Nocturnal warming increases photosynthesis at elevated CO₂ partial pressure in Populus deltoides

•  We measured night-time respiration and daytime photosynthesis of leaves in canopies of 4 m tall cottonwood (Populus deltoides) trees to investigate the link between leaf respiration and photosynthetic capacity. •  Trees were grown at three CO₂ partial pressures [p(CO₂ )_a ] (42, 80, 120 Pa) and experimentally exposed to differing nocturnal temperatures (15, 20 or 25°C), but constant daytime temperatures (30-32°C), in a short-term whole-ecosystem environmental manipulation. •  Rates of night-time leaf dark respiration (R_d ) increased significantly at all growth CO₂ partial pressures when nocturnal temperatures were increased from 15 to 25°C. Predawn leaf nonstructural carbohydrate (soluble sugars and starch) content was significantly lower at the higher night temperatures. Photosynthetic capacity (A_max ) during the day increased significantly between 15 and 25°C at 42 and 80 Pa, but not at 120 Pa. •  These findings indicate that the previously determined relationships between elevated night-time temperature, dark respiration and increased photosynthetic capacity may also hold at elevated p(CO₂ )_a . This response may have a significant influence on plant and ecosystem carbon exchange under global change scenarios.