Context-dependent conflicting selection on flowering phenology.

Climate change will affect precipitation and water availability in natural plant populations, potentially influencing phenology, fitness and natural selection. To examine how water availability affects non-pollinator- and pollinator-mediated selection on flowering phenology in Primula tibetica, we manipulated pollination at three sites that differed in soil water content (low, medium and high) in a single common garden. We detected conflicting selections on phenology. At low water content, there was pollinator-mediated selection for earlier flowering start and non-pollinator-mediated selection for later start. At medium water content, pollinators selected for shorter flowering duration and non-pollinator agents for longer duration. The opposing selection resulted in no statistically significant net selection on phenology. Non-pollinator-mediated selection differed between sites, and changes in trait-fitness relationships among hand-pollinated plants were mainly driving variation in selection on phenology. The results indicate that soil water content primarily affects selection on phenology via resource uptake, and are consistent with higher pollinator abundance or constancy early in the flowering season. The study highlights that both flowering start and duration can be targets of selection, that phenology may be subject to conflicting selection from pollinators and other agents, and that the evolution of flowering time in response to pollinator-mediated selection can be constrained by climate.