The effects of CO₂ and nutrient fertilisation on the growth and temperature response of the mangrove Avicennia germinans

In order to understand plant responses to both the widespread phenomenon of increased nutrient inputs to coastal zones and the concurrent rise in atmospheric CO₂ concentrations, CO₂–nutrient interactions need to be considered. In addition to its potential stimulating effect on photosynthesis and growth, elevated CO₂ affects the temperature response of photosynthesis. The scarcity of experiments testing how elevated CO₂ affects the temperature response of tropical trees hinders our ability to model future primary productivity. In a glasshouse study, we examined the effects of elevated CO₂ (800 ppm) and nutrient availability on seedlings of the widespread mangrove Avicennia germinans. We assessed photosynthetic performance, the temperature response of photosynthesis, seedling growth and biomass allocation. We found large synergistic gains in both growth (42 %) and photosynthesis (115 %) when seedlings grown under elevated CO₂ were supplied with elevated nutrient concentrations relative to their ambient growing conditions. Growth was significantly enhanced under elevated CO₂ only under high-nutrient conditions, mainly in above-ground tissues. Under low-nutrient conditions and elevated CO₂, root volume was more than double that of seedlings grown under ambient CO2 levels. Elevated CO₂ significantly increased the temperature optimum for photosynthesis by ca. 4 °C. Rising CO₂ concentrations are likely to have a significant positive effect on the growth rate of A. germinans over the next century, especially in areas where nutrient availability is high.