'Fire seeders' during early post-fire succession and their quantitative importance in south-eastern Spain

摘要

Resilience against fire disturbance of Mediterranean vegetation has been frequently described. However, fire regimes change due to abandonment of local land use practices and climatic change. Thus, it is useful to know the importance of fire-specific and unspecific mechanisms during regeneration in order to predict changes in species resilience under an altered fire regime. In six burnt areas in a mountainous and in a coastal region in south-eastern Spain we collected information on fire-related germination characteristics (impact of smoke, charred wood or heat) of all abundant species. We excluded those species that predominantly recover by sprouting. According to these results (germination tests and literature research) we classified species that showed a positive reaction to any of the fire-related treatments studied as potential "fire seeders". Germination of seven out of a total of 21 tested species was significantly increased by heat whereas germination of 11 hard-seeded species was mainly triggered by mechanical and/or chemical scarification. However, none of the tested species reacted positively to the treatments of ash, charred wood, and smoke. According to a quantitative plot-based vegetation analysis we then compared the coverage of "fire seeders" on (a) fire sites at the coast (2-3 years old) with sites of similar age in the mountains and (b) fire sites in the mountains of mid-successional stages (7-9 years) with undisturbed reference sites and areas of different types of disturbance (i.e. logging and fire break areas) but of comparable age and location. Results of comparison (a) showed that "fire seeder" coverage is below 4% and even lower in the coastal area. Comparison (b) showed similar coverage (about 15%) of "fire seeders" on the fire sites and on the fire breaks (strongly disturbed sites) whereas their abundance on logging and undisturbed reference sites was significantly lower. Thus, the term "fire seeder" might be misleading as fire impact is not essential for inducing germination of heat-triggered seeds. In south-eastern Spain, the low abundance of "fire seeders" and their successful regeneration on other disturbed sites are in line with historically early and strong human disturbance and low fire frequencies as the fuel load is limited due to the dry conditions. The tested species are not dependent on a certain regular fire impact though strong disturbance is very favourable for the creation of dense populations.