Fire and the dynamics of allochthonous detritus in a South African mountain stream

Abstract

SUMMARY. Input of allochthonous material, standing stocks of benthic organic matter (BOM) and suspended paniculate organic matter (POM) were measured in a south‐western Cape mountain stream from March 1986 to February 1988. The surrounding fynbos‐dominated catch‐ment was subjected to a prescribed burn in March 1987. Litter‐fall in the pre‐burn year exhibited a distinct seasonal pattern, with peak falls during the early summer. Although the riparian canopy was not directly affected by the fire, in that it did not burn, a heavy, aseasonal leaf‐fall occurred shortly afterwards. The following summer, litter‐fall was less than half that of the pre‐burn summer. Standing stocks of BOM were significantly higher in autumn than in winter in the pre‐burn year and were inversely related to discharge. Despite the heavy post‐burn leaf‐fall and low litter‐fall during the post‐burn summer, there was no significant difference between pre‐ and post‐burn BOM standing stocks. Proportions and quantities of fine benthic organic matter (FBOM) in the soft BOM fraction were significantly higher in the post‐burn spring, and monthly accumulation of ultra‐fine benthic organic matter (UBOM) was also significantly higher in the post‐burn spring and summer. These results may reflect accelerated decay rates of BOM in response to enhanced post‐burn nitrate concentrations in stream water. Export of CPOM was low in comparison to FPOM and particularly to UPOM, and the stream appears to be highly retentive of CPOM. The natural resilience of the riparian vegetation minimizes the potentially disturbing effects of fire on the stream environment. As a result, the prescribed burn had a less than expected effect on both standing stocks of BOM and the stream environment in general.