The interactions between plants and their insect herbivores are among the most common ecological interactions. By feeding on plant tissue, insects can influence plant physiology and chemistry, with potential implications for other herbivores associated with the same host plant and for ecosystem-level processes mediated by the plant. In this thesis, I studied the community and ecosystem-level effects of insect herbivores associated with the pedunculate oak (Quercus robur, L.) in temperate woodlands in Oxfordshire, UK. First, I studied how early-season moth caterpillars affect photosynthesis and selected physical and chemical leaf traits. I then examined how herbivore-induced changes in the host plant affect tree performance, the structure of the sessile herbivore community associated with the host tree, canopy-level carbon sequestration, and forest-level primary productivity. My work shows that moth caterpillars can substantially reduce the photosynthetic rate of their host plant. These leaf-level changes can reduce canopy-level carbon sequestration, and therefore affect the primary productivity of the ecosystem. In this system, insect herbivory did not affect any of the studied leaf traits, tree performance, or the structure of the herbivore community. Leaf chemical composition varied substantially between individual trees, was affected by the growth location of the tree, and covaried significantly with the associated insect community. My results suggest that insect herbivores can have important ecosystem-level effects by inducing small-scale changes in the primary metabolism of their host plant. My work also emphasises the importance of abiotic conditions in influencing plant chemistry, and plant chemical composition in shaping the plant-associated insect community. I stress the need to quantify the effects of insect herbivores on carbon cycling in other systems and to include these effects in models on ecosystem productivity and biosphere-atmosphere interactions.