The sign and magnitude of the effects of thermal extremes on an intertidal kelp depend on environmental and biological context

Predicted shifts in mean and extreme temperatures associated with climate change can have variable impacts on organisms, and the sign and magnitude of these impacts may depend upon local context. For Hedophyllum sessile, a habitat-forming intertidal kelp, the impacts of warming may vary with local density and position in the intertidal zone. To assess the potential context-dependence of warming, we manipulated H. sessile densities across an intertidal gradient and experimentally imposed periodic thermal stress in the field. The recruitment of H. sessile juveniles was unimodally related to shore level, peaking near the center of the species’ vertical distribution and falling off at the upper and lower distributional limits. Experimental warming tended to have mildly positive effects on recruitment lower on the shore regardless of adult density, and in upper zone, high density plots. However, warming had strongly negative effects on recruitment in upper zone, low density plots. Temperature manipulations also had context-specific effects on adult plant growth; seasonal increases in blade number and canopy cover were slightly enhanced by warming in high-density plots but greatly reduced by warming in low-density plots. Finally, experimental heating had context-dependent effects on an understory herbivore, the chiton Katharina tunicata, which increased in abundance following heating in high density plots but decreased in low density plots. Our results demonstrate that extreme temperature events can affect multiple species and multiple life history stages, and that the impacts of such events can depend upon both environmental (e.g. intertidal height) and biological (e.g. adult density) context.