Stage Description
Stage 0 Tree growth and litter decomposition are limited by nitrogen. No nitrate leaches into stream water because the forest has sufficient capacity to retain nitrogen.
Stage 1 When the nitrogen supply increases chronically, the foliar nitrogen content increases temporarily. Although the fine root mass changes a little, foliar biomass increases gradually, and trees grow. The forest can retain more nitrogen with increased biomass, and little nitrate leaches into stream water.
Stage 2 The nitrogen supply exceeds the forest’s absorption capacity, and forest ecosystems are limited by water and phosphorus instead of nitrogen. Foliar nitrogen content increases, and fine root mass decreases. Rapidly increased nitrification rates accelerate nitrate leakage into stream water and N2O emissions into the atmosphere.
Stage 3 Nitrate leakage into stream water and N2O emissions into the atmosphere further increase, and the forest declines because of a decrease in foliar biomass and fine root mass.