Impacts on stream and groundwater quality during the inter-rotation phase of a Pinus plantation in the coastal lowlands of south-east Queensland

Abstract

The impacts of exotic Pinus plantation management on water quality in the coastal lowlands of south-east Queensland were studied for 6 years in a large catchment. The study utilised nested catchments with a 395 ha upper ‘control’ sub-catchment which drained to a lower 904 ha sub-catchment. Stream monitoring stations were constructed at the outlet of each sub-catchment. A combination of regular grab samples, automated sampling (during flood events) and real time sampling with in situ instruments was carried out to monitor trends in the key water quality parameters of nitrogen, phosphorus and suspended solids. Water quality in surrounding regional streams was also monitored for a 12-month period. A network of piezometers within the study area was used to monitor groundwater depth and quality. Cross-sectional surveys of watercourses and adjoining riparian zones were carried out before and after a harvest treatment to assess the stability of these important areas of the drainage basin.

After a 4-year calibration, part of the lower sub-catchment was harvested by clearfelling 80 ha of a mature 36-year-old stand and thinning 250 ha of a 24-year-old stand. The clearfelled area was subsequently re-established with Pinus plantation. Both streamflow and export loads of nitrogen, phosphorus and suspended solids were highly episodic in keeping with the ephemeral nature of streams originating in the coastal lowlands region. Annual export loads for nitrogen, phosphorus and suspended solids ranged from 0 to 8.4, 0 to 0.5 and 0 to 606 kg ha-1 respectively. These loads were within the range reported for forest-dominated catchments both in Australia and overseas and are recognised as the lowest ranked land-use with respect to impacts on water quality per se. Despite a major rainfall and flood event shortly after the clearfelling and thinning operations, harvest did not affect key water quality parameters measured, nor groundwater quality. Cross-sectional surveys of the watercourse protection zones found these areas to be stable in the harvested area, but subject to a degree of erosion in the upper ‘control’ sub-catchment; this was thought to be due to a combination of the inverse relationship between sediment yield and catchment size as well as poor management practices in the 1970s.

The results of this study should serve as a benchmark for important ‘best management practices’ in relation to: observance of watercourse protection zones; harvesting techniques based on processing at stump and retention of in situ harvest debris; and appropriate site preparation practices with regard to erosion hazard and maintenance of in situ harvest debris.