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Estimates of above-ground biomass, net primary production and energy flows in 8 to 10 year old red alder (Alnus rubra Bong.) ecosystems Smith, Nicholas John


Estimates of the above-ground net primary production, standing crop biomass and above-ground autotrophic energy flows were made for eight 0.04 ha plots in naturally seeded 8 to 10 yr old red alder (Alnus rubra Bong.) ecosystems near the University of British Columbia. All plots had closed canopies and covered a range of stem densities (8,204 to 42,971 stems per ha.) and site classes (good, medium and poor). The mean overstorey biomass was 82 mt/ha (range: 40-112 mt/ha) and current overstorey net productivity was 28,601 Kg/ha/yr (range: 17,802-40,831 Kg/ha/yr). Understorey biomass and productivity were 1.5 percent and 2 percent of the overstorey values respectively. Salmonberry (Rubus spectabilis Pursh) formed 68 percent of the understorey biomass and required at least 6 percent full-sunlight before growth was significant. Litter fall averaged 4,454 Kg/ha (range: 3,019-5,713 Kg/ha) and the undecomposed forest-floor (L layer), 5,811 Kg/ha. The plot averages for the biomass accumulation ratio of 2.86 Kg/Kg, for the leaf area index (one side only) of 7.64m²/m², for the foliage assimilation efficiency of 4.9 Kg/Kg and for the standing crop density index of 1.06 Kg/m³ were all in the upper ranges for cool-temperate deciduous forests. The mean caloric content of the above-ground tree biomass was 4.53 Kcal/g ash-containing matter. There was a significant increase in caloric values with increasing height in the tree and during the fall. Net conversion efficiency of visible radiation for the growing season (April-Oct.) was 2.9 percent (range: 1.8-4.2 percent). Energy invested in biomass (biocontent) averaged 37,203 2 Kcal/m² (67 percent stem, 33 percent crown) and energy invested in net primary production (production content) averaged 13,064 Kcal/m² (54 percent stem, 46 percent crown). Seventeen percent of production content was lost to tree-litter fall. The forest floor energy turnover was 1.2 yrs. Denser stands were characterized by a narrower range of dimensional classes, a higher mortality, a greater current net primary production and foliage efficiency, a higher solar energy conversion rate but a similar cumulative biomass to the less dense stands. Density-dependent growth models are hypothesized in which denser stands achieve full site occupancy at a later stage than less dense stands. The potentials of alder as a fuel and biomass-farm crop are discussed.

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