Impacts of Pinus plantation management on selected physical properties of soils in the coastal lowlands of south-east Queensland, Australia

Abstract

The soil physical environment is a major determinant of site productivity, which must be protected if crop yields are to be maintained and sustainable land management objectives achieved. Since the mid 1960s, the risk of soil physical degradation by compaction in forest plantations has increased due to the greater use of heavy plant and equipment and the tendency to operate machinery in all seasons. The periods of heaviest traffic in plantation management, and hence the periods of greatest compaction risk occur: (i) during establishment, (ii) during thinning and (iii) during clearfelling. This paper reports preliminary studies in which changes in surface and profile bulk densities, penetration resistance and surface and profile hydraulic conductivities were investigated during each of these periods of Pinus plantation management in south-east Queensland, Australia. The magnitude of compaction impacts, the area affected and the depth of profile affected all increased as the degree of disturbance and the level of soil moisture at the time of disturbance increased. Severe compaction was observed in the furrow bases of areas where the site was prepared with continuous high mounds. In areas thinned by removing every fifth row of Pinus and selectively cutting in the remaining rows, the area severely affected by compaction increased from 10% to 46%, and rutting depths following five round forwarder trips increased from 15 cm to 35 cm, as the soil moisture at the time of harvest increased. In two clearfall areas, 31% of the surface was severely compacted by multiple tractor passage, and a further 14% disturbed to a lesser extent by the extraction of single logs. The studies highlighted the need for improved understanding of compaction processes and consequences, of the options for minimising the magnitude and extent of compaction effects, and of techniques for compaction amelioration.