Results/Conclusions: Carbon isotope discrimination varied between 9.6 and 22.4 ‰ (VPBD). Discrimination decreased non-linearly as a function of light, approaching an asymptote above ~ 500 umol m-2 sec-1. In contrast, we observed no consistent response to temperature. Carbon isotope discrimination reached steady state rapidly (70 ± 7 s) in response to a step change in light, whereas the photosynthetic gas exchange response was significantly slower (1205 ± 288 s). The consistent differences in the temporal response of photosynthesis versus carbon isotope fractionation raise compelling questions about the underlying biochemical and physical processes driving the processes, as well as how to account for such differences in other isotope applications. Constraining the magnitude and time of the isotopic response to changing environmental conditions can contribute to our understanding of underlying physiological processes at the scale of the leaf and plant, as well as how to model patterns at the scale of the ecosystem.