Heatwaves decrease fitness and alter maternal provisioning in natural populations of Mimulus guttatus

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