The role of vegetation structure and fuel chemistry in excluding fire from forest patches in the fire-prone fynbos shrublands of South Africa

Fuel mass in fynbos was less than half that in the forest. The forest was taller and had a pronounced separation between the litter layer and tree canopies. The ratio of fuel volume to fuel-bed volume of forest species was half that of fynbos species, reflecting the sparse nature of the crowns. Foliar moisture contents of forest trees were 50-100% higher than in fynbos plants. Heat yields were marginally higher (21 860 J g-1) in fynbos than in forest species (20 703 J g-1). Crude fat contents of forest species averaged c3% of dry mass. Fat contents were more variable among the fynbos species, but some species had high (6-10%) fat contents. A modified Rothermel fire model predicted flame lengths of 0.7-4.9 m in fynbos under low to extreme fire hazard conditions, but predicted that fires would fail to burn in forest vegetation under the same conditions. The physical and chemical make-up of fynbos favours fir. It is more flammable than Chilean mattorral, but is less flammable than Californian chaparral or Australian Eucalyptus woodlands due to lower crude fat contents and higher foliar moisture contents. Although narrow forest strips may be scorched by intense fires in adjacent fynbos, it seems unlikely that extensive forest patches would burn under the conditions usually selected for prescribed burning operations. -from Authors