GENETIC-RELATIONSHIPS BETWEEN WOOD DENSITY COMPONENTS AND CAMBIAL GROWTH RHYTHM IN YOUNG COASTAL DOUGLAS-FIR

To better understand the genetic control of wood formation in coastal Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) France), and to assess the potential impact of selecting for increased wood density on adaptation of trees, genetic relationships of wood density, and its components, with cambial growth rhythm traits were examined in a 15-year-old progeny test. Timing of diameter growth during the 1987 growing season was available from an earlier study, and wood formation traits were estimated by X-ray densitometry of increment core samples. Wood formation traits were under weak genetic control (h(i)(2) < 0.20). lengths of earlywood and latewood formation were mostly determined by timing of latewood transition. Overall core density was negatively correlated with the dates of cambial growth initiation (r(A) = -0.41) and latewood transition (r(A) = -0.62), and positively correlated with the date of cambial growth cessation (r(A) = 0.40). As a result of these relationships, higher wood density was associated with a longer duration of cambial growth (r(A) = 0.67) and a slower rate of wood formation (r(A) = -0.37). All density components showed similar relationships with cambial phenology and wood formation traits. Selection for increased wood density is expected to cause only a slight extension of the cambial growth period, but it would also cause an earlier transition to latewood formation, negatively affecting growth rate.