(1) To study the population-level effects of fire on the savanna grass Anaropogon semiberbis, size-classified matrix population models were constructed for an annually burnt population and for a population protected from fire. These models were used
to examine the effects of fire on population growth rate, stable size distributions and reproductive value, and to simulate different fire regimes. (2) The burnt population is capable of increasing rapidly (I.-I = 1·2524, r = 0·2251), whereas the unburnt population is unable to persist (I.-I =0·2762, r = -1·2886). Most of this difference is due to effects on the growth, survival and reproduction of the smallest two size classes, which are shown by elasticity analysis to be the most important to population growth in both populations. The stable size distributions and reproductive values are similar in the two populations.
(3) Both deterministic and stochastic analyses reveal a critical frequency of fire ("" 0'85) below which this species is unable to maintain itself. This apparent reliance on fire frequency suggests that the spread and evolution of this species has been
closely related to human occupation of neorropical savannas.