Density-dependent pollinator visitation and self-incompatibility in upper Great Lakes populations of Trillium grandiflorum

Trillium grandiflorum is a widely distributed, non-clonal, long-lived, white-flowered perennial herb. We examined the pollination biology and self-compatibility of Trillium grandiflorum in five populations distributed across northern Wisconsin and the western Upper Peninsula of Michigan. Self-pollination was almost entirely ineffective in producing seeds in all populations, suggesting strong self-incompatibility. Natural seed set varied slightly across populations, with an average of 71% of ovules fertilized per fruit. At the population level, seed set was not limited by receipt of pollen. At the plant level, seed set per flower first increased, then decreased in response to an increasing density of flowering plants within 2 m. This suggests that neighboring flowers facilitate pollinator visitation at low densities then compete for visits at higher densities. Observed pollinator visitation rates support this interpretation. The presence of self-incompatibility and the absence of clonal spread could make Trillium grandiflorum vulnerable to disruptions in pollinator service.