The germination process in vernal pools: sensitivity to environmental conditions and effects on community structure.

Variation in timing and amount of rainfall in California has been noted to cause strong year-to-year differences in the composition of vernal pool communities. We explored the effects of possible variation of the "rainy season" in a seedbank germination study conducted with three factors: monthly timing of first soil moistening, length of moist period before inundation, and length of inundation. Monthly timing was the most important factor in determining the number of plants and number of species that germinated in the pots. Many species, mostly generalist wetland and pool-endemic species, showed sharp decreases in number in later trials. Five species did not germinate in the final trial, and two of these were abundant only in the first trial. Inundation was required for the germination of two species. Treatments with a combination of inundated and moist conditions resulted in higher numbers of species and numbers of plants compared to treatments with only moistness or only inundation. Our results indicate that vernal pool species have mechanisms that keep them from emerging under unfavorable conditions. The sensitivity of vernal pool species to out-of-season germination suggests that they could be negatively affected by climatic change, with generalist species like non-native Lythrum hyssopifolium having a potential advantage.