Trillium recurvatum Beck is a long lived perennial with its northernmost distributional range in southern Wisconsin and is listed as Special Concern. The breeding system and reproductive ecology of this purple sessile flowered Trillium have not been previously studied. To test the hypothesis that pollen limitation is a primary contributing factor to the rarity of T. recurvatum, its breeding system and reproductive ecology were studied. Pollinator activities, pollen to ovule ratios, fruit and seed set and breeding system dynamics via field crossing studies were assessed. Observations of insect visitations revealed a single effective pollinator Coleomegilla maculata De Greer. The estimated pollen to ovule ratio was 3255 to 1 suggesting strongly obligate outcrossing. Seeds were small weighing less than any species in the genus. Fruit set in hand pollinated, outcrossed flowers was significantly greater than in open pollinated controls, demonstrating pollen limitation. Autogamous crosses yielded less than 2% fruit set, indicating self incompatibility. Seed set was greater in open pollinated controls than in manipulated flowers, indicating high pollen load depositions. This study demonstrates that pollen limitation is present in T. recurvatum but loss of habitat, habitat fragmentation and effects of distributional range limitation may not be discounted as forces influencing its rarity.
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