Timing of fire during summer determines seed germination in Mediterranean Cistaceae

Abstract

Abstract Background Mediterranean-type ecosystems are fire-prone environments where species have evolved in the presence of seasonal summer conditions and frequent fires. In these environments, many species have seeds with impermeable hard coats that impose physical seed dormancy which prevents seed germination until it is broken by any factor such as fire or high summer temperatures. Most studies have usually focused on the isolated effects of fire or summer temperatures on the release of physical seed dormancy. However, both factors frequently operate together because when a fire occurs, usually in summer, seeds in the soil suffer not only the heat shock during the fire, but also the high summer temperatures. Nevertheless, the combined effect of both factors is almost unknown. In this investigation, we studied the isolated and combined effects of heat shock and the high temperatures of summer. The main objective was to evaluate whether the timing in which seeds are exposed to a heat shock during the period of high summer temperatures affects seed germination. Results We exposed the seeds of 12 Cistaceae species to three different treatments, in which heat shock (100 °C for 10 min) was applied before, after, or before + after they were exposed to summer temperatures (50/20 °C for 1 month). Although fire was the main factor breaking physical dormancy, its effects were regulated by the timing when it was simulated. We found that summer temperatures following heat shock had negative effects on seed germination since a high proportion of seeds remained dormant. On the contrary, when summer temperatures preceded heat shock, physical dormancy was broken, and germination increased. Conclusions Timing of fire during summer plays an important role in controlling seed germination of Cistaceae. Our results provide a novel explanation for the higher Cistaceae seedling emergence found in the field after late season fires. Additionally, we can carefully draw conclusions for fire management since prescribed burnings would have different effects depending on when they were carried out.