GROWTH OF A DECIDUOUS TREE SEEDLING COMMUNITY IN RESPONSE TO ELEVATEDCO2 AND NUTRIENT SUPPLY

In a FACE experiment, one year old alder, ash, beech, and oak seedling s were planted together in tubs containing calcareous sandy alluvial s oil with or without a slow release NPK-fertilizer, and exposed to ambi ent and elevated CO2 concentrations (360 and 600 mu l mol(-1)) for 20 weeks. In addition to the fertilizer, all tubs received N-15-ammonium nitrate as a marker. Elevated CO2 significantly increased biomass prod uction in alder, but had no effect on oak and ash. In beech, due to di sease and mortality in all treatments, any possible effects were obscu red. 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 synergis tic CO2 and fertilizer effect in alder may be explained by a fertilize r induced reduction of the N-fixing root-nodule biomass concurrent wit h a switch of the N-assimilation from atmospheric N to soil N supply, as the delta(15)N measurements in the leaves of alder as opposed to oa k indicate. Although elevated CO2 resulted in a significant biomass in crease in alder, it did not lead to an appreciable increase in the pro portional presence of the species as measured on total plant biomass i n the tub. Increasing the nutrient supply in the soil, however, did le ad to appreciable gains in the proportional presence of oak and ash.