Rangewide responses of Mimulus cardinalis to an extreme heat event

Extreme events are an understudied aspect of ongoing anthropogenic climate change that could play a disproportionate role in the threat that rapid environmental shifts pose to natural populations.

We exposed plants originating from seeds that were harvested before (ancestors) and after (descendants) multiple extreme heat events from six populations across the range of Mimulus cardinalis (Phyrmaceae) to a short-term heat-wave treatment in controlled growth chamber environments. We assessed physiological, performance, and functional responses (stomatal conductance, leaf temperature deficit, photosystem II efficiency, relative growth rate, specific leaf area, and leaf dry matter content) to the heat-wave treatment, along with evolutionary responses (differences between ancestors and descendants) of M. cardinalis populations to the recent natural extreme heat event.

Plants in the heat-wave treatment increased their overall performance, and the magnitude of increase was generally greatest among trailing-edge populations. Despite limited overall trait differences between ancestors and descendants, there was some evidence of divergent evolutionary responses among regions to the natural extreme heat event. However, we did not find evidence of adaptive evolution that affected how M. cardinalis populations responded to the heat-wave treatment.

These results demonstrate that many M. cardinalis populations may reside in environments that are below their optimum average temperature, revealing potential resiliency to future warming. However, limited evolutionary responses in M. cardinalis to the recent extreme heat wave could still indicate potential for future vulnerability to extreme climate events of increased intensity, frequency, and duration.