Changes in leaf nutrient traits and photosynthesis of four tree species: effects of elevated [CO2], N fertilization and canopy positions

Leaf traits of trees exposed to elevated [CO2] in association with other environmental factors are poorly understood in tropical and subtropical regions. Our goal was to investigate the impacts of elevated [CO2] and N fertilization on leaf traits in southern China. Four tree species, Schima superba Gardn. et Champ. (S. superba), Ormosia pinnata (Lour.) Merr (O. pinnata), Castanopsis hystrix AC. DC. (C. hystrix) and Acmena acuminatissima (Blume) Merr. et Perry (A. acuminatissima) were studied in a factorial combination of atmospheric [CO2] (ambient at ∼390 μmol mol−1and elevated [CO2] at ∼700 μmol mol−1) and N fertilization (ambient and ambient + 100 kg N ha−1 yr−1) in open-top chambers in southern China for 5 years. Leaf mass per unit leaf area (LMA), leaf nutrient concentration and photosynthesis (Asat) were measured. Results indicated that leaf traits and photosynthesis were affected differently by elevated [CO2] and N fertilization among species. Elevated [CO2] decreased LMA in all species, while N fertilization did not affect LMA. Leaf mass-based N concentration (NM) was significantly greater in O. pinnata and C. hystrix grown in elevated [CO2] but was lower in S. superba. Leaf mass-based P concentration (PM) was significantly greater in C. hystrix and A. acuminatissima exposed to elevated [CO2] but was lower in S. superba. N fertilization significantly increased PM in O. pinnata but decreased PM in S. superba. Photosynthetic stimulation in O. pinnata, C. hystrix and A. acuminatissima was sustained after 5 years of CO2 fumigation. N fertilization did not modify the effects of elevated [CO2] on photosynthesis. Leaf traits (NM, NA, PM, PA) and light-saturated photosynthesis were decreased from the upper to lower canopy. Canopy position did not alter the responses of leaf traits and photosynthesis to elevated [CO2]. Results suggest that photosynthetic stimulation by elevated [CO2] in native species in subtropical regions may be sustained in the long term.