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Influence of slash burning on the establishment and initial growth of seedlings of Douglas-fir, western hemlock and western redcedar : a study of the effect of simulated slash burn on soil blocks from some sites of the Coastal Western Hemlock Zone Jablánczy, Alexander


Laboratory and greenhouse experiments were carried out with controlled burning and with seedling growth correlated to soil chemical changes. The surface of soil blocks from three sites of the Coastal Western Hemlock Zone were burned at two intensities and planted separately with seeds of Douglas-fir, western hemlock, and western redcedar. Growth data were recorded periodically and dry weights of the seedling crops were obtained to determine treatment differences. The burning procedure showed the insulating and cooling effects of the vaporizing soil moisture. The burning slightly increased germination of Douglas-fir and western hemlock, generally promoted fungal population, and initiated different chemical changes in the soil on each site. Dry matter production, for all species combined, varied with treatment for each site in the following decreasing order: Swordfern site - severely burned, moderately burned, unburned control; Moss site - unburned control, moderately burned, severely burned; Salal site - moderately burned, unburned control, severely burned. Comparisons of dry matter production on control blocks with that in nature indicated that the removal of blocks from the natural environment had significantly changed the original conditions. In consequence, new artificial sites were created. Consistent evidence of the rhizosphere effect was produced on soil pH by seedlings, especially by Douglas-fir. Dormancy was successfully broken in all plants and there was evidence of different responses in photo-periodism with each species. The highest dry matter production was directly related to increased soil pH, to increased partial cation saturation, and to increased concentration of available phosphorus but to a decreased cation exchange capacity. Cation exchange capacity was inversely related to the availability of nutrients. Decrease of cation exchange capacity proved to be a beneficial effect of fire. In this experiment, where the ash was not supplied as usual in a slash burn, the increased base saturation resulted from the decreased cation exchange capacity. As compared with field samples in August 1959, total nitrogen was lower in all blocks in June 1960. Nitrogen increased in the following year in all variants of the Swordfern site and somewhat in unburned control blocks of the Moss and Salal sites. The Swordfern site benefited from burning by accelerated mineralization, which substituted for the deprived seepage. Fire caused the least damage to this habitat. The Moss site suffered heavily by burning, which reduced humus, the main source of nutrition. The unburned blocks were benefited by fast decomposition of humus in the greenhouse. The Salal site's thick raw humus benefited from moderate burn, which removed part of the humus and acted as a fertilizer on the remainder. Severe burning was most harmful on this site by the destruction of the large part of humus. Rich soils, usually with seepage water, are less damaged by fire than poor soils with strong drainage. It is mainly because in rich soils organic matter is at least partly incorporated into the mineral horizon and acts readily after fire especially for nitrogen supply.

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