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Slashburning effects on tree growth and nutrients in the Coastal Western Hemlock zone, Southern British Columbia : longer term trends Curran, Michael Patrick


To investigate regional (southern Coastal Western Hemlock biogeoclimatic zone) trends in fire effects on longer-term tree growth and nutrient levels, 52 research blocks were resampled in five subzones, from Long Beach (west coast of Vancouver Island) to Mission Tree Farm (50 km east of Vancouver). These were on 1938 to 1985 burns studied by the Canadian Forest Service (CFS), B.C. Ministry of Forests, industry, and the University of British Columbia. Five new 1985 research blocks were monitored in cooperation with MacMillan Bloedel Ltd. and CFS. Blocks studied in detail had up to 40 impact monitoring plots (IMPlots) in each block; over 900 IMPlots were sampled for tree growth and nutrient status. Overall regional (zonal) burned versus unburned trends (38 block pairs) were analyzed using ANOVA and contrasts. Different from what had been reported in the literature, a new finding is that strong regional trends are evident and the null hypothesis (no regional trends) has been rejected for Douglas-fir (Pseudotsuga menziesii) total height growth, three year height increment, foliar B, Ca, and K; and for western hemlock foliar B, Mg, and Fe. Longer-term effects, evident or suggested, are positive for Douglas-fir (the seral species on the study sites), and negative for the climax species, western hemlock (Tsuga heterophylla) and western redcedar (Thuja plicata)). Few regional soil nutrient trends existed on two research blocks (15 and 30 years postburn). Five IMPlot fire impact rating systems were developed for different determining factors, ranging from simple measures of forest floor reduction to relative estimates of mineral ash loading. The rating systems and IMPlot approach are new to this type of study, enabling the analysis of differing studies as one large, regional data set. There was a lack of significant regional trends with the rated impact levels, similar to results reported in the few literature examples. Some general trends were evident; for example, Douglas-fir growth and nutrition appear negatively related to forest floor depth. In conclusion, slashburning appears to be a useful tool in terms of longer-term Douglas fir growth and nutrient levels on the site conditions studied. For western hemlock and western redcedar, results suggest negative effects.

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