AbstractFew studies have investigated the effects of elevated CO2on the physiology of symbiotic N2‐fixing trees. Tree species grown in low N soils at elevated CO2generally show a decline in photosynthetic capacity over time relative to ambient CO2controls. This negative adjustment may be due to a reallocation of leaf N away from the photosynthetic apparatus, allowing for more efficient use of limiting N. We investigated the effect of twice ambient CO2on net CO2assimilation (A), photosynthetic capacity, leaf dark respiration, and leaf N content of N2‐fixingAlnus glutinosa(black alder) grown in field open top chambers in a low N soil for 160 d.At growth CO2, A was always greater in elevated compared to ambient CO2plants. Late season A vs. internal leaf p(CO2) response curves indicated no negative adjustment of photosynthesis in elevated CO2plants. Rather, elevated CO2plants had 16% greater maximum rate of CO2fixation by Rubisco. Leaf dark respiration was greater at elevated CO2on an area basis, but unaffected by CO2on a mass or N basis. In elevated CO2plants, leaf N content (μg N cm−2) increased 50% between Julian Date 208 and 264. Leaf N content showed little seasonal change in ambient CO2plants. A single point acetylene reduction assay of detached, nodulated root segments indicated a 46% increase in specific nitrogenase activity in elevated compared to ambient CO2plants. Our results suggest that N2‐fixing trees will be able to maintain high A with minimal negative adjustment of photosynthetic capacity following prolonged exposure to elevated CO2on N‐