A comparison of the impacts of winter versus summer burning of slash fuel in alien-invaded fynbos areas in the Western Cape: scientific paper

Abstract

Dense slash from cleared stands of invasive alien trees in mountain catchment areas poses a hazard to fynbos recovery and soil stability, if it burns under extreme summer weather conditions. Experimental burning of alien slash fuel was done to investigate whether winter burning of fuel may be used as a practical alternative to risking a summer wild fire. Fire intensity, heat release at the soil surface and indigenous post-fire plant recruitment were compared between different blocks of the same felled alien stand, which burnt during a controlled winter burn and a summer wild fire. Both fires consumed nearly all the fuel, but fire intensity and heat penetration into the soil were much greater for the summer wild fire than the winter control burn. Fynbos plant recruitment following the main winter establishment season was extremely low in the summer burn, and indicated that propagules in the upper soil had been killed in the fire. Species that emerged following the summer burn were predominantly resprouters or species with larger seeds that could emerge from deeper in the soil. Plant recruitment, species richness and cover were much higher following the winter burn. Half the species (mainly forbs, resprouters and legumes) appeared during the summer after the fire, but the remainder appeared only after the main winter establishment season. Small-seeded species were few and indicated that all germination cues were not optimized by burning in winter. Alternatively, soil-heating during the winter burn was sufficient to kill small seeds located close to the surface. Comparisons between summer slash fires on granite versus sandstone parent materials suggest that the impacts are much greater on the granite. The impacts were also greater where fuel loads are high. It is recommended that summer burning of slash fuel be avoided in all areas supporting old (>8 yr), dense (>75% cover) alien stands. Furthermore, summer slash fires also should be avoided for moderately dense (>50%) alien stands on shale bands or granite-derived soils. As an interim strategy in these areas, winter burning of fuel should be considered to prevent excessive soil and propagule damage during a summer wild fire. However, future alternatives, such as the immediate targeting of low-density stands and the implementation of biological control agents, urgently should be sought.


Southern African Forestry Journal No.192 2001: 41-50