Canopy microclimate modification in central and marginal populations of a marine macroalga

The effects of environmental changes on species distribution are generally studied at large geographical scales. However, aggregations of individuals can significantly moderate the impact of the environment at smaller, organismal scales. We focused on the intertidal macroalga Fucus guiryi and carried out field and laboratory common garden experiments to evaluate how the different individual morphologies and canopy densities typical of central and peripheral populations modify microhabitat conditions and associated levels of stress. We show that F. guiryi canopies significantly alter environmental conditions (i.e., temperature, humidity and light regimes) and mitigate the levels of stress experienced by individuals within the group. Southern algae are more branched and form denser canopies but, unexpectedly, despite these considerable differences, the mitigating effects of northern and southern canopies did not differ significantly. Microhabitat conditions beneath canopies were more stressful at marginal locations, indicating that southern populations are not more effective than northern algae at mitigating the harsher climate at the edge of the species distribution. Our findings highlight the importance of assessing structural changes in aggregating species across their distribution and relating these to local climates to understand the impact of environmental changes at scales relevant to individual organisms.