Is long primary growth associated with stem sinuosity in Douglas-fir?

Abstract

Stem sinuosity is a highly visible stem-form trait in the leaders of fast-growing Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) trees, yet its cause is unknown. We tested the hypotheses that sinuous stems have longer expanses of primary growth than nonsinuous stems (putting the leader at higher risk for curvature, induction of compression wood formation, and possibly overcorrection) and higher leader angle using 4- to 5-year-old saplings in raised beds. As hypothesized, sinuous stems had longer expanses of primary growth than did nonsinuous stems (13.5 vs. 12.3 cm, respectively). However, for the dates for which growth (length/day, primary growth, secondary growth, and total growth) differed significantly among sinuosity class, sinuosity class only explained 15%–21% of the variation in growth rate. There were no significant differences in leader angle for saplings of the three sinuosity classes. Contingency tables indicated some consistency in the category of sinuosity to which we assigned the stems in 2001 and 2002 (χ2 = 11.2, p < 0.004). When we used a more quantitative measure, the ratio of stem length/stem distance, there was a tendency toward a significant relationship between the two years (r = 0.272, p = 0.0893). These data suggest that, counter to expectation, the rate of stem growth was not a large factor in determining whether leaders become sinuous for this population of trees.