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A Study of slash burning and its effect on a British Columbia forest soil Rideout , Eldon Fowler

Abstract

Slash burning has been practiced in British Columbia since 1930 with little regard for the regeneration of forests or the soils which support them. A study of burned soils on Vancouver Island was carried out in order to determine whether or not the inherent site quality of forest soils is altered through burning and whether any such alteration is permanent or exhibits a cyclic trend. For this purpose samples were taken from the surface and subsurface of sites with the same soil type, burned in different years, and were compared to similar samples from an adjacent virgin site. It was concluded that slash burning on coniferous forest soils of Vancouver Island causes: (1) Lack of natural regeneration for 10 - 15 years. (2) A gradual increase in surface soil pH due to the "pumping" action of fast-growing herbs and shrubs which causes bases to be brought to the surface from the subsoil. (3) Increased exchangeable hydrogen in the sub-soils of burned sites apparently due to action of herbaceous growth in removing bases to the surface and also to increased biological activity. (4) An initial increase in exchangeable base content of the burned surface which begins to disappear due to leaching in 3 or 4 years. (5) Increased magnesium and potassium in the sub-soils of burned sites as a result of leaching these elements from the accumulated ash. (6) Initial increase in phosphorus content of the surface burned soil with a subsequent removal of this element to the subsoil due to the solvent action of percolating rain water. This phenomenon is especially true in the case of severely burned soils. (7) Increase in ammonia content in both surface and subsurface soils after burning due to stimulated ammonification. (8) Increased nitrification and loss of nitrate by leaching with severe burning. (9) Migration of colloidal organic matter and clay particles to the subsoil. (10) Increased total nitrogen content and a consequent decrease in the carbon-nitrogen ratio of the subsoil. (11) A reduction in the moisture holding capacity of the surface soil immediately following burning. As a result of this study the following recommendations were made: (1) Due to the inherent variability of all soils it is advisable to carry out any further studies of slash burning on several sites over a period of 15 to 20 years with annual sampling and analysis of the soil from each site. Only in this way is it possible to eliminate soil variability so that the effects of burning are elucidated. (2) In conjunction with soil studies after burning, it is important to analyze the herbaceous growth invading burned sites as a means of correlating changes in soil nutrients with changes in vegetative growth. (3) Careful studies with respect to seedling survival and response on burned soil in this area are necessary to determine whether or not artificial regeneration will be economically feasible.

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