The frequent fires characteristic of tropical savannas can have a major impact on the regeneration of plant species. This study investigated the effect of Unburnt, Early and Late regimes on seed production and seedling establishment at Kapalga. Results are presented for two species - Eucalyptus miniata, a dominant overstorey species, and Acacia oncinocarpa, a common midstorey species. Seed production in both species occurs during the dry season, coinciding with fire season.

For E. miniata, the Late regime substantially reduced both the proportion of flowering trees per stand, and the density of flowers in the canopy. By contrast, the Early and Unburnt regime had a similarly high proportion of flowering trees, and a high density of flowers in the canopy. Although the Early regime did not significantly reduce flowering in E. miniata, it did substantially reduce the development of the flowers through to mature seed-bearing capsules. Successful flower development was 2-3 times higher in the Unburnt and Late regimes compared to the Early regime. Due to the effect of the Late regime on the abundance of flowers, and the Early regime on flowering success, the amount of E. miniata seed produced and dropped from the canopy was at least 5 times higher in the Unburnt regime than in the Early and Late regimes. A similar pattern was found for A. oncinocarpa, with 4 to 8 times the amount of seed produced per plant in the Unburnt regime compared to Early (this species did not occur in the Late regime compartments).

Therefore, fire reduced the amount of seed available for regeneration in both E. miniata and A. oncinocarpa. Experiments have shown that the number of seedlings establishing at Kapalga is limited by the availability of seed, which suggests that the reduction in seed production might reduce seedling recruitment. However, could this effect be countered by burning encouraging higher numbers of seeds to germinate and seedlings to establish? This question was answered by establishing an experiment in which seeds of E. miniata and A. oncinocarpa were artificially applied to quadrats in the Unburnt, Early and Late regimes, and the number of emergent seedlings monitored. The experiment showed that the Early and Late regimes reduced seedling emergence compared to Unburnt sites. The differences in establishment are probably due to higher canopy cover and lower grass cover in the Unburnt regime, and the higher abundance of seed harvester ants in the burnt regimes.

This study shows that annual burning can have a significant impact on population recruitment in E. miniata and A. oncinocarpa. Frequent fire reduces the amount of seed produced, and reduces the chances of seedling establishment. This suggests that a fire-free period of a few years would promote seedling regeneration of these common tropical savanna species.