The population dynamics of four grass species in relation to burning in the KwaZulu-Natal Drakensberg

Abstract

Marked tillers of Heteropogon contortus, Trachypogon spicatus, Tristachya leucothrix and Harpochloa falx were sampled regularly over a two-year period in Highland Sourveld burnt annually in winter and biennially in spring. Parameters recorded were height of shoot apex, tiller mass, number of new lateral tillers and time of flowering. Monitoring of marked tillers before and after each burn enabled the life history of populations of tillers to be followed. In all species investigated, flowering was found to be of minimal importance with most tillers remaining vegetative until death. Shoot apices remained close to the soil surface (<20 mm) enabling them to survive frequent
burns. All species exhibited smooth survivorship curves, suggesting that dramatic fluctuations in climate and severe defoliation, as by fire, had little impact on mortality. The differential response of the four grass species was explained by their different reproductive capacities and reproductive rates. The predicted responses to fire regime were verified by changes that took place at the study area from 1981 to 2019, showing the importance of applying appropriate burning regimes to maintain the bud bank. A biennial spring burn was best for maintaining these important grass species at current levels of abundance in the KwaZulu-Natal Drakensberg.

Keywords: bud banks, Leslie matrix, mortality, survivorship