Physiological responses of Abies faxoniana populations from different elevations to increased CO2 and N application

The altitude-related responses to the increased application of CO₂, N, and their combination were investigated in two Abies faxoniana populations, which originated from a subalpine coniferous forest at elevations of 2,580 and 3,200 m using closed-top chambers. The two contrasting populations were subjected to two CO₂ regimes (350 and 700 µmol mol^?1) and two N levels (0 and 5 g N m^?2 year^?1). Their net photosynthetic rate, non-structural carbohydrate concentration, and photosynthetic N use efficiency (PNUE) increased under elevated CO₂. However, the increases detected in the high-elevation (HE) population were significantly greater than those found in the low-elevation (LE) population. Under elevated CO₂ and N application, the maximal carboxylation rate (V _cmax) increased in HE population, whereas no effects were found on V _cmax in LE population. The C to N ratio decreased under N application in both populations. N application also induced the HE population to show greater increases in free amino acids, soluble proteins, N concentration, and PNUE than LE population. These results suggested that the population from HE was more sensitive to elevated CO₂ and (or) N application than LE population. Results of this study provided valuable knowledge for predicting forest development under increased atmospheric CO₂ concentration and (or) N deposition.