The d18O composition of water and CO2 exchange at smaller scales (leaf and ecosystem) can be affected by changes in atmospheric water vapour at larger (regional) scales. During a sampling campaign in a beech forest in Germany in August 2002, we encountered such a large scale change when dry sunny weather was followed by a large storm system with heavy rains leading to floods across Europe. During the transition, canopy vapour d18O decreased at least 2 permil, and the d18O composition of leaf water then reflected isotopic exchange with this depleted vapour due to the high humidity. Hence, bulk leaf water was substantially more depleted at night compared to the first, sunny period, and showed virtually no evaporative enrichment during the day. Values of 18O discrimination during CO2 exchange for photosynthesis strongly decreased from the sunny to the wet period, whereas those for nocturnal respiration increased by a factor of 2 to 4. Model simulations indicated that the small positive foliage isoflux during the day was offset by the negative isoflux at night. As a consequence, the d18O of CO2 in canopy air decreased by about 3 permil. The d18O signatures of canopy water and CO2 thus reflected the transition from local water to the regional regime of depleted water deposited across the area by the storm.