Effects of elevated CO2 concentration and nutrition on net photosynthesis, stomatal conductance and needle respiration of field-grown Norway spruce trees

To study the effects of elevated CO2 on gas exchange, nonstructural carbohydrate and nutrient concentrations in current-year folia,ae of 30-year-old Norway spruce (Picea abies (L.) Karst.) trees, branches were enclosed in ventilated, transparent plastic bags and flushed with ambient air (mean 370 mu mol CO2 mol(-1); control) or ambient air + 340 mu mol CO2 mol(-1) (elevated CO2) during two growing seasons. One branch bag was installed on each of 24 selected trees From control and fertilized plots. To reduce the effect of variation among trees, results from each treated branch were compared with those from a control branch on the same whorl of the same tree. Elevated CO2 increased rates of light-saturated photosynthesis on average by 55% when measured at the treatment CO2 concentration. The increase was larger in shoots with high needle nitrogen concentrations than in shoots with low needle nitrogen concentrations. However, shoots grown in elevated CO2 showed a decrease in photosynthetic capacity compared with shoots grown in ambient CO2. When measured at the internal CO2 concentration of 200 mu mol CO2 mol(-1), photosynthetic rates of branches in the elevated CO2 treatments were reduced by 8 to 32%. The elevated CO2 treatment caused a 9 to 20% reduction in carboxylation efficiency and an 18% increase in respiration rates. In response to elevated CO2, starch, fructose and glucose concentrations in the needles increased on average 33%, whereas concentrations of potassium, nitrogen, phosphorus, magnesium and boron decreased. Needle nitrogen concentrations explained 50-60% of the variation in photosynthesis and CO2 acclimation was greater at low nitrogen concentrations than at high nitrogen concentrations. We conclude that the enhanced photosynthetic rates found in shoots exposed to elevated CO2 increased carbohydrate concentrations, which may have a negative feedback on the photosynthetic apparatus and stimulate cyanide-resistant respiration. We also infer that the decrease in nutrient concentrations of needles exposed to elevated CO2 was the result of retranslocation of nutrients to other parts of the branch or tree.