Losses from the seed bank of Mimosa pigra: soil micro-organisms vs. temperature fluctuations.
Field and laboratory experiments were conducted to investigate the relative contributions of the soil microflora and soil temperature fluctuations to the loss of dormant seeds from the seed bank of Mimosa pigra, an invasive tropical shrub. The field experiment was carried out during the dry season at 2 sites near Darwin, Northern Territory, with differing soils subject to seasonal inundation, one a heavy black cracking clay, the other solodic. The distinction was drawn between causes of seed bank depletion - the external environmental factors resulting in seed losses - and the processes of depletion - death or germination - through which the various causes operate. Overall, 66 and 59%, respectively, of seeds remained viable in the two regions after the 7 months of the dry season had elapsed. The causes of seed loss were diurnal temperature fluctuations of up to 50°C and soil micro-organisms. Both caused germination and death of seeds in the seed bank, germination stimulated by temperature fluctuations being the more important. Fungicide (benomyl at 0.5-50 g/litre Benlate) reduced the overall loss of seeds significantly in both regions, but only by 10-16%. The results did not support the contention that many seeds are lost from the soil through the depredations of pathogens. However, it is possible that the most important impact of pathogens is on germinating, not dormant, seeds.