Cross-scale patterns of structure and maximum biomass in late-seral Douglas-fir-dominated rainforests

Abstract

This study of late-seral Douglas-fir forests in coastal Washington examined scales from 0.1 to 65–80 ha to quantify persistence of the Douglas-fir overstory and patterns of aboveground biomass (henceforth biomass) distribution. Plot-based biomass estimates were scaled from 0.1–1 ha via tree-level allometry in four forests with Douglas-fir 180–600 yr old. At the 0.1 ha scale, biomass ranged from 69–2905 Mg ha⁻¹ live and 12–792 Mg ha⁻¹ dead with magnitude and spatial variability increasing with dominant tree age. Some 0.1 ha areas had > 2000 Mg ha⁻¹ live Douglas-fir alone. Our 1–3 ha plots held 1100–1700 Mg ha⁻¹ biomass with Douglas-fir always accounting for > 70 % of live biomass and > 46 % of crown volume despite having only 23 trees ha⁻¹ when trees exceeded 500 yr old. Dead biomass in plots ranged from 10–25 % of the total with 50–85 % being Douglas-fir. A segmented canopy height model from airborne laser scanning (i.e., ALS segments) was mapped to 191 measured trees to create an allometric equation for predicting live biomass. This equation was applied to all ALS segments across 288 ha of forest with dominant trees ranging from 340–620 yr old. Within plot boundaries, ALS-predicted live tree biomass errors ranged from 10–23 %. At 65–80 ha spatial scales, patterns of biomass revealed a breakup of high-biomass forest into aggregations of < 1-ha patches. The largest trees (> 50 Mg) were associated with high-biomass areas when Douglas-fir were younger, but beyond 500 yr, such elite trees increasingly occurred in relatively low-biomass areas. Estimated live tree biomass ranged from 112–1600 Mg ha⁻¹ with median values decreasing with Douglas-fir age from ∼900 Mg ha⁻¹ at 350 yr to ∼600 Mg ha⁻¹ at 600 yr. In Douglas-fir-dominated rainforests, restoration management can promote long-lived high-biomass forests and arboreal biodiversity through actions that enhance development of elite trees across all overstory age cohorts, emulating patterns of biomass distribution observed here.