On the variability of the ecosystem response to elevated 1 atmospheric CO 2 across spatial and temporal scales at the 2 Duke Forest FACE experiment

12 While the significance of elevated atmospheric CO2 concentration on instantaneous leaf13 level processes such as photosynthesis and transpiration is rarely disputed, its integrated 14 effect at ecosystem level and at long-time scales remains a subject of debate. In part, the 15 uncertainty stems from the inherent leaf-to-leaf variability in gas exchange rates. By 16 combining 10 years of leaf gas exchange measurements collected during the Duke Forest 17 Free Air CO2 Enrichment (FACE) experiment and three different leaf-scale stomatal 18 conductance models, the leaf-to-leaf variability in photosynthetic and stomatal 19 conductance properties is examined. How this variability is then reflected in ecosystem 20 water vapor and carbon dioxide fluxes is explored by scaling up the leaf-level process to 21 the canopy using model calculations. The main results are: (a) the space-time variability of 22 the photosynthesis and stomatal conductance response is considerable as expected. (b) 23 Variability of the calculated leaf level fluxes is dependent on both the meteorological 24 drivers and differences in leaf age, position within the canopy, nitrogen and CO2 25 fertilization, which can be accommodated in model parameters. (c) Meteorological 26 variability is playing the dominant role at short temporal scales while parameter variability 27 is significant at longer temporal scales. (d) Leaf level results do not necessarily translate to 28 similar ecosystem level responses due to indirect effects and other compensatory 29 mechanisms related to long-term vegetation dynamics and ecosystem water balance. 30