age, but there was a limited effect of ring width. After accounting for these variables, samples from stands of irregular structure had greater dimensional stability than those from stands with a regular structure, although G_V and R-ratio differences were more pronounced in the upper stem compared with breast height. The fixed effects of the models explained between 44 and 60% of the variation in G_V, and between 7 and 44% of the variation in R-ratio. A higher presence of mild reaction wood or lower within-ring variation in wood density in trees of layer origin from irregular stands may explain the observed differences between stand structures.