Allocation in tree seedlings

A major limitation to understanding tree growth processes is our poor grasp of the mechanisms which control the allocation of recent assimilates to different tree components. Understanding allocation is important as an accurate model of growth depends on the sizes of the various tree components and, because relative changes in allocation may lead to shifts in the global carbon and nutrient cycles. The effect of increased CO2 concentrations on the processes of growth and allocation are of particular importance in our understanding of the impacts of the continued increases in global atmospheric CO2 concentrations. Recent data suggest that the results of experiments on the impact of elevated CO2 concentrations can only properly be interpreted where the interactive effects with nutrient availability are known It was hypothesised that the allocation process is regulated by a balance between the rate of assimilation of two or more interdependent resources. The rate of carbon uptake depends on the nitrogen status of the leaves and the amount of nitrogen uptake depends on the amount of carbon assimilate available for root growth. The concept of a functional balance suggests that the plant requires resources to be in a proportional balance. In this study growing environments were manipulated to change the relative uptake rate of three key resources carbon dioxide, nutrients and water to see whether expected shifts in allocation were observed. 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-yearold 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 CO 2 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 CO 2 concentrations. Leaf starch concentrations were larger at elevated CO 2 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. Peach seedlings (Prunus persica (L.) Batsch) were grown in elevated and ambient CO2 concentrations and at two levels of water availability for one year to test the hypotheses that carbon dioxide and water availability control the relative allocation between the root and leaves, and between the leaves and stem in accordance with the functional balance. Half of the plants were grown with soil water at field capacity while water was withheld from the other half during a four week drying cycle. Elevated CO2 concentrations increased the biomass of all components with the exception of the fine roots. The impact of withholding water on the dry mass was much larger than the CO2 treatment effect although growth at elevated CO2 concentrations ameliorated the effect of low water availability. There were only