Nitrogen storage and availability during stand development in a New Zealand Nothofagus forest

Stemwood production, N pools, and N availability were determined in even-aged (10, 25, 120, and >150-year-old) stands of a monospecific mountain beech (Nothofagus solandri var. cliffortioides (Hook. f.) Poole) forest in New Zealand recovering from catastrophic canopy disturbance brought about by windthrow. Nitrogen was redistributed among stemwood biomass, coarse woody debris (CWD), the forest floor, and mineral soil following disturbance. The quantity of N in stemwood biomass increased from less than 1 kg/ha in seedling stands (10 years old) to ca. 500 kg/ha in pole stands (120 years old), but decreased in mature stands (>150 years old). In contrast, the quantity of N stored in CWD declined rapidly with stand development. Although the mass of N stored in the forest floor was greatest in the pole stands and least in the mature stands, N availability in the forest floor did not vary greatly with stand development. The mass of N in the mineral soil (0-100 mm depth) was also similar for all stands. Foliar N concentrations, net N mineralization, and mineralizable N in the mineral soil (0-100 mm depth) showed similar patterns with stage of stand development, and indicated that N availability was greater in sapling (25 years old) and mature stands than in seedling and pole stands. We conclude that declining productivity in older stands is associated more with reductions in cation availability, especially calcium, than N availability.