Relationships Between Fire Severity and Atmospheric and Leaching Nutrient Losses in British Columbia's Coastal Western Hemlock Zone Forests

Abstract

The relationships between fire severity and fire-induced nutrient losses to the atmosphere and through soil leaching were investigated using small (4m2) plots in logging slash. The study utilized (Pseudotsuga menziesii - Tsuga heterophylla - Thuja plicata) slash in southwestern British Columbia, Canada, in an area where overland flow was negligible. Twenty-two plots containing a range of slash fuel loads were burned, nutrient (N, P, S, K, Mg, and Ca) losses to the atmosphere were measured, and nutrient (N, P, K, ME, and Ca) losses in soil leachate were quantified for the first year postburn. For a given nutrient, total (atmospheric plus leachate) fire-induced losses were similar to atmospheric losses and could be reliably predicted from them due to the relatively low magnitude of leaching losses; Leaching losses were generally poorly related to atmospheric losses. Total, atmospheric, and most leaching losses increased as fire severity (defined as fuel consumption) increased. Nutrient losses were better estimated from fuel consumption variables than they were predicted from fuel load variables. As most of the results of the study were consistent with those of studies conducted elsewhere, these results likely apply to a wider geographic area and range of fire situations than those of the present study. The effort and cost of assessing total fire-induced losses in, at least, North American conifer forests can be minimized, without sacrificing much accuracy, by not measuring fire-induced soil leaching losses, but assuming these are 5-20 kg/ha, depending on the nutrient and the severity of the fire. If nutrient leaching into water bodies is to be quantified, then measurement of leaching losses would be necessary.