Effects of radial growth rate on outerwood properties of coastal Douglas-fir in healthy stands versus stands impacted by Swiss needle cast

Abstract

A common belief in forestry is that rapid growth in coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) results in decreased outerwood quality. In Oregon, the opposite pattern has been reported for stands affected by Swiss needle cast, whereby a native fungus causes premature needle drop and an increase in latewood proportion (LW%), wood density, stiffness (MOE), and strength (MOR). Using a combination of new and existing data, we compared the properties of ∼25-year-old outerwood from 18 healthy and 14 diseased stands using direct tests (6–8 beams from 7–12 trees per stand, 2614 beams total) and indirect SilviScanII tests (1 sample for each of the 366 trees). As seen before, diseased stands showed a decrease in wood quality with growth rate: ring count was strongly and positively correlated with density, MOE, and MOR (r2 = 0.74, 0.65, and 0.63), and LW% was positively correlated with ring count, density, MOE, and MOR (r2 = 0.50, 0.62, 0.30, and 0.44). In contrast, healthy stands showed no significant effect of ring count on density, MOE, or MOR. LW% was weakly and significantly correlated with MOE (r2 = 0.25) but not with ring count, density, or MOR. Among healthy stands, growth acceleration had no adverse effects on outerwood properties.