Repeated harvest residue burning reduces Eucalyptus pellita productivity on sandy soils in tropical monsoonal Australia

Abstract

Plantation forest site preparation through pushing and heaping of harvest residues to create windrows that are then burnt redistributes surface biomass and topsoil organic matter and reduces site nutrient capital. On sandy low nutrient soils of Melville Island (Australia), harvest residue burning has been applied to prepare sites for planting, creating areas of poor growth in non-ash bed areas and good growth in ash-bed areas. This study was established to quantify the ash-bed effect on Eucalyptus pellita growth at two sites, to investigate soil properties associated with it, and to trial corrective nutrient additions to non-ash bed areas at nine years after establishment. More rapid initial tree growth in areas where topsoil and biomass had been heaped and burnt was countered by poor growth between windrows in non-ash-bed areas. E. pellita growth in ash-bed soils averaged 25.4–26.9 m³ ha⁻¹ yr⁻¹ across the two sites (Yapilika and Kilu Impini), while in the non-ash bed soils it ranged between 11.9 and 23.0 m³ ha⁻¹ yr⁻¹. At the Yapilika site, nutrient applications (macro nutrients +/- micro nutrients) to non-ash bed E. pellita increased diameter by 18–23 % and macro nutrients applications increased tree height by 11 %. Soils sampled eight years after site preparation showed that soil organic matter, pH, and most nutrients were higher in ash-bed soils relative to non-ash bed soils. To maintain the sustainability of E. pellita plantations grown on sandy soils with low nutrient capital, it is strongly recommended to conserve harvest residues and topsoil organic matter and avoid topsoil redistribution and nutrient loss through windrowing and burning. The avoidance of burning reduces the loss of nutrients directly to the atmosphere (e.g. nitrogen (N) and sulphur (S)) and to the hydrosphere (e.g. N, phosphorus (P), calcium (Ca), and S) and optimizes nutrient retention on site.