Improving productivity and sustainability of successive rotations of Acacia auriculiformis plantations in South Vietnam §
We studied the productivity of Acacia auriculiformis plantations in South Vietnam over three successive rotations covering 15 years. The focus of our study was on the effects of inter-rotation management on stand growth and soil properties. Contrastin g slash and litter management treatments were applied at the start of the sec ond rotation, and reapplied at the start of the third rotation with an additional phosphorus fertiliser treatment. There were improvements in the genetics of planting stock , weed control and stocking with each rotation. Average growth rates (mean annual increment) increased from 10.6 m3 ha−1 y−1 in the first rotation (age 7 years) to 28.3 m3 ha−1 y−1 in the second rotation (age 6 years) and to 33.9 m3 ha−1 y−1 at age 5 years in the third rotation. Removal of slash and litter after harvesting the first rotation removed 20.2 Mg ha−1 biomass, containing 169.6, 13.9, 76.3 and 25.1 kg ha−1 of nitrogen (N), phosphorus (P), potassium and calcium, respectively, from the site. Greater amounts were removed after the second rotation commensurate with higher amounts of biomass produced. Slash and litter removal reduced growth of the second rotation by 13% compared to their retention. Soil organic carbon (C) in the 0–10 cm soil layer increased from 16.7 g kg−1 at the end of the first rotation to 22.8 g kg−1 at the end of the second rotation with a corresponding increase in soil N from 1.2 g kg−1 to 1.7 g kg−1. Over the same time, soil organic C and N contents were 26% and 40% greater, respectively, in treatments with slash and litter retained compared to initial levels before treatment application. In the second rotation, extractable soil P declined and in the third rotation there was a response to added P. Overall results demonstrate that there is an opportunity to increase and sustain production of A. auriculiformis over at least three rotations by integrated management practices promoting better stocking, planting of genetically improved stock, organic matter and nutrient conservation, and judicious weed management.
Keywords: harvests, inter-rotation, site management, soil carbon and nutrients, tropical plantations