A DEMOGRAPHIC-STUDY OF AN ANNUAL GRASS (ANDROPOGON-BREVIFOLIUS SCHWARZ) IN BURNT AND UNBURNT SAVANNA
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We used demographic models to examine the effects of fire and fire exclusion on the annual grass Andropogon brevifolius in Venezuelan savannas. Seed germination, vegetative survival, seed production, and seed survival were measured in experiments in burned and unburned plots. From these data we calculated the population growth rate, stable age distribution, reproductive value, growth rate sensitivities, and the critical fire frequency necessary for persistence of the population. The annual growth rate of the population in the burned plot was 2.81, whereas the growth rate of the population in the unburned plot was only 0.66. Thus A. brevifolius will decline to extinction under fire exclusion. Population growth rate is most sensitive to changes in survival, but most of the effect of fire exclusion was contributed by reductions in fertility, including both seed production and seedling recruitment. We estimate that a fire frequency of at least 0.29 is needed for A. brevifolius to persist.