Phenotypic Plasticity in Sub-Antarctic Plants: Costs, Environmental Complexity, and the Facilitative Role of Trait Integration

Phenotypic plasticity is one plant response to climate warming, but it may incur costs and be either limited or facilitated by trait integration or simultaneous environmental shifts such as shading. Empirical studies addressing these costs and limitations across populations or maternal lineages, and how they vary with environmental context, remain scarce. We studied three plant species from the sub-Antarctic Kerguelen Islands, a region undergoing rapid warming, growing seeds from multiple source populations under different temperature and light treatments. We measured plasticity, performance, and morphological integration in young plants. Plants from more plastic source populations performed worse, while those from more integrated populations were more plastic, consistent with the facilitation hypothesis. Shading reduced plasticity in response to warming, and combined shading and warming reduced performance more than single treatments, contrasting with recent findings in Mediterranean plants, where multiple stressors did not limit adaptive plasticity. Our results suggest that phenotypic integration may favour rather than limit plasticity. However, plasticity under climate warming may also be constrained by increased shading from competitors, and does not guarantee success given induced performance costs.