Does nitrogen limit the growth of native eucalypt forests: some observations for mountain ash (Eucalyptus regnans)

It is often stated that the availability of N limits the rate of growth of native forests. We discuss this hypothesis with particular reference to the mountain ash (Eucalyptus regnans) forests of south-eastern Australia. The abundance of N-15 in leaves and soil of mountain ash forest is in accord wi th data for Northern Hemisphere temperate forests and for tropical forests, and indicates that N availability is relatively high. None of the nutrient elements has limited the rate of growth of mountain ash forest regeneratin g after major disturbance (clear-felling and intense wild-fire). There is s ome evidence that P may be limiting to some ecological processes (e.g. the rate of litter decomposition). We conclude that phosphorus is more likely t o be limiting than nitrogen in mountain ash forest because nitrogen cycling is conservative and continual inputs of N through biological fixation supp lement this conservative N supply, and the stands never become N-deficient. The development of methodologies to determine the rate of N-2-fixation in forests should be of high priority in ecological research.