Seed production and germination in two rare and three common co-occurring Acacia species from south-east Australia

Seed set, size, viability and germination requirements were investigated for two rare (Acacia ausfeldii and A. williamsonii) and three common (A. pycnantha, A. genistifolia and A. paradoxa) co-occurring congeners in box-ironbark eucalypt forests near Bendigo, south-east Australia to investigate correlates of rarity. Seed size was significantly smaller for the two rare species and germinants were less able to emerge from deeper sowing depths than were the larger seeded common congeners. All species had a strong heat-stimulated germination response. While the rare A. ausfeldii showed strong germination only at the highest temperature treatment (100degreesC), the common and widespread A. pycnantha showed strong germination across a broad range of temperatures (60-100degreesC) likely to be experienced by soil-stored seeds during a fire. Seed viability, number of seeds per plant, and number of firm, aborted and eaten seeds per pod varied between species, but the pattern of variation was not related to rarity. Small seed size and a very specific temperature requirement for germination may help to explain rarity in A. ausfeldii, and to a lesser extent in A. williamsonii. Fires are often patchy and heating of the soil is likely to be highly spatially variable, so species with germination responses to a broad range of temperatures have an advantage over those that respond only to a narrow range. A narrower range of soil depths from which seeds can emerge will further reduce the proportion of the seed bank that might recruit following fire. Human impacts on species habitats, such as fragmentation, loss of topsoil through mining, timber harvesting, grazing and urbanization, and consequent reduction in fire intensity, are likely to have further contributed to rarity in these species. The role of pollination and other factors in relation to population size is the subject of further investigation.