Carbon dioxide stimulation of photosynthesis in Liquidambar styraciflua is not sustained during a 12-year field experiment

Elevated atmospheric CO₂ (eCO₂) often increases photosynthetic CO₂ assimilation (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 (Ci); or A-Ci curves) were contrasted with earlier measurements using leaf photosynthesis model equations. Relationships between light-saturated photosynthesis (Asat), maximum electron transport rate (Jmax), maximum Rubisco activity (Vcmax), chlorophyll content and foliar nitrogen (N) were assessed. In 1999, Asat for eCO₂ treatments was 15.4 ± 0.8 μmol m⁻² s⁻¹, 22 % higher than aCO₂ treatments (P < 0.01). By 2009, Asat declined to