EFFECT OF ELEVATION ON SEXUAL REPRODUCTION IN ALPINE POPULATIONS OF SAXIFRAGA-OPPOSITIFOLIA (SAXIFRAGACEAE)

Self-compatibility in high arctic and alpine areas is regarded as an a daptation to low pollinator abundance, However, high genetic variabili ty as a consequence of outcrossing is, with regard to population persi stence, favorable in highly stochastic environments such as tundra hab itats, To evaluate these contradictory scenarios, I performed in situ pollination experiments to examine the breeding system of the predomin ant out-crosser Saxifraga oppositifolia in ten populations at two diff erent elevations in the Swiss Alps. Pollinator limitation was detected at both elevations, but fruit set in naturally pollinated flowers was only slightly less at the higher elevation. Increased pollinator limi tation at high compared with low elevation thus could not be demonstra ted in this experiment. Hand-crossings yielded equal mean proportion s eed set at both elevations, and so did hand-selfings. This constant pa ttern of the breeding systemin S. oppositifolia indicates selective fa ctors that lead to the maintenance of a high level of outcrossing even in high-elevation populations. Based on sex allocation models, it was expected that a high ovule number should be selectively advantageous in a plant-pollinator system where chance visitation or selfing play i mportant roles. However, female reproductive offer in terms of ovule n umber per flower did not change from low to high elevation. Since neit her increased pollinator limitation nor increased seed set in selfed f lowers was found at high compared with low elevation, the prerequisite s for testing the hypothesis were not given. This study contradicts th e hypothesis that inimical environmental factors in alpine or arctic h abitats necessarily select for increased selfing rates in a preferenti ally outcrossing species like S. oppositifolia.