The temperature dimension of the seed germination niche in fen wetlands

The seed germination niche partly determines adaptation, ecological breadth and geographic range in plant species. In temperate wetlands, environmental temperature is the chief regulator of germination timing, but the ecological significance of high and low temperatures during dormancy break and germination is still poorly understood. Our aim was to characterize the temperature dimension of the germination niche in mountain base-rich fens, determining (1) the effect of different temperatures on dormancy break and germination, and (2) whether different germination strategies may be identified at the species level. We conducted laboratory germination experiments with seeds of 15 species from these habitats, collected in 18 fen sites in the Cantabrian Mountains (Spain) for two consecutive years. In all the species, the seeds were totally or conditionally dormant at dispersal and stratification produced a significant increase of germination. In most cases, there was not an obligatory requirement for cold temperatures during dormancy break, since warm stratification promoted germination as well. Although the optimal germination thermoperiod was generally high (30/20 °C), most species could also germinate at lower temperatures after cold-stratification. We also identified a group of species associated to cold-water springs that germinated only at low temperatures. Our results demonstrate that dormancy break in mountain base-rich fens does not obligatorily depend on cold temperatures during overwintering. Furthermore, germination at cool temperatures may be more widespread in wetland habitats than previously thought. The existence of two distinctive germination strategies, ‘warm’ and ‘cool’, can potentially give rise to divergent species responses to climate change.