Phenological Variation and Evolution Across Space and Time in the Scarlet Monkeyflower (Erythranthe cardinalis)

Climate change poses an existential threat to global biodiversity, especially in the California Floristic Province, where selective pressures differ across the ranges of latitudinally distributed species. Lower latitude populations face extreme heat and drought stress but may be older and more likely to harbor alleles adapted to these conditions. Conversely, poleward populations are likely younger and face a rapidly changing climate. Quantifying intraspecific variation in traits across latitudinal gradients is vital for predicting evolutionary responses to climate change. Flowering phenology, the seasonal timing and intensity of plant reproduction, varies across a species' range and is closely tied to plant strategies for dealing with drought. In this study, we combine a resurrection study and a common garden experiment at the southern edge of the scarlet monkeyflower's (Erythranthe cardinalis) range to investigate how its flowering phenology and life history have evolved in response to recent climate change. We grew seeds from the northern, central, and southern regions of its range, collected in 2010 (ancestors) and 2017 (post-drought descendants). We measured height at first flower, rhizome production, and constructed flowering curves from June to September for full-sibling seed families. We estimated additive genetic variance and heritability for phenological metrics and found that both decreased regionally from south to north. Southern populations began flowering later and at a larger size compared to central and northern populations, although southern descendants were shorter relative to ancestors. Conversely, northern populations produced substantially more rhizomes and began flowering earlier and shorter overall. All populations exhibited greater heritability for early-season flowering; however, latitudinal differences were most pronounced for late-season flowering, which shifted substantially in descendants of one central population. Northern and southern populations represent opposite phenotypic extremes regarding late-season flowering but may exhibit increasingly greater synchrony in early-season flowering.