The effect of fire and ants on the seed‐bank of a shrub in a semi‐arid grassland

Abstract

Abstract Invasion of grasslands by woody plants following the introduction of domestic stock is a worldwide phenomenon. Burning is frequently recommended as a remedial measure but for a pastoral enterprise it is costly and the frequency of the fires required is of critical economic importance. The size and longevity of the soil seed‐bank is an essential part of the response of shrub populations to prescribed fire regimes. In this study the seed‐bank of the semi‐arid zone shrub Dodonaea attenuata in Eragrostis eriopoda tussock grassland was examined in relation to harvesting by ants and the burning history of the sites. On unburnt sites, more than 3500 seeds per m2 entered the seed‐bank in the summer 1984–85 but sites burnt 5 years previously produced less than one‐third of that number. Burnt shrubs did not flower for 5 years and no seeds survived in the soil through to 1985. Burning immediately prior to seed ripening destroyed the seed crop but burning after seed‐fall stimulated a greater germination in the following spring than on unburnt treatments. Ants rapidly harvested most of the seeds produced and after 20 months the combined effects of ant harvesting and germination had reduced the seed‐bank at unburnt sites to 8–21 seeds per m2 and at sites burnt 7 years previously to less than two seeds per m2. Seeds were initially harvested in summer by Pheidole spp. of ants for their elaiosome and then discarded in middens outside the entrance to the ant nest. During the subsequent cool season the seeds were taken back into the nest and stored at depths ranging from 2 to 30cm. It was concluded that ants provided short‐range dispersal (< 10 m) and promoted the contagious distribution of D. attenuata, which is advantageous for a fire‐susceptible, arid‐zone shrub invading a grassland liable to be burnt: seedlings derived from seeds in ant storage chambers near the soil surface and in sparse grass situations caused by competition from shrubs may obtain some survival advantage. Prescribed fire has potential as a management tool for controlling population density of D. attenuata because, depending upon season and frequency, it reduces seed rain by killing shrubs, suppresses flowering activity and destroys seed crops on the plant. Under the influence of a regime of regular burning, such as prevailed prior to the European pastoral industry, the limited soil seed‐bank would have been a major constraint on D. attenuata populations.