Sexual reproduction in Saxifraga oppositifolia L and Saxifraga biflora all (Saxifragaceae) in the Alps

Plant life in high alpine or arctic areas is strongly limited by abiotic rather than biotic environmental factors. In climate warming, invading plants from lower altitudes may outcompete indigenous alpine species, which might only survive by colonizing new habitats. Because diaspores are most effective for long-distance dispersal, colonizing species mostly depend on reproduction by seeds. The breeding systems of Saxifraga oppositifolia and Saxifraga biflora were investigated in the field. Saxifraga oppositifolia was found to be mostly cross-pollinated, and S. bipora showed a mixed mating system. Relative seed set (seed:ovule ratio) was higher in S, biflora than in S. oppositifolia. Saxifraga biflora also showed a higher relative reproductive success (RRS) in naturally cross-pollinated plants, because of high fruit set, and in geitonogamous selfing compared with S, oppositifolia. Germination percentage was lower in S, biflora than in S. oppositifolia, but only a few seeds germinated in both species. Variation of germinability could not be attributed to the pollen source of the various pollination treatments. Saxifraga oppositifolia is among the earliest plants to flower in spring, at a time when low temperatures Limit pollinator activities. Since pollen transfer is essential in this preferentially outbreeding species, S. oppositifolia is a typical ''pollen risker'' Late-flowering species such as S, biflora have numerous pollinators at their disposition and usually display a high selfing rate. Early snowfall, however, can destroy the whole seed crop. In this context, S. biflora resembles a ''seed risker.'' Because of a high outcrossing rate, ''pollen riskers'' are expected to have a high genetic variability, which is regarded as preadaptation to stochasticity in tundra habitats and to climate fluctuations. A possible future climate warming will reduce the chances that ''seed riskers'' lose their whole seed yield, which could be advantageous for colonizing new habitats. This demonstrates the need of learning about species-specific reproductive strategies in order to make predictions on the vulnerability of tundra plants in a changing climate.