Elevated atmospheric CO₂ (eCO₂) often increases photosynthetic CO₂assimilation (A) in field studies of temperate tree species. However, there is evidence that A may decline through time due to biochemical and morphological acclimation, and environmental constraints. Indeed, at the free-air CO₂ enrichment (FACE) study in Oak Ridge, Tennessee, A was increased in 12-year-old sweetgum trees following 2 years of ∼40 % enhancement of CO₂. A was re-assessed a decade later to determine if the initial enhancement of photosynthesis by eCO₂ was sustained through time. Measurements were conducted at prevailing CO₂ and temperature on detached, re-hydrated branches using a portable gas exchange system. Photosynthetic CO₂ response curves (A versus the CO₂ concentration in the intercellular air space (C_i); or A–C_i curves) were contrasted with earlier measurements using leaf photosynthesis model equations. Relationships between light-saturated photosynthesis (A_sat), maximum electron transport rate (J_max), maximum Rubisco activity (V_cmax), chlorophyll content and foliar nitrogen (N) were assessed. In 1999, A_sat for eCO₂treatments was 15.4 ± 0.8 μmol m⁻² s⁻¹, 22 % higher than aCO₂treatments (P < 0.01). By 2009, A_sat declined to <50 % of 1999 values, and there was no longer a significant effect of eCO₂ (A_sat = 6.9 or 5.7 ± 0.7 μmol m⁻² s⁻¹ for eCO₂ or aCO₂, respectively). In 1999, there was no treatment effect on area-based foliar N; however, by 2008, N content in eCO₂ foliage was 17 % less than that in aCO₂ foliage. Photosynthetic N-use efficiency (A_sat : N) was greater in eCO₂ in 1999 resulting in greaterA_sat despite similar N content, but the enhanced efficiency in eCO₂ trees was lost as foliar N declined to sub-optimal levels. There was no treatment difference in the declining linear relationships between J_max or V_cmax with declining N, or in the ratio of J_max : V_cmax through time. Results suggest that the initial enhancement of photosynthesis to elevated CO₂ will not be sustained through time if N becomes limited.