Benjamin J. LeBlanc

Premise: The increasing frequency and severity of heat waves across the globe is well known. However, few longitudinal studies have tracked demographic change and fitness within natural populations, and fewer still have spanned multiple extreme events. Determining how organisms tolerate, respond, and potentially adapt to extreme events is key for assessing long‐term population viability.Methods: We examined how mortality, fecundity, seed provisioning, and offspring germination differed across 12 populations of annual common monkeyflower (Mimulus guttatus) over 5 years.Results: Two heat waves occurred within the study: a heat wave within the first 10 days of the growing season in 2019 and a heat dome event nearly a month into the growing season in 2021. Mortality was high, and fecundity decreased in a population‐specific manner following each heat wave. However, the 2021 heat dome caused declines of 34.5% in seed size and 22.1% in the ability to germinate, while the 2019 heat wave did not significantly affect either. Structural equation models indicated that similar climatic factors—including early‐season maximum temperatures and late‐season precipitation—are associated with declines in fecundity and slower germination.Conclusions: These results suggest that the consequences of heat waves will depend on the relative severity and timing of the heat wave in the growing season, and further suggest that this timing may have amplified longer‐term impacts because offspring have less provisioning. Specifically, with growing seasons shifting earlier into the spring, later or slower germination could exacerbate population extirpation risk.