Effects of Climate and Water Limitation on Reproductive Traits and Trait Divergence Across Soil Boundaries in a Serpentine‐Tolerant Annual Herb

ABSTRACT

Biodiversity hotspots are often found in areas with high precipitation or diverse microhabitats. For plants, geographic areas with high soil variability tend to have high species richness or functional trait diversity. Serpentine soil has driven the diversification of numerous lineages, and many taxa show phenotypic variability across soil types. Precipitation can increase effects of soil on divergence, but this has been examined only for growth and vegetative traits. Using a greenhouse common garden, we characterized reproductive phenology for the serpentine‐tolerant annual herb Antirrhinum vexillocalyculatum along a moisture gradient and evaluated whether trait divergence varies with precipitation. Furthermore, we conducted a water limitation experiment and examined whether water limitation changes trait expression or divergence in trait expression. Finally, through field observations we assessed whether reproductive phenology, divergence in reproductive phenology, or the potential for gene flow differs between wetter and drier areas. We found strong effects of precipitation regime on flowering phenology, and some support for the hypothesis that precipitation enhances divergence across soil types. Experimental water limitation reduced floral displays, caused phenological delays, and increased divergence across soil types for some traits. In the field, the proportion of plants flowering was consistently greater in the wetter area, and the potential for gene flow was higher from non‐serpentine to serpentine populations. Despite phenological differences among ecotypes, flowering distributions overlapped to a large extent along the precipitation cline and under water limitation. This suggests that gene flow across soil types is a dominant force in this system but may be lower in wetter areas and under future drought. Shortened flowering durations and floral displays under drought suggest that plant reproductive potential will decrease in the future, with the possibility of lower adaptive gene flow from serpentine to non‐serpentine populations should it occur.