Eucalyptus Pellita Substantially Outperforms Acacia Mangium in Tropical Savannah Ecosystem of Australia, but Strategies Are Needed to Maintain Soil Nutrients

Eucalyptus pellita and Acacia mangium are widely planted in the tropics and managed over five -to -fifteen -year rotations for wood and wood products. This study compares the growth of routine plantings of A. mangium with trial plantings of E. pellita and nearby native forests across six broadly distributed locations on Melville Island, northern Australia. In addition to tree growth measurements, litter mass and chemical properties, and topsoil (0 - 10 cm) physical and chemical parameters were assessed. Melville Island has a pronounced seasonal rainfall and soils that range from loamy sand to sandy loam textures; across the sites sampled soils comprised 65 - 85% sand and 8 - 16% clay. Across our sites, the maximum mean annual increment of E. pellita was 31.12 m 3 / ha/year compared to 14.64 m 3 /ha/year for A. mangium , with stand volume strongly positively correlated with clay content. On a site basis tree volume growth correlated strongly with soil clay content, with lower survival on sites with less clay in the predominantly sandy soils. The higher E. pellita growth compared with A. mangium reduced available phosphorous and calcium in topsoil, indicating that addition of P and Ca in subsequent rotations will be important to maintain and increase E. pellita productivity. The N 2 -fixing species, A. mangium , significantly increased nitrogen in litter (1.88 +/- 0.04%) and in top soil (0.15 +/- 0.01%) compared to E. pellita (litter 0.93 +/- 0.05%; topsoil 0.11 +/- 0.01%) so that carbon to nitrogen (C/ N) ratios in Acacia litter (23.90 +/- 0.81) and soil (23.06 +/- 0.91) were significantly lower than in Eucalyptus (litter 46.30 +/- 2.66; top soil 27.66 +/- 1.65). Topsoil C/N also decreased with increasing soil clay content in all land uses ( A. mangium, E. pellita, native forests) indicating better conditions for decomposer activity and turnover in sites with higher clay content soils. The Eucalyptus plantation drawdown of N and PO 4 3- in topsoil suggests that future plantings of E. pellita may benefit from additional N fixed into biomass and soil organic matter by the preceding A. mangium plantations. Tree growth correlations with rainfall and soil characteristics showed growth is best explained by annual rainfall, soil texture (as a percentage of clay or of sand), soil electrical conductivity, and soil bulk density. Overall, in tropical long dry season regions, for plantation development on sandy soils, E. pellita is a better choice than A. mangium .