The performance of Acacia angustissima, A-auriculiformis and A-mangium as potential agroforestry tree species in the highlands of Papua New Guinea

In this study Acacia auriculiformis A. Cunn. ex Benth., Acacia mangium Wild. and Acacia angustissima Mill. were evaluated for their growth performance at three sites (Tabibuga, 1300 m above sea level (asl); Aiyura, 1700 m asl; and Gumine, 1950 m asl) in the highlands of Papua New Guinea (PNG). Based on the early results of this study, A. angustissima was tested in a hedgerow intercropping system at Aiyura to determine the optimum alley spacing and to assess the sustainability of growing sweet potato, the main highland staple crop. The evaluation trial used a randomised block design at all sites. The hedgerow intercropping study used systematic design and the treatments were replicated four times. The treatments were arranged in decreasing order down the slope in one replicate, while in the other replicate the order was reversed. In the growth evaluation study, A. angustissima had early vigorous growth at all sites and at 11 months, it had a height range of 1.0-3.0 m at all sites. Growth rates at Gumine and Aiyura were initially very slow for A. auriculiformis and A. mangium. However, later assessment shows the performance of these species was better at Tabibuga followed by Aiyura. A. auriculiformis never recovered at Gumine and was either static or dying. The hedgerow intercropping study showed that a 5 m alley was a better prospect because the hedges produced more biomass (20 t/ha) and returned more nutrients to the soil; the 5 m alleys also yielded more sweet potato tubers (14 t/ha). This study shows that A. angustissima, due to it's vigorous growth and higher biomass production, may be suitable for intercropping/agroforestry systems where biomass can be cut back and applied as mulch. A. auriculiformis and A. mangium could be more suitable at lower altitudes.