Growth response to step‐decrease in nutrient availability in small birch (Betula pendula Roth)

Changes in the uptake and allocation of carbon and nitrogen, after a step‐decrease in nutrient availability, were investigated in small birch ( Betula pendula Roth). By demonstrating stable nutrition, before and after the decrease in nutrient supply, it was possible to eliminate the effects of plant size and age. Immediately following the step‐decrease in nutrient availability, net nitrogen uptake to leaves and the relative rate of increase in shoot area tended to zero. Although photosynthetic rate per shoot area decreased, carbon uptake remained in excess of that used in structural growth and respiration. More of the excess carbon was accumulated as starch in leaves than in roots. After a lag phase, the relative rates of increase in plant dry matter, starch amount, net nitrogen uptake to leaves and shoot area development equalled that of the reduced rate of nutrient supply. It is concluded that the reduction in plant relative growth rate was much more attributable to the reduced allocation of photosynthate to leaf area growth than to the reduction in photosynthesis per shoot area.