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Relationships between coastal Douglas-fir stand biomass and stand characteristics Wong, Ann Yone

Abstract

The purpose of the study was to investigate how the above-ground tree biomass components (stem, branches, and foliage) and the ratios between these biomass components change with age, site quality, initial spacing, and total stand volume. The species studied was coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). To examine stands of various stand characteristics, virtual stands were generated from the Tree and Stand Simulator (TASS), a provincial growth and yield model. The stands ranged in site index (height at 50 years breast-height age) from 20 to 40 m at 5 m site index intervals, initial plant spacing of 1.0 to 5.0 m at 0.5 m spacing intervals, and from 10 to 200 years at 10-year intervals, for a total combination of 900 stands. Published tree biomass equations from the Pacific Northwest were applied to these generated stands to produce a stand-level biomass estimate for each stand. Stand-level biomass regressions were also developed from the modelled stands. Over the modelled 200-year horizon, stand-level stem and branch biomass is expected to increase, whereas foliage biomass is predicted to increase then plateau or peak then decline. At a given age, predicted biomass was higher for denser stands and richer sites. However, for a given total stand volume and initial spacing, predicted biomass was slightly higher for poor sites. At a given volume, more dense stands also had lower predicted stem and branch biomass, and higher predicted foliage biomass. In terms of the proportion ratios among components of biomass, the proportion of foliage and branch biomass in total aboveground tree biomass over time was predicted to decrease then plateau, whereas the proportion of stem biomass was predicted to increase then plateau. For a given stand volume, denser stands had proportionally less predicted above-ground tree biomass in stem biomass and proportionally more in foliage and branch biomass. From the stand-level biomass regression analysis, it was found that in addition to total stand volume, other stand variables, especially basal area, improved the predictive abilities of the stand-level biomass equations.

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