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 with 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 regenerating after major disturbance (clear-felling and intense wild-fire). There is some evidence that P may be limiting to some ecological processes (e.g. the rate of litter decomposition). We conclude that phosphorus is more likely to be limiting than nitrogen in mountain ash forest because nitrogen cycling is conservative and continual inputs of N through biological fixation supplement 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.