Effect of fire on hard-coated Cistaceae seed banks and its influence on techniques for quantifying seed banks

The impact of fire on hard-coated Cistaceae (Halimium ocymoides, Cistus lad anifer, and C. salvifolius) soil seed banks in a Mediterranean 'maquis' shr ubland, and its effect on seed germinability were studied. The study also c ontrasts the effectiveness of two widely used techniques for quantifying se ed banks, the seedling emergence and the physical separation methods, in re lation to fire. The null hypothesis that a massive enhancement of physicall y-dormant Cistaceae seed germination by fire would make use of the time-con suming physical separation technique unnecessary was tested. Fire reduced C istaceae seed banks in the 0-2 cm deep soil layer by both seed fire-consump tion and lethal temperatures, revealed by the significant decreasing of the seed bank density and by the increase of apparently-intact but soft-unviab le seeds, respectively. In contrast, no damage was recorded in the 2-5 cm s oil layer. A dramatic seed bank depletion (> 90%) in both soil layers was r ecorded one year after fire in the burnt area, coinciding with a significan t increase of seedling density confined to the first post-fire year. The ec ological consequences of this massive post-fire seed bank input are discuss ed. A germinability test revealed that germination of surviving Cistaceae s eeds was significantly enhanced in all cases except for the C. salvifolious seed bank in the deeper soil layer. However, final germination levels (60- 75%) did not correspond to the magnitude of seed bank depletion, especially for C. salvifolious, which suggests that other environmental factors not e xclusively associated with fire may also be important in softening Cistacea e seeds. Germination enhancement by fire soil-heating was not high enough t o reject the physical separation technique, at least in the deeper soil lay er. The simultaneous use of both seedling emergence and physical separation is recommended for reliable seed bank estimates when a physically-dormant hard-seeded component can be expected in the soil, as in many Mediterranean ecosystems, regardless of fire occurrence.