Untangling the effects of water availability on flower size: Insights from the largest tropical wetland

Flower size plays a crucial role in the reproduction of animal-pollinated plants. Larger flowers usually increase pollinator visitation and reproductive success, but they are also associated with higher water investment for production and maintenance. Thus, water availability is a relevant factor in determining flower size, which may be consequently reduced in hot and dry environments. However, the phylogenetic relationships across plants may limit the variation in flower size, a strongly conserved trait among closely related species. We tested this hypothesis across 97 animal-pollinated species in the Pantanal, the world's largest tropical wetland. To determine how phylogeny and water availability affect community-level flower size, we quantified the variation of this trait between seasons (dry and rainy), substrates (terrestrial and aquatic), and species (phylogenetic signal). We observed a high variability in flower size, which showed a significant phylogenetic signal. Conversely, we did not detect a significant influence of season and substrate on flower size, although we observed a trend toward larger flowers in aquatic species, especially during the rainy season. Our results demonstrate that phylogeny mostly determines flower size variation in the Pantanal. Our findings also suggest that water availability may still exert evolutionary pressures on flower size, even in a context of phylogenetic conservatism. This study highlights the need to investigate the multiple interacting factors shaping flower size and contributes to a deeper understanding of its variation in response to environmental factors and phylogeny.