Stem mortality following fire in Kalahari sand vegetation: effects of frost, prior damage, and tree neighbourhoods

Fire is a key factor affecting the survivorship and dynamics of woody plants in savannas, but few empirical studies in savanna vegetation have investigated correlates of mortality following fire at the level of individual stems. A study of stem mortality as a function of size, neighbourhood effects, and prior damage (mainly caused by elephants) was undertaken in an area of Kalahari sand vegetation in western Zimbabwe. Stem and whole-plant mortality were quantified for the dominant stems of 557 trees in 4 plots following dry season unplanned fires in 2001. Two plots were located in areas that had been affected by frost earlier in the season, and 2 in areas that had not. Mortality was also recorded for 1762 trees in 20 unburned reference plots, also classified according to the presence of frost damage. Mortality estimators were constructed with a maximum likelihood regression method. Whole-plant mortality was low (on the order of 1-2%) compared with stem mortality, which in burned plots approximated 100% for the smallest size classes and declined as a function of stem diameter. Fire-driven mortality was lower in stems protected by the crowns of larger trees than in stems that were in the open. There was also evidence to suggest that the effects of fire are exacerbated by the prior action of frost and elephant herbivory.