Growth of a deciduous tree seedling community in response to elevated CO2 and nutrient supply

In a FACE experiment, one year old alder, ash, beech, and oak seedlings were planted together in tubs containing calcareous sandy alluvial soil with or without a slow release NPK-fertilizer, and exposed to ambient and elevated CO2 concentrations (360 and 600 μl mol−1) for 20 weeks. In addition to the fertilizer, all tubs received 15N-ammonium nitrate as a marker. Elevated CO2 significantly increased biomass production in alder, but had no effect on oak and ash. In beech, due to disease and mortality in all treatments, any possible effects were obscured. The addition of fertilizer had no effect on biomass production in alder, but increased production in oak and ash significantly. In oak a treatment synergism may be present. The non-appearance of a synergistic CO2 and fertilizer effect in alder may be explained by a fertilizer induced reduction of the N-fixing root-nodule biomass concurrent with a switch of the N-assimilation from atmospheric N to soil N supply, as the δ15N measurements in the leaves of alder as opposed to oak indicate. Although elevated CO2 resulted in a significant biomass increase in alder, it did not lead to an appreaciable increase in the proportional presence of the species as measured on total plant biomass in the tub. Increasing the nutrient supply in the soil, however, did lead to appreciable gains in the proportional presence of oak and ash.