The advantage of using the wind as the carrier gas, as in FACE, is that the perturbation of the natural microclimate is minimal in contrast to enclosure methods. The disadvantages are that a dilution gradient is generated across the treatment plot and the system is dependent on continuous air movement. So, although the centre of the plot is maintained close to the target, the upwind site may be 100 μmol mol−1 above and the downwind 100 μmol mol−1 below the target. With a strong prevailing wind, a gradient effect would occur across each plot. However, analysis of isotopic composition across FACE plots shows a remarkable uniformity. This suggests that although transient gradients occur, when averaged over growing seasons, these gradients are not detectable