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Comparative study of nutrient cycling in the subalpine mountain hemlock zone of British Columbia Krumlik, Jiri George


This study was undertaken to compare the overstory above-ground biomass, net primary production, and nutrient cycle in three common types of subalpine coastal forests near Vancouver, B.C. Canada. Twelve sample plots, representing three plant associations of different moisture regime, were established along an elevation transect. The following parameters were determined on each plot: overstory litterfall biomass and its macronutrient content, overstory throughfall volume and its macronutrient content, above-ground tree biomass and its macronutrient content, tree bole wood increment, annual net primary production and its macronutrient content, mean depth of forest floor and its biomass. The quantity of macronutrients supplied in incident precipitation was measured in three forest openings in the vicinity of the sample plots. Litterfall was sampled for 24 months, while throughfall and incident precipitation were sampled during the summers of three consecutive years. Diameter increment for the last 20 years was measured on increment cores obtained from 95 randomly selected trees. Increment of tree boles was calculated from allometric volume equations and combined with data on litter production to provide the estimate of net primary production. Distribution of biomass and macronutrients in the above-ground tree layer was calculated by logarithmic equations prepared in a preliminary study. Sample plots ranged in elevation from 1250 to 1450 m. Tree cover consisted of mountain hemlock and Pacific silver fir in various proportions with some yellow-cedar at the top and some western hemlock at the bottom of the elevation transect. Mean age of trees on the sample plots ranged from 295 to 440 years. The above-ground tree standing biomass on the sample plots was 389-731 t/ha, with the largest volumes on the mesic sites. The annual net primary production was 1.77-3.35 t/ha. The biomass of overstory above-ground litterfall was 1.48-3.02 t/(ha*a); the amount of macronutrients in litterfall was 24-41 kg/(ha*a). The largest litter production was on mesic sites. There was a considerable amount of epiphytic lichens in the litterfall (71-426 kg//(ha*a)). The amount of nitrogen in incident precipitation was greater than in throughfall, indicating that the tree canopy extracted nitrogen from rainwater. More than 1 kg/ha of nitrogen was extracted from rainwater during the summer sampling period. In contrast, up to 3 kg/ha of potassium, 1 kg/ha of calcium and 10 kg/ha of sulphur were leached from the tree canopy during the 13 weeks summer sampling period. It is possible that the high value for sulphur reflects the presence of a pulpmill about 20 km southwest of the study area. The results of the study were used to test the hypothesis that differences in phytosociological characteristics occurring on a topographic sequence along relatively short elevation transects are accompanied by sufficiently large changes in patterns of ecosystem function to distinguish these sites on a functional basis. Analysis of the data supported this hypo thesis.

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