Ecophysiology of fire-stimulated seed germination in Cistus incanus ssp. creticus (L.) Hey wood and C. salvifolius L.

Abstract. Seed germination characteristics were investigated in the most common Cistus species in Greece, namely C. incanus ssp. creticus and C. salvifolius. In addition to the soft seed subpopulation, both species produce a large fraction of hardcoated, water-impermeable seeds which can be softened and, thus, promoted to germinate by mechanical scarification and thermal pretreatment. Temperature and light control of seed germination are unimportant. In the ecological context of the Mediterranean ecosystems, the eventually advantageous, opportunistic strategy of germination is based on: (a) seed heterospermy (which allows the smaller, softcoated fraction to germinate promptly each year while the majority of the seeds, the hard ones, accumulate in the soil); (b) the seed population heterogeneity in relation to coat hardness (so that any heat conditions produced by fire induce the softening and germination of a certain seed fraction); (c) the notably low germination rate (which suppresses premature germination); (d) the wide, Mediterranean-type (relatively cool), temperature range of germination (while higher temperatures simply inhibit but do not induce any dormancy); and (c) the apparent lack of photo-sensitivity (enabling germination under every light regime). In non-fire years, the temporal distribution of field germination and seedling appearance might be partly determined by the seed dispersal strategy of the individual Cistus species. Nevertheless, the post-fire regeneration response is manifested in the form of a huge wave of germination (of practically all seeds softened by the fire heat), shortly after the onset of the rainy season.