Demographic consequences of an extreme heatwave are mitigated by spatial heterogeneity in an annual monkeyflower

Heatwaves are becoming more frequent and intense with climate change, but the demographic and evolutionary consequences of heatwaves are rarely investigated in herbaceous plant species. We examine the consequences of a short but extreme heatwave in Oregon populations of the common yellow monkeyflower (Mimulus guttatus) by leveraging a common garden experiment planted with range-wide populations and observational studies of eleven local populations. In the common garden, 89% of seedlings died during the heatwave including >96% of seedlings from geographically-local populations. Some populations from hotter and drier environments had higher fitness, however others from comparable environments performed poorly. Observational studies of local natural populations drastically differed in the consequences of the heatwave - one population was completely extirpated and nearly half had a >50% decrease in fitness. However, a few populations had greater fitness during the heatwave year. Differences in mortality corresponded to the impact of the heatwave on soil moisture - retention of soil moisture throughout the heatwave led to greater survivorship. Our results suggest that not all populations experience the same intensity or degree of mortality during extreme events and such heterogeneity could be important for genetic rescue or to facilitate the distribution of adaptive variants throughout the region.