We evaluated the combined effects of fire after drought on the seed bank composition and its role in the postfire recovery of NW Patagonia grasslands. During three years, we monitored the seed bank and the aboveground vegetation. Species were arranged in functional groups and Detrended Correspondence Analysis was used to separate sites according to species and functional groups. Similarity between aboveground vegetation and seed bank was calculated with SØrensen Index. In the first year, the seed density was similar in the control and burned sites and was lower than following years in all the sites. The species that survived the high temperatures were all annuals with the exception of the perennial species Fabiana imbricata and Rumex acetosella. In the second postfire year, the diversity and seed density increased due to the contribution of fugitive species (rare in the community) and exotic annual species. Seed bank of perennial species was the most affected by fire and just recovered in the third year. Drought did not affect the similarity between the seed bank and vegetation. Fire had low impact on the total seed bank, probably due to the heat buffering nature of the soil, whereas drought reduced significantly seed bank size and richness. Seed bank contributes to grassland richness maintenance.

There are two main methods for estimating seed density and species composition of soil seed banks: manual seed extraction and seedling emergence. These methods were used to determine and compare seed density and species composition in the soil of a natural grassland in Patagonia. Additionally, known amounts of seeds of different sizes from Patagonian grassland species were mixed with soil to evaluate the efficiency of the seed extraction method, and determine their recovery percentage. Seed density found in the grassland soil with the extraction method was four times higher than that found with the seedling emergence method. Through the use of these two methods, there was very little overlap found in species composition. Small seeds (< 1 mm) were only found with the seedling emergence method, whereas the seeds of species with specific germination requirements were found with the seed extraction method. Seed recovery of grassland species varied from 2.5% for smaller seeds (Erophila verna) to 100% for larger seeds (Rumex acetosella) with the seed extraction method. This method was more effective in detecting seeds of large-seeded species. Discrepancies in seed detection between both methods may be related to seed dormancy, specific germination requirements, seed size and mass. These two methods are necessary to describe seed density and seed bank composition.Nomenclature: Correa (1969Correa ( -1999.

Abstract. Postfire recruitment of seedlings has been attributed to a stimulation of germination by fire-related cues. The germination response to heat shock (80 • C -5 min), smoke (60 min), the combination of both factors and no heat no smoke (control) was studied in six native species (two dominant grasses, two dominant shrubs and two annual fugitive herbs) of northwestern Patagonian grasslands. Seeds of the grasses Festuca pallescens and Stipa speciosa and the shrub Senecio bracteolatus (Asteraceae) germinated when they were exposed to heat shock, whereas seeds of the other shrub, Mulinum spinosum (Apiaceae), were killed by this fire cue. In grasses, probably the glume of caryopsis protected embryos from heat. Possibly, the seed size could explain the different responses of the two shrubs. Heat combined with smoke reduced seed germination for S. speciosa and S. bracteolatus. The heat could have scarified seeds and the longer exposure to smoke could have been toxic for embryos. The same treatment increased germination of the annual fugitive herb Boopis gracilis (Calyceraceae). We concluded that fire differentially affects the seedling recruitment of the studied species in the northwestern Patagonian grasslands.

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