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Initial effects of slashburning on the nutrient status of two sub-boreal spruce zone ecosystems Taylor, Stephen William


A study was carried out to investigate the effects of slashburning on the nutrient status of two Sub-Boreal Spruce zone ecosystems in the west central interior of British Columbia. The slash, forest floor and mineral soil (0-15 cm depth) in these ecosystems were sampled for mass and nutrient content before and after burning on a recently clearcut site. The forest floor and mineral soil were also sampled for nutrient concentrations nine months following burning. Average losses of organic matter, N, P, S, K, Ca, and Mg to the atmosphere due to slashburning were 11.1 kg/m² and 563, 55, 87, 60, 252, and 16 kg/ha, respectively, from the mesic ecosystems and 11.5 kg/m² and 345, 52, 74, 16, 289, and 177 kg/ha, respectively, from the subhygric/hygric ecosystems. These amounts corresponded to 51, 47, 41, 56, 40, 40 and 20%, respectively, of the total quantities of these nutrients in the slash and forest floor in the mesic ecosystems before burning, and 25, 9, 13, 14, 5, 6 and 17%, respectively, of the cjuantities in the subhygric/hygric ecosystems. There were substantial losses of organic matter from coarse (>8 cm diam.) and fine ≤ 1 cm diam.) slash and the forest floor components. However, nutrient losses were largely attributed to fine slash and forest floor consumption. Nutrient losses from the fine slash appeared to be independent of fire severity, although losses of most nutrients from the forest floor increased with fire severity; in plots in the mesic ecosystems which received moderate impact burns, and plots in the subhygric/hygric ecosystems which received low and moderate impact burns, net gains in forest floor Mg, K, and K and Ca content, respectively, were found. Nine months following burning there were significant increases in pH and total Mg concentrations and decreases in exchangeable K concentrations in the forest floor in the mesic ecosystems and S concentration in the forest floor in plots which had received low impact burns in both ecosystems. Effects of burning on some nutrient concentrations were confounded by the inherent seasonal variability in labile nutrient forms. There were no significant changes in mineral soil nutrient concentrations that could be attributed to burning. However, any such changes of small magnitude would have been difficult to detect due to the high spatial variation in soil nutrient concentrations. The survival and growth of interior spruce seedlings in the first season following outplanting was better in burned than in unburned areas in both ecosystems. However, seedling foliar N and P concentrations were lower in the burned areas. It is concluded that if slashburning is carried out in the mesic ecosystems, low to moderate severity fires should be prescribed to preserve the nutrient capital present in the forest floor. Slashburning would not substantially reduce the nutrient capital of subhygric/hygric ecosystems even with much higher fire severities than were observed in this study.

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