THE IMPACT OF EXPERIMENTAL FIRE REGIMES ON SEED PRODUCTION IN 2 TROPICAL EUCALYPT SPECIES IN NORTHERN AUSTRALIA

This study investigated the effect of three experimental fire regimes on the fecundity, ovule development and seedfall of two common wet-dry tropical savanna eucalypts, Eucalyptus miniata and Eucalyptus tetrodo nta, in northern Australia. Both species flower early in the dry seaso n and ovule development occurs during the dry season. This coincides w ith a period of frequent fires. The three fire regimes considered were applied for four years between 1990 and 1994. These regimes were (i) Unburnt, (ii) Early, fires lit early in the dry season, and (iii) Late , fires lit late in the dry season. The treatments were applied to nin e catchments (15-20 km(2)) with each fire regime replicated three time s. Fire intensity typically increases as the dry season proceeds. Ther efore, early dry season fires generally differ from late dry season fi res in both their intensity and their timing in relation to the reprod uctive phenology of the eucalypts. Late dry season burning significant ly reduced the fecundity of both species, whereas Early burning had no significant effect. Ovule success was significantly reduced by the Ea rly burning for both species. The Late burning significantly reduced o vule success in E. tetrodonta, but not in E. miniata. The results sugg est that fire intensity and fire timing may both be important determin ants of seed supply. Fire intensity may be a determinant of fecundity, whereas fire timing in relation to the reproduction phenology may hav e a significant impact on ovule survival. Both fire regimes resulted i n a substantial reduction in seed supply compared with the Unburnt tre atment. This may have a significant impact on seedling regeneration of these tropical savanna eucalypts.