Impacts of harvesting on nutrients in a eucalypt ecosystem in southeastern Australia

Abstract

Eucalypt forests in the foothills of eastern Victoria, Australia, are being managed for sustained production of hardwood timbers on rotations of at least 80 years. Distribution of N, S, P, K, Ca, Mg, Fe, Mn, Zn, Cu and B in a eucalypt ecosystem and removals during intensive harvesting, i.e. removal of sawlogs and residual roundwood (pulpwood), were determined. Atmospheric losses of these nutrients from burning of loggin residues were estimated from published data. Total nutrient content of soils to bedrock was determined, and estimates of nutrient reserves available for plant uptake were obtained based on empirical soil tests. The likely impacts of intensive harvesting on soil reserves of these nutrients were then investigated. Nutrient removals in wood (sawlogs and pulpwood) generally represneted only a small percentage of available soil reserves. Nutrient content of bark is high compared to stemwood, and exports of nutrients (Ca and Mn in particular) as a result of harvesting are significantly reduced by debarking logs on site. Estimated losses to the atmosphere due to volatilisation and ash convection during burning of logging residues are substantially greater than removals in wood. The development of alternative methods of regeneration of these forests that excluded burning of residues would conserve considerable quantities of organic matter and nutrients. Removals of S, P, K, Mg, Fe, Mn, Zn and Cu from harvesting and burning are small compared with soil reserves and are unlikely to cause a significant decline in availability of these nutrients over a number of 80-year rotations. Removals of N are likely to be balanced by inputs of N from rainfall and biological N 2 fixation, provided there is adequate regeneration of native legumes after prescribed fires during each rotation. However, removals of Ca and B due to harvesting and burning represent a substantial proportion of exchangeable Ca and extractable B in the soil, and there is a potential risk that serious depletion of these nutrients may limit future long-term productivity of these forest ecosystems. High intensity burning of logging residues in these mixed species eucalypt forests should be avoided wherever possible.