To study the effects of rising CO₂ levels on water usage of old-growth temperate oak forest, we monitored trees in an open-air elevated CO₂ experiment for 5 years. We found 4 %–16 % leaf-on season reduction in daylight water usage for ~35% increase in atmospheric CO₂. July-only reduction varied more widely. Tree water usage depended on tree size, i.e. stem size and projected canopy area, across all treatments. Experimental infrastructure increased the water usage of the trees in leaf-on season.