Fire behaviour a key factor in the fire ecology of Africangrasslands and savannas

摘要

The effect of fire on natural ecosystems involves the response of living organismsrnto the release of heat energy through the combustion of plant material. The manner in which andrnthe factors that influence the release of heat energy, involves the study of fire behaviour. In Africarnthere is a serious deficiency of knowledge concerning the behaviour of fires and this is particularlyrnapplicable to the savanna and grassland areas of the continent. Virtually no attempt hasrnbeen made to quantify the dynamics of the release of heat energy during a fire and the subsequentrnresponse of plants to it. The determination of such relationships helps explain many of thernapparently inexplicable effects of fire that are often cited in the literature. Research on the effectsrnof fire has been conducted throughout the grassland and savanna areas of Africa, particularly inrnsouthern Africa, since the early period of the 20th century. An interesting feature about thesernearly investigations and subsequent research up until 1971, was that it focused on the effects ofrnseason and frequency of burning on the forage production potential of the grass sward and the ratiornof bush to grass in savanna areas However, in 1971 a conference was convened in the UnitedrnStates of America in Florida, by the Tall Timbers Research Station on the theme of "Fire in Africa".rnThe major benefit that accrued from this conference was the realization that in Africa thernstudy of fire behaviour and its effects on the ecosystem, as described by type and intensity ofrnfire, had been largely ignored in all the fire research that had been conducted up until that time.rnAs a consequence a research program was initiated in South Africa and later extended to EastrnAfrica, to characterise the behaviour of fires burning in savanna and grassland vegetation and determinernthe effect of type and intensity of fire on the vegetation. The overall effect of type ofrnfire is that surface head fires and crown fires have the least effect on grasses and have the potentialrnto have the greatest effect on trees. This is because the heat is released above ground levelrnaway from the growing points of the grasses and closest to the growing points of the trees. Conversely,rnsurface back fires have the opposite effect on grass and tree vegetation. This is becausernthe heat is released close to ground level where the growing points of grasses are located andrnaway from the growing points of the trees in the canopy. The overall effect of fire intensity isrnthat grasses are generally not sensitive to increasing fire intensities because their growing pointsrnare located close to ground level away from the release of heat energy. Conversely trees are sensitivernto increasing fire intensities because their growing points are generally exposed to the releasernof heat energy in the canopy of the trees. These effects explain why intense fires favour therndevelopment of grassland and open savannas and vice versa. The behaviour of surface head andrnback fires is currently being compared in southern African savannas and tall grass prairies inrnNorth America and results indicate that fires burning in grass dominated communities behavernsimilarly and may therefore have similar effects on grass and tree vegetation globally.