Estimating nutrient budgets for prescribed thinning in a regrowth eucalyptus forest in south-west Australia

This paper reports an approach for estimating thinning-induced changes in N and P budgets in jarrah (Eucalyptus marginata) forest in the Wungong catchment of Western Australia. Two thinning strategies, herbicide injection and selective removal, were tested and nutrient budgets were constructed for soil, litter and tree biomass. The effects of thinning were evaluated based on pre-thinning biomass allocation and on reductions in biomass after thinning. Tree above ground biomass was 399 ton ha(-1), from which the selective logging removed 18.7 ton ha(-1) or 5 per cent of the N and 4 per cent of the P. Thinning residues from stem injection of herbicide contained fivefold more nutrients than the ground litter. Top soil was the primary nutrient store but only 1-2 per cent of total N and P were in available forms. In contrast, fine litter materials in thinned sites may release 4.8-5.7 kg P ha(-1) via leaching over the rainy months. Cut branches and dead stems stored 176 kg N ha(-1) and 7.0 kg P ha(-1) but would decompose over many decades. Our results indicate that both thinning strategies would increase nutrient cycling in the forest, while the implications of thinning-induced nutrient supply for the growth of remaining vegetation, understorey competition and ecosystem health need further examination.