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The Role of Vegetation Structure and Fuel Chemistry in Excluding Fire From Forest Patches in the Fire-Prone Fynbos Shrublands of South Africa

B. W. van Wilgen, K. B. Higgins and D. U. Bellstedt
Journal of Ecology
Vol. 78, No. 1 (Mar., 1990), pp. 210-222
DOI: 10.2307/2261046
Stable URL: http://www.jstor.org/stable/2261046
Page Count: 13
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The Role of Vegetation Structure and Fuel Chemistry in Excluding Fire From Forest Patches in the Fire-Prone Fynbos Shrublands of South Africa
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Abstract

(1) South African fynbos is fire-prone, but contains non-flammable forest patches which seldom burn. Although fuel structure and foliar chemistry are often hypothesized to be important in determining flammability, the fuel properties of these vegetation communities are poorly known. (2) Biomass, the size and vertical distribution of plant parts as fuel, foliar moisture contents, heat yields, and crude fat contents were determined in the two vegetation communities, to test if differences would explain the inability of fires to penetrate forest patches. The data were also used to define fuel models and to simulate fire behaviour in the two communities, using a modification of Rothermel's fire model for multi-layered vegetation. (3) Fuel mass in fynbos was less than half that in the forest. The forest also differed from fynbos in being taller, and having 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. (4) 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 about 3% of dry mass. The fat contents were more variable among the fynbos species, but some species had high (6-10%) fat contents. (5) The modified Rothermel 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. (6) The physical and chemical make-up of fynbos favours fire. It is more flammable than Chilean mattorral, but is less flammable than California 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

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