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Structure and function of western red cedar and western hemlock forests on northern Vancouver Island Keenan, Rodney


Two forest types predominate on middle or upper-slope situations on northern Vancouver Island: an old-growth type dominated by western red cedar (Thujaplicata Donn) and western hemlock (Tsuga heterophylla (Raf.) Sarge) (the CH type), and a windstorm-derived, second-growth type dominated by western hemlock and amabilis fir (Abies amabilis (Dougl.) Forbes) that largely originated following a widespread windstorm in 1906 (the HA type). Distribution of the two types has no obvious relationship with geology, topography or mineral soil. However, seedlings regenerated following cutting on the two types exhibit large differences in growth. Seedlings on cutovers in the CH type grow slowly and have symptoms of nutrient defiency; those in the HA type grow relatively rapidly with no sign of nutrient deficiency. This difference in productivity is partly due to lower nutrient availability in the forest floors of the CH type, and to competition from the ericaceous shrub salal, that dominates the CH type following clear cutting. The objectives of this study were to describe the structure of the two forest types, and to investigate aspects of their functioning, in particular the cause(s) of differences in rates of nutrient mineralisation between the two types. It was hypothesised that nutritional differences may have been due to: (i) regular windthrow in the HA type, (ii) a greater accumulation of forest floor organic matter in the CH type, which may lead to detrimental changes in soil physical properties; or (iii) differences in foliar and woody litter quality of the dominant coniferous species, and the way that this affects rates of decomposition and nutrient mineralisation. The diameter-class structure of the CH type suggested it was a self-replacing, climax community. The diameter-distribution of western hemlock indicated continuous recruitment, while that of western red cedar suggested more periodic recruitment, at a slower rate than hemlock. The diameter class distribution of the HA type was unimodal, suggesting an even-aged stand, but a sample of tree ages indicated that many trees established some time before, or after, the 1906 windstorm. There were substantial quantities of detrital biomass in and on the forest floor in both forest types (642 Mg/ha in the CH type, and 436 Mg/ha in the HA type). This suggested that differences in nutrient availability were not due to changes in soil physical properties brought about by higher organic matter accumulation. Detrital biomass was generally greater in the CH type, but the total amount of N contained in detrital biomass was similar, because of higher N concentrations in the HA type. To investigate the wind throw disturbance hypothesis, soil properties were measured in an experiment that partly intended to simulate the effect of windthrow, by mixing organic and mineral soil horizons. The mixing treatment had no significant effect on soil nutrient availability in the HA type, and generally reduced nutrient availability in the CH type. Thus, windthrow was probably not the direct cause of higher nutrient availability and productivity in the HA type. Above-ground litter fall was lower in the CH than the HA type. Cedar resorbed higher proportion of foliar N at the time of foliage senescence (76%) than hemlock in the CH (64%), or hemlock in the HA type (51%), resulting in poorer quality of cedar foliar litter. Decomposition rate of a standard substrate, lodgepole pine needles was almost the same in the two types, suggesting that environmental conditions for decomposers are similar in the two types. Cedar foliar litter in the CH type decomposed more slowly than the foliar litter of other species in either type, and salal leaves lost mass significantly more rapidly than coniferous foliage. Using an ecosystem-level computer model, it was found that much of the difference in N mineralisation between the two forest types could be explained by differences in litter quality of the dominant species. Structural and functional differences between the two forest types can be ascribed to differences in life history, physiological, and biochemical characteristics of the dominant tree and understorey species, and way these features interact with the environment and the disturbance regime. In particular, differential responses to N availability may be an important mechanism in the successional dynamics that determine forest composition in this environment.

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