Along temperate to polar rocky shorelines, large brown algae known as kelps form marine forests which provide a three-dimensional habitat for many associated species. Ocean warming is posing an increasing threat to kelps at their warm distributional edges and first range shifts have been recorded. For these sessile species unable to migrate, trait variability due to phenotypic plasticity and genetic variation is an important mechanism of response to environmental change. The aim of this thesis was to produce a comprehensive assessment of the variation and plasticity of thermal traits across populations and life cycle stages of a keystone marine forest species, the cold-temperate to Arctic kelp Laminaria digitata. Using physiological response parameters, population genetics and transcriptomics, I present evidence for four levels of thermal trait variability in a marine forest key species. I describe (1) genetic and physiological differentiation along the species' distribution range, (2) genetic variation for phenotypic plasticity among genotypes, (3) carry-over effects over reproduction and individual ontogeny, and (4) the production of new phenotypes by outbreeding among distant lineages. Integrating these responses into a framework of seasonal temperature variation and predictions of ocean warming showed that L. digitata, as a species with a cold-temperate thermal profile, is adapted well to the current conditions along its distributional range, but may not be equipped to respond to rapid climate change at its warm range edges. The concepts investigated in this thesis provide further insight into trait variability as a mechanism of marine forest resilience and offer intriguing features for mariculture and conservation efforts.