Light intensity triggers different germination responses to fire-related cues in temperate grassland species

Fire is a widespread disturbance that affects plant individuals and populations. In fire-prone environments, such as Mediterranean-type ecosystems, many species are stimulated to germinate by fire-related cues, for example heat and smoke. However, little is known about the effect of fire on seed germination of species from herbaceous communities like temperate grasslands. In this study we assessed the germination response to direct (heat shock and smoke) and indirect (light intensity) fire-related cues of five herbaceous and one shrubby species that occur in natural grasslands of eastern Uruguay. All species are native and belong to the Poaceae (2 species) and Asteraceae (4 species) families. Seeds were subjected to smoke and heat shock treatments (50°C, 100°C) under two light intensities (high-light and low-light). We found that direct fire-related cues did not stimulate germination in any of the species studied. Most of the species showed a reduced percentage of germination or a delayed mean germination time, relative to the control treatment, when exposed to heat shock at 100°C or smoke. However, the seeds survived the exposure to high temperatures, indicating that they can tolerate a fire event. Light was found to be a key germination cue. Treated seeds responded to fire-related cues mainly when we simulated the high-light environment after fire. Our results contribute to the growing body of evidence that many species of South American grasslands do not rely on recruitment by seeds after a fire event, but rather on the ability to resprout by means of subterranean or protected basal buds.