Eucalyptus pellita harvest residue management in sandy tropical soils – nutrient content, decomposition and potential emissions from burning

Plantation forestry on sandy, nutrient poor soils has been developed over the last few decades in seasonally dry tropical monsoon environments. Despite the known benefits of harvest residue conservation between rotations in temperate climes, there are very few studies around the world on Eucalyptus harvest residue management in tropical areas. In Northern Australia on Melville Island 30,000-hectares of Acacia mangium is planned to be converted to Eucalyptus pellita. E. pellita harvest residues comprise high loads of stringy bark, leaves and twigs, posing a fire risk throughout the annual 6-to-8-month dry season in Australia's monsoonal tropics. In this environment, quantifying and demonstrating the anticipated benefits of residue retention is important for determining appropriate responses to fire risk.

A 4-hectare experimental trial of 12-year-old E. pellita was cut with trees moved intact to the coupe edge and processed for woodchips. The tree residues from the roadside chipper were pushed back over the site. A total of 35 Mg ha⁻¹ of harvest residue was left on the site, mainly comprised of bark (24.5 Mg ha⁻¹ or 60 % of the total residue mass). Although the leaf and branch residues contained higher concentrations of N, Ca, Cu, K, Mg, Mn, P, and Zn than bark, stem and woodchips, bark N (51.5 kg ha⁻¹) was twice foliage N (26.9 kg ha⁻¹), and contributed to more than 50 % of residue N.

Harvest residue bark, leaves and twigs were sampled for a litterbag decomposition study of mass loss on the harvested site. After 10 months about 80 % of bark original weight remained on site (20 % mass loss), compared to about 55 % of the leaves/twigs remaining (45 % mass loss).

The potential non-CO₂ emissions from the burning of E. pellita harvest residues were estimated at 12.3 Mg CO₂-eq ha⁻¹ or 3.35 Mg C ha⁻¹, with the majority of emission arising from the smoldering of bark residues. This study discusses the implications of either retaining or burning harvest residues – for plantation long-term sustainability on sandy tropical sites, and for greenhouse gas emissions.