AN EXPERIMENTAL TEST FOR REPRODUCTIVE INTERACTIONS BETWEEN 2 SEQUENTIALLY BLOOMING SAXIFRAGA SPECIES (SAXIFRAGACEAE)

The blooming period of Saxifraga reflexa on south-facing bluffs in interior Alaska precedes and slightly overlaps that of Saxifraga tricuspidata. We tested the hypothesis that reproductive interference maintains this phenological separation by measuring seed set of potted S. reflexa during the normal and S. tricuspidata blooming periods, with and without S. tricuspidata nearby. During the S. reflexa blooming period we placed arrays of four pots of S. reflexa in sites with and without S. tricuspidata. Several weeks later, during the S. tricuspidata blooming period, we repeated the experiment with potted S. reflexa whose flowering had been artificially delayed. Seed set of S. reflexa was decreased by retarding blooming time, but not by the presence of blooming S. tricuspidata. Although both species were visited by the same pollinators, primarily syrphid flies, the presence of S. tricuspidata did not reduce the rate of visitation to S. reflexa. In mixed plots, visitation rates by syrphid flies were much higher to S. reflexa than to S. tricuspidata. Interspecific floral transitions by syrphid flies were uncommon. It appears that in 1986 at this site S. reflexa experienced no negative effects from flowering synchronously with, and occurring in close proximity to, S. tricuspidata.