Effects of site preparation on runoff, erosion, and nutrient losses from Pinus plantations established on the coastal lowlands of south-east Queensland, Australia

Abstract

Site preparation practices used during establishment of second rotation Pinus plantations on the coastal lowlands of south-east Queensland became more conservative during the 1990s. Traditionally, site preparation included stick raking of residue into windrows, followed by burning. Areas subject to periodic saturation were then cultivated to form continuous high mounds, while better drained sites were contour strip cultivated. These practices were replaced by retention of residue biomass on hillslopes. Similarly, intensive site disturbance was increasingly replaced by tillage of the immediate planting zone only and, as far as possible, avoidance of continuous furrows, major drainage structures, and high mounding works.

In the study reported here, simulated rainfall and overland flows were used to assess the impacts of the earlier, less-conservative site preparation practices on hillslope soil loss, nutrient loss, and the potential for off-site nutrient movement.

Where residue from clearfell harvesting of the first rotation was not removed, runoff, erosion, and nutrient loss were negligible. Harvesting tended to distribute residue widely across hillslopes, and, together with naturally occurring understorey biomass, it formed a largely continuous groundcover.

Highest concentrations of sediment in overland flow were recorded for hillslopes in the newly stick-raked condition. Sediment concentrations in overland flows applied to properly designed and constructed high mounds were low, even for large flows simulating relatively long furrow lengths. However, sediment in rainfall-induced runoff from mounded areas was observed to be nutrient enriched, indicating potential for an off-site nutrient pollution risk, at least prior to establishment of grass cover along furrows. Nutrient enrichment was correlated with enrichment of finer size classes of sediment in runoff.

The work reported in this paper vindicated the move away from traditional residue removal and site preparation practices to an approach based on residue retention, minimum site disturbance, and, wherever possible, ‘spot’ rather than continuous mounding/cultivation.