Distribution of the Soil Seedbank and Influence of Fire on Seedling Emergence in Acacia saligna Growing on the Central Coast of New South Wales

Abstract

The distribution of seed in the soil and its response to simulated fires were examined in a population of Acacia saligna (Labill.) H.L.Wendl. on the central coast of New South Wales (NSW). Soil seed density was measured by excavating and sieving soil samples, and was estimated to be 1389 ± 297 seeds m^–2 and 3600 ± 279 seeds m^–2 at two sites within the population. Seed density declined with depth, with 80% of the seedbank located in the upper 6 cm of soil. The passage of fire was simulated by heating the soil surface with a propane burner over an area of 0.25 m² for durations of 2 or 4 min. A large percentage (88–94%) of the seedbank remained dormant following heating. Temperatures measured in the soil during heating showed that the 4-min treatment approximated the level of soil heating expected during a fire in which all fine ground fuel was consumed; therefore, the seedbank of A. saligna is unlikely to be significantly depleted following a single fire. A frequency distribution of the depths from which seedlings emerged was constructed using measurements of the distance between the soil surface and the hypocotyl–radical junction. A zone of lower than expected seedling emergence occurred from 0–2 cm, but the depth of this zone was not consistent with seed mortality due to excessive heating. Reduced emergence from 0–2 cm may be the result of seedling mortality due to temperature or water stress, which may be influenced by changes in the soil structure or chemistry caused by heating.