Changes in intrinsic water-use efficiency (WUE), the amount of carbon gain per unit transpiration during photosynthesis, will be an important part of the physiological response of plants to altered atmospheric composition and climate. Our knowledge of how WUE will adapt to increasing atmospheric CO₂ comes from two sources: plant chronologies (particularly tree rings) and experiments conducted under projected atmospheric CO₂. Several studies of projected CO₂ have found results which suggest that gains in WUE that occur under dry conditions are enhanced under elevated CO₂, a phenomenon which has not been observed in examinations of WUE in plant chronologies. However, these two types of studies typically use different methods to assess WUE: instantaneous measurements for projected CO₂ studies and measurements of ¹³C discrimination in plant chronologies. To bridge this gap, we measureed  ¹³C discrimination in a five year chronology of leaf litter collected from three species (Populus tremuloides, Betula papyrifera, Acer saccharum)  growing at the free air CO₂ enrichment (FACE) experiment in Rhinelander, WI, USA. Each of the ¹³C discrimination chronologies was then fit with several different environmental variables including soil moisture, precipitation, and humidity. We hypothesized that if instantaneous measurements were correct, we would see larger gains in WUE under dry conditions for plants growing under elevated CO₂ than for plants growing in ambient CO₂.

Results/Conclusions

Overall, growth under elevated CO₂ increased ¹³C discrimination by 8% (P < 0.04) but did not significantly affect WUE. However, the effect of elevated CO₂ on ¹³C discrimination and WUE was subject to strong annual variation, particularly for Populus and Acer. For these two species the effect of elevated CO₂ on WUE was negatively correlated with several moisture related variables, particularly average volumetric soil moisture values during the fall of the previous year and the spring of the current year.  In contrast, changes in ¹³C discrimination and WUE for Betula were more strongly related to spring humidity. Given that growth under elevated CO₂ has been known to increase moisture availability, the large increases in WUE in response to moisture stress were somewhat surprising. However, the results are in agreement with instantaneous measurements at this and other sites which suggest reduced stomatal limitation to photosynthesis, diminishing the effect of reduced stomatal conductance on carbon assimilation.