Family Variation in Age Trends of Wood Density Traits in Young Coastal Douglas-Fir

Changes in ring density and its components with increasing distance from the pith (i.e., age trends) were examined in 15-year-old trees from 60 open-pollinated families of coastal Douglas-fir [Pseudo-tsuga menziesii var. menziesii (Mirb.) Franco]. Earlywood, latewood, and overall densities of each annual ring, obtained by X-ray densitometry of increment cores, were weighted by the area of the ring occupied by each trait, relative to the total stem cross-sectional area at breast height for the trait. Age trends in weighted values differed among traits but, with the exception of earlywood density, family variation was not detected. Weighted earlywood density (WED) steadily increased from pith to bark in some trees, while in other trees a plateau occurred at age 11 or later. Significant family differences were found in the proportion of trees reaching a plateau in WED by age 12. This proportion was under moderate genetic control (family h2 = 0.30) and was not genetically correlated with overall core density or stem growth at age 15. Although there are reasons to hypothesize that the plateau in WED is an indication of transition from juvenile to mature wood formation, this hypothesis needs to be verified in older trees