Contrasting responses in growth, photosynthesis and hydraulics of two subtropical tree species to cadmium contamination as affected by elevated CO2 and nitrogen addition.

Plant growth, photosynthesis, and hydraulics are affected by heavy metals but also by elevated atmospheric CO2 concentration (e[CO2]) and nitrogen (N) deposition. However, few studies have investigated the response of woody species to the combined effects of these three factors. We conducted an open-top chamber experiment with two common subtropical trees (Acacia auriculiformis and Syzygium hainanense) to explore the effects of cadmium (Cd)-contamination, e[CO2], and N addition on plant eco-physiological traits. We found that the growth of A. auriculiformis was insensitive to the treatments, indicating that it is a Cd-tolerant and useful afforestation species. For S. hainanense, in contrast, e[CO2] and/or N addition offset the detrimental effects of Cd addition by greatly increasing plant biomass and reducing the leaf Cd concentration. We then found that e[CO2] and/or N addition offset the detrimental Cd effects on S. hainanense biomass by increasing its photosynthetic rate, its N concentration, and the efficiency of its stem water transport network. These offsetting effects of e[CO2] and/or N addition, however, came at the expense of reduced xylem hydraulic safety resulting from wider vessels, thinner vessel walls, and therefore weaker vessel reinforcement. Our study suggests that, given future increases in global CO2 concentration and N deposition, the growth of Cd-tolerant tree species (like A. auriculiformis) will be probably stable while the growth of Cd-sensitive tree species (like S. hainanense) might be enhanced despite reduced hydraulic safety. This also suggests that both species will be useful for afforestation of Cd-contaminated soils given future global change scenarios.