The influence of N addition on nutrient content, leaf carbon isotope ratio, and productivity in a Nothofagus forest during stand development

To test whether increased nitrogen (N) availability might increase productivity in maturing mountain beech (Nothofagus solandri var. cliffortioides (Hook. f.) Poole) forest in central South Island, New Zealand, we applied N to 25-year-old sapling and 125-year-old pole stands. Nitrogen fertilizer increased foliar and fine-root N concentrations, fine-root growth, and leaf litter production in both sapling and pole stands but had no effect on stem basal area increment or individual leaf area, and it decreased individual leaf mass marginally. Heavy flowering and seeding occurred in the second year after fertilizer was applied, and N increased production of both. Leaf litter production and flowering responded similarly to N in sapling and pole stands, but N increased fine-root and seed productivity more in pole stands than in sapling stands, confirming our hypothesis that productivity of pole stands was more limited by low N availability. Resource allocation to fine roots and seed production may have restricted stem basal area increment response to N in the short term. Pole stands had higher leaf (delta)13C values than sapling stands. It is concluded that both low N availability and moisture stress may contribute to the decline in productivity and wood biomass previously found in mature mountain beech stands.