Light and Pollination Limitation Alter Patterns of Fitness and Phenotypic Selection in Sagittaria trifolia L.:...

Seed production in plants is constrained by both light and pollination. Studies often focus on the effect of each factor separately on subsets of plants, leaving the combined constraints of light and pollination on individual fitness poorly understood. Here, we placed arrays of the monoecious herb Sagittaria trifolia L. in three distinct treatments, namely, a "mesh-enclosed" treatment where plants were covered with a fine mesh, simultaneously restricting light availability and pollinator access, a "shaded" treatment with limited light due to canopy cover, and an "open" treatment with abundant sunlight. Flowering traits and reproductive fitness were measured at both the inflorescence and individual levels. Plants in the open treatment produced the most inflorescences, flowers, and seeds. While initial fruit-set was the lowest in the mesh-enclosed treatment, it increased markedly in later inflorescences. Despite severe pollinator limitation, mesh-enclosed plants offset early fitness costs by producing more inflorescences, ultimately achieving total seed output similar to shaded plants. Phenotypic selection on five measured traits was stronger under limited conditions directed toward more female flowers per inflorescence in shade, but toward more inflorescences in mesh-enclosed plants. The results suggest that the effect of pollinator limitation in S. trifolia could be mitigated by the allocation of more inflorescences. Our study emphasizes the importance of considering temporal dynamics across sequential inflorescences within a single flowering season for accurately quantifying phenotypic selection and whole-plant reproductive strategies.