Response to fire of a predominantly transient seed bank in a Mediterranean weedy pasture (eastern-central Spain)

The impact of fire on the viability of the entire soil seed bank in a Mediterranean pasture dominated by weeds originating from arable fields, and its recovery in the first post-fire year, were investigated. Soil samples were excavated separately in 0-2 cm and 2-5 cm deep layers, and their seed content was assessed using the seedling emergence method. Emergent seedling counts were carried out in the field during the spring following the fire. Analysis of samples from an unburnt stand revealed that annuals (83% species, 86% seeds), and weeds (50% species, 30% seeds) were a major component in the seed bank. Seed banks of most species (85%) were accumulated in the upper soil layer, denoting their transient nature. Fire severely reduced the density and species richness of the seed bank, as expected for a mainly transient soil seed bank in a community not exposed to a recurrent wildfire regime throughout its evolutionary history. One year after the fire, the seed bank increased in density and species richness, particularly in the upper soil layer. The recovery occurred in two ways: i) in-site seed production From seeds which survived the fire and germinated, and ii) wind-borne seed invasion from adjacent unburnt vegetation. The first type of recovery was mainly the result of relatively large-seeded species with short-distance seed dispersal mechanisms, whereas the second H as observed for anemochorous and/or tiny-seeded species. We conclude that the ability of weedy communities to respond to fire is based on the colonizing and invasive nature of the species. Consequences for the use of fire in the management of such communities are discussed.