Seed longevity and fire: germination responses of an exotic perennial herb in NW Patagonian grasslands (Argentina).

Fire affects grassland composition by selectively influencing recruitment. Some exotic species can increase their abundance as a consequence of fire-stimulated seed germination, but response may depend on seed age. Rumex acetosella L. (Polygonaceae, sheep's sorrel) is a cosmopolitan herb that has invaded NW Patagonia's grasslands. This species forms persistent soil seed banks and increases after disturbances, particularly fire. We studied how fire and seed longevity influence R. acetosella germination. In 2008, we conducted laboratory experiments where we exposed different-aged seeds (up to 19 years old) to heat, smoke, charcoal, ash and control treatments. Total percentage germination and mean germination time depended on both seed age and fire treatment. Germination of younger seeds decreased with increasing temperature. There was no general pattern in germination responses of different-aged seeds to smoke, charcoal and ash. While smoke improved the germination of fresh seeds, charcoal decreased germination. Germination of untreated seeds was negatively correlated with seed age, and mean germination time increased with seed age. In most treatments, fresh seeds had lower germination than 1-5-year-old seeds, indicating an after-ripening requirement. Smoke stimulates R. acetosella germination, causing successful recruitment during post-fire conditions. Fresh seeds are particularly responsive to fire factors, possibly because they have not experienced physical degradation and are more receptive to environmental stimuli. Knowing the colonisation potential from the soil seed bank of this species during post-fire conditions will allow us to predict their impact on native communities.