Background/Question/Methods: Photosynthetic down-regulation in plants exposed to elevated [CO₂] has been observed in several ecosystem types worldwide. However, the interactive effect of climate warming on CO₂-induced down-regulation is uncertain. Understanding the pattern and extent of down-regulation associated with rising [CO₂] is essential because it ultimately will determine the potential terrestrial feedback to global change. We measured several leaf-level photosynthetic properties, including light-saturated CO₂ assimilation rate (A_sat), in the C3 perennial grass Pascopyrum smithii after three years of exposure to elevated [CO₂] and warming in the Prairie Heating and CO₂ Enrichment (PHACE) experiment in southeastern Wyoming, USA. This factorial experiment combines FACE technology (ambient and elevated [600 ppm] CO₂ concentration) and experimental warming with infrared heaters (1.5°C daytime, 3°C nighttime) to evaluate direct, indirect and interactive effects of global change factors on semi-arid grassland structure and function. Here “control” treatments refer to both ambient temperature and [CO₂].

Results/Conclusions: Warming by itself did not influence A_sat. During the peak growing season in June, 2009, A_sat averaged 19.8 (SE = 0.4) µmol m^-2 s^-1 in the warmed treatment plots and 18.4 (SE = 1.2) µmol m^-2 s^-1 in control plots. However, A_sat in the elevated [CO₂] treatment (12.4 [SE = 1.3] µmol m^-2 s^-1) was 33 and 37 % lower than in control and warmed treatment plots, respectively. Plants exposed to combined elevated [CO₂] and warming had only slightly lower A_sat values (17.4 [SE = 0.9] µmol m^-2 s^-1) compared to plants in warmed only or control treatments. These patterns of A_sat were similar to those observed earlier and later in the growing season, and differences among treatments in water relations could not account for differences in A_sat; plants in elevated [CO₂] treatments had the highest leaf water potential values. We hypothesize that reduced A_sat under elevated [CO₂] is driven by non-stomatal photosynthetic limitations, and that warming nearly eliminated this down-regulation. These results will be presented in addition to an analysis of the biochemical properties explaining photosynthetic responses. The potential consequences of warming-induced photosynthetic stimulation are complex, and may ultimately depend on plant nitrogen use and nitrogen availability.