The hypothesis that allocation between the main tree components is controlled by the relative uptake rates of carbon dioxide and nutrient was tested by growing one-year-old sycamore seedlings (Acer pseudoplatanus L.) at elevated and ambient CO₂ concentrations and at two nutrient addition rates which were modelled in accordance with a predicted optimal growth function. Elevated CO₂ concentration significantly increased the leaf dry mass but did not significantly change the dry mass of any other component. In contrast nutrient addition rate had large effects on all biomass components with the exception of the root. Allocation was strongly affected by nutrient addition rate although CO₂ treatment only affected the allocation to the stem. Nutrient concentrations were reduced by growth at elevated CO₂ concentrations. Leaf starch concentrations were larger at elevated CO₂ concentrations. There were no differences in soluble leaf carbohydrate concentrations between treatments. There was evidence of downregulation of photosynthesis at the low nutrient addition rate but not at elevated CO₂ concentrations. In conclusion, the growth concentration of CO₂ and nutrient and water availabilities are important in the control of allocation. Increases in atmospheric CO₂ concentrations are likely to be accompanied by shifts in allocation towards the root and stem although the magnitude of these changes will be determined by the nutrient availability of soils. The effects of perturbations in these variables are also determined by differences in growth and allocation between clones of the same species, between species and by the development of allocation patterns which are dependent on phenology. The functional balance is useful in predicting changes in allocation, although it does not fully explain the observed patterns.