Evaluation of the use of pre- and post-harvest bulk density measurements in wet Eucalyptus obliqua forest in Southern Tasmania

Abstract

Abstract Criteria and indicators under the Montreal process are intended to provide a common framework for describing and evaluating progress towards sustainable forest management. In 1998 a nationally coordinated project was initiated to evaluate proposed Montreal indicators 4.1.d and 4.1.e and to provide advice to state agencies on how these indicators could be applied. Interim indicator 4.1.e quantifies ‘ Proportion of harvested forest area with significant change in bulk density of any horizon of the surface (0–30 cm) soil ’. Application and evaluation of this interim indicator was conducted as part of the Warra Silvicultural Systems Trial, in Tasmania, Australia. This forest was composed of wet mixed regrowth/mature Eucalyptus obliqua forest with a dense rainforest understorey. Pre-harvest soil sampling was conducted in four stands followed by post-harvest sampling at two of the sites. Pre-harvest sampling according to soil drainage class, showed that there was a high natural variation in bulk density within individual stands and between neighbouring stands. Post-harvest sampling was stratified according to major operational areas and disturbance classes, including snig tracks (major and minor), firebreaks, harvested and unharvested areas, landings and access roads. Forest harvesting substantially increased the variability of soil bulk density. Though moderate and severe disturbance of the snig tracks and firebreaks increased average soil bulk density, changes in soil bulk density were not uni-directional. Many of the post-harvest samples showed a marked decline in soil bulk density due to the incorporation of litter and slash in the upper soil layer. Bulk density changed most in the upper 0–100 mm layer of soil. Hence there is reasonable argument that monitoring changes in bulk density should be limited to this depth. This would substantially reduce the cost of replicate sampling to the 300 mm depth. The high natural variation found for soil bulk density and the fact that changes are not uni-directional means that it is unlikely that the interim indicator proposed can be implemented in a practical, sensitive and cost-effective manner in the forest type examined in this study.