Accelerated phenology fails to buffer fitness loss from delayed rain onset in a clade of wildflowers

The timing of early life cycle events has cascading effects on phenology and fitness. These effects may be critical for climate resilience of plant populations, especially in Mediterranean environments, where delayed rainfall onset causes delayed germination. To examine impacts of germination timing on ten species of the Streptanthus/Caulanthus clade, we induced germination across a range of dates in ambient seasonal conditions and recorded phenological and fitness traits. Later germination cohorts accelerated flowering, partially stabilizing flowering date, but the degree of this compensatory plasticity differed across species. Fitness declined with later germination; the magnitude of this decline depended on the balance between direct negative effects of later germination and compensatory positive effects of accelerated flowering. The resulting species differences in fitness responses suggest differential vulnerability to climate change. Species from wetter, cooler, less variable habitats accelerated flowering more and declined less in seed set with later germination, suggesting climate adaptation. However, other fitness responses to germination timing, like first year fitness, were evolutionarily labile across the clade and unrelated to climate. Although compensatory phenological plasticity may buffer the impacts of delayed germination, it cannot prevent long term declines in population fitness as fall rains come later with climate change.