Development and dominance of Douglas-fir in North American rainforests

Abstract

One of the five tallest tree species, Pseudotsuga menziesii has enormous economic and ecological importance, but rainforests dominated by this species are not as well understood as their drier montane counterparts. We climbed and measured 30 trees up to 97 m tall growing in coastal forests of the Olympic Peninsula and northern California to quantify structural attributes—leaves, bark, cambium, sapwood, heartwood, deadwood, biomass, growth increments, and age—and combined these with an equal number of trees up to 85 m tall growing in forests of the Cascade Mountains to develop allometric equations based on ground-level predictors. After comparing new equations to those previously published, we applied the best available equations for tall forests to predict aboveground quantities of all vascular plant species in 12 ha of Olympic and Cascade forests. The largest (117 Mg) and one of the oldest (615 years) trees we studied had the highest biomass increment (305 kg yr−1), but age had a negative effect on current and long-term growth increments. After accounting for variation in tree size and aboveground vigor, older trees produced less wood annually and grew less efficiently than younger trees. Size of P. menziesii trees increased more rapidly, and the proportion of biomass and leaf area in P. menziesii decreased more slowly, in Olympic than Cascade forests over six centuries following stand-replacing fire. Maximum aboveground biomass (1999 Mg ha−1) and carbon density (994 Mg ha−1) occurred in a Cascade forest with high abundance of three conifer species (P. menziesii, Tsuga heterophylla, Thuja plicata), but maximum P. menziesii biomass (1289 Mg ha−1) occurred in an Olympic forest with 50 trees ha−1 up to 90 m tall. Vulnerability to wood decay fungi and dependence on fire for stand dominance limit P. menziesii biomass accumulation in rainforests.