Rising CO_2 influences terrestrial ecosystems through enhanced photosynthesis and changing water use efficiency. How these proximal physiological effects cascade through multiple complex ecosystem processes ultimately can change competition between species, ecosystem productivity, water cycling, and nutrient cycling, change crop production and nutrition and strongly influence how the land carbon sink changes in the future. These physiological and ecological effects of elevated CO_2 have the potential to impact how ecosystems respond to extreme events, modify ecosystem resilience to agents of global change, and has the potential to impact human systems through changing water cycling and influencing agricultural yield and nutritional value. Despite the ubiquity of CO_2 as an agent of global change and its influence on all ecosystems and agroecosystems worldwide, our current state of knowledge remains incomplete. Evidence is often conflicting and context dependent. Some forest Free Air CO_2 Enrichment (FACE) Experiments show biomass gains, while others show none. Many tree-ring studies show historical changes in water use efficiency but no detectable change in growth, while global scale atmospheric composition studies demonstrate an increasing global carbon sink in which it is though that CO₂ fertilisation plays a leading role. In agricultural systems increased photosynthesis and water use efficiency is thought to be a good thing, but studies show that the nutritional value of crops may be adversely affected. In this session we aim to bring together speakers from diverse disciplines to begin to synthesize conflicting evidence and integrate current knowledge of CO_2 effects on terrestrial ecosystems.