Fire, aridity and seed banks: What does seed bank composition reveal about community processes in fire-prone desert?

Abstract

Questions: The relationship between fire, aridity and seed banks is poorly understood in plant community ecology. We tested whether there was a close correspondence between the seed bank and standing vegetation composition with time-since-fire in a desert. We also examined whether longer-lived species showed seed limitation relative to more ephemeral species, as this could influence grass-woody ratios in a major biome. Location: Dune hummock grasslands/shrublands of central Australia. Methods: The effects of time-since-fire on floristic and functional group composition were examined by comparing plots unburned since 1984 against plots that had been burned in 2002. Three methods were used to quantify seed abundances: a germination trial using heat and smoke application, a flotation method, and a sieving method. Results: Seed bank densities were very low (<3000m⁻²). Species similarity between the seed bank and standing vegetation was high at sites recently burned (0.86) and low in sites long-since burned (0.52). The relative abundance of ephemeral species in the seed bank peaked in recently burned plots, but the relative abundance of seeds of woody species did not match the pattern of abundance in the standing vegetation. Remarkably, the dominant perennial grasses and woody species were either absent from the seed bank or present at extremely low abundances. Discussion: Differences in the relative abundance of ephemeral species between standing vegetation and seed bank relate to the post-fire succession process. The small soil pool of seed from woody species may be explained by allocation to belowground carbohydrate storage over seed production. Field observations suggest, however, that production of strongly dormant seed can be prolific and that high levels of seed predation make this system strongly seed-limited. The discovery of this seed bank syndrome indicates that shifts in grass-woody ratios can be driven by the juxtaposition of unpredictable seed rain and fire events in these desert dunes. However, estimates of grass-woody ratios due to changing fire regimes will be difficult to predict.