Intraspecific facilitation in an intertidal foundation species plays fundamental role in promoting resistance to extreme climatic events

Atmospheric and marine heatwaves are becoming more frequent, and severe, increasingly pushing organisms past their critical thermal limits and threatening the persistence of entire populations. Under greater levels of stress, positive species interactions (e.g. facilitation) are predicted to become increasingly important in promoting population and community-level resistance and resilience. While ecological interactions are important in driving the structure of marine benthic communities, the strength and nature of facilitative interactions under realistic stress conditions have rarely been empirically tested in these systems. Using an intertidal rocky shore dominated by the canopy-forming macroalga Fucus serratus as a model system, we conducted a novel manipulative field experiment to simulate a realistic three-day atmospheric heatwave and examined physiological responses of F. serratus fronds under both high and low-density treatments. We recorded a prominent facilitative interaction between canopy-forming plants and their sub-canopy counterparts, which almost completely ameliorated thermal stress. This positive interaction was evident at low density treatments and despite overlying air temperatures far exceeding previous estimates of the critical thermal maximum of F. serratus. However, despite sustained intraspecific facilitation, consecutive exposures to high air temperatures led to reduced resilience of canopy-forming plants. Our results have direct implications for assessing population-level vulnerability to current and future warming trends. We show 1) intraspecific facilitation will likely play a pivotal role in mediating stress, even at lower canopy densities, allowing the persistence of foundation species under extreme warming, and 2) consecutive exposures to stressful temperatures will reduce thermal resilience, jeopardizing positive ecological interactions. More broadly, incorporating facilitative interactions under extreme climatic events into predictions of responses to climate change will improve the accuracy of future vulnerability assessments.