Seed dynamics and factors limiting recruitment of the mallee Eucalyptus incrassata, in semi-arid, south-eastern Australia.

Abstract

(1) Seedlings of mallee eucalypts occur infrequently after fire and are absent from long-unburnt stands. In this study, seedling recruitment in long-unburnt stands of yellow mallee, Eucalyptus incrassata, was found to be limited by a low availability of viable seed in the soil and by the inability of new seedlings to survive. (2) Despite a large reserve of seeds retained in woody capsules in the canopy for several years (maximum seed load of 8.0 (S.D. = 1.0) × 106 seeds ha-1 and a seed rain of up to $2.1 (S.D. = 0.9) \times 10^6 \text{seeds} ha^{-1} \text{year}^{-1}$, there was little or no storage of seed in the soil. Seed-harvesting ants rapidly removed 65-100% of newly-fallen seed. The exponential half-life of an isolated seed on the soil surface was 5.2 days. Seed harvesting activity was spatially and temporally variable. Naturally-fallen seeds were occasionally present at low densities on the soil surface. (3) The hypothesis that fire-induced mass release of seed may facilitate recruitment through the establishment of a seed bank in the soil was tested and supported. The experimental addition of seeds to the soil surface of an unburnt stand at likely post-fire densities (300 seeds m-2) resulted in an increased store of seed in the soil. Seeds remained viable in the soil for at least 300 days. Depletion of surface-added seed was gradual, indicating that high seed densities may have satiated the seed predator populations. The rate of field germination of this surface-added seed was similar to that of natural recruitment (10 000 seedlings ha-1) following a fire in December 1977. (4) Sufficient numbers of seedlings germinated in unburnt stands during the winter of 1980 to maintain population densities, despite the activities of seed-harvesting ants. However, all seedlings, both natural and induced, had died by the late spring of 1980, indicating that the conditions were not suitable for establishment. The successful establishment after fire, and the general absence of seedlings from unburnt stands, strongly suggest that the effects of fire in modifying environmental factors are crucial for seedling recruitment of mallees.