Inter-rotational strategies for sustaining site fertility and productivity of acacia and eucalyptus plantations planted on steep slopes in northern Vietnam

In Vietnam, approximately 1.3 M ha of acacia and eucalypt plantations have been established in the past three decades to supply a growing local and international demand for pulp and sawlog. Wood production over successive rotations does not appear sustainable as yields are declining and soils, especially on steep sites, are being eroded and negatively impacted. There is concern that current practices, i.e. burning harvest residues and only applying a small dose of fertilisers at planting are in part responsible for this situation and need to be changed. This thesis examines if changes in the current practices i.e. the retention of harvest residues and increased P fertilisation will improve the soil properties and productivity of Eucalyptus hybrid (Eucalyptus urophylla × E. pellita) and Acacia mangium plantations planted on steep slopes in northern Vietnam. The first study investigated whether the decomposition of retained A. mangium harvest residues (branches, leaves and bark) could provide sufficient nutrients for the next rotation. The biomass and nutrient content of above-ground stand components of the previous 7-year-old Acacia mangium rotation were examined at harvest, and the rates of decomposition and nutrient release from the harvest residues determined. The decomposition constant k, half-life t0.5 and release of nutrients (N, P, K, Ca and Mg) were monitored by using the litterbag technique for a 1.5-year-period. At harvesting, the total above-ground stand biomass of the previous rotation was 60.8 t ha\(^{-1}\), comprising stemwood (42.7 t ha\(^{-1}\)), bark (8.9 t ha\(^{-1}\)), branches (6.6 t ha\(^{-1}\)) and leaves (2.5 t ha\(^{-1}\)). The retained bark on site made up one-third of the mass of all residues (harvest residues + litter + understorey vegetation) and conserved 6% Mg, 14% K, 18% P, 30% N and 41% Ca content for recycling. The decomposition rate of the leaves was the most rapid (k = 1.47 year\(^{-1}\); t0.5 = 0.47 year), then branches (k = 0.54 year\(^{-1}\); t0.5 = 1.29 year) and bark (k= 0.22 year\(^{-1}\); t0.5 = 3.09 year). During decomposition, the loss of nutrients from harvest residues was K ≈ Ca > N > P> Mg. Over 1.5 years of the study period, as much as 137.1 kg N ha\(^{-1}\), 4.7 kg P ha\(^{-1}\), 20.8 kg K ha\(^{-1}\), 94.5 kg Ca ha\(^{-1}\) and 2.2 kg Mg ha\(^{-1}\) were recycled. The N, Ca and K, though not P and Mg released from decomposing A. mangium harvest residues are potentially able to meet a significant part of the demand by trees growing in the next rotation. The second study examined whether the retention of residues, and application of phosphorus fertiliser at higher rates than the current practice, can increase rates of growth and vigour of trees, i.e. better tree form and lesser crown damage, of eucalypt and acacia plantations. A factorial combination of residue management (burning vs. retention) and phosphorus (P) fertiliser application at planting (15 vs. 100 kg ha\(^{-1}\)) treatments was applied at a steeply sloping site (slope rages from 8 – 40 ̊). Two adjacent experiments were established, one with A. mangium and the other with E. hybrid. Standing volume (V) and leaf area index (LAI) in A. mangium were greater following burning; this was mostly attributable to the significantly higher survival rate of seedlings. Burning of residues was associated with increases in the number of large branches per tree, and a higher crown damage index (CDI). In the E. hybrid, diameter and height responses to the higher rate of fertiliser were observed at age 6 and 12 months, but not beyond. High P application also led to higher CDI. Standard fertiliser treatment, applied in amounts equivalent to 17, 15 and 8 kg ha\(^{-1}\) of N, P, K, respectively, was adequate to meet the early growth requirement of eucalypt and acacia plantations at this site. The third study examined the effect of two contrasting harvest residue management treatments (burning vs. residue-retention) on soil properties i.e. soil total carbon (TC), total nitrogen (TN), extractable P (ext-P), exchangeable K (exch-K) and soil bulk density (BD) of E. hybrid and A. mangium plantations. In this study, soil samples were collected in plots treated with similar amounts of fertiliser (applied at the current rate) but subject to contrasting residue management treatments (burning vs. residue-retention). The soil properties were assessed at pre-establishment, and at age one and two years following planting. The results showed that the soil properties in either E. hybrid or A. mangium plantations were not significantly different between residue retention and residue burning inter-rotational treatments. However marked variations observed in soil TC, exch-K and ext-P suggest that position on the slope masked any overall trends. In summary, the relatively low amounts of harvest residues and high fertility levels at the site may be associated with the lack of significant growth and soil responses to the silvicultural treatments applied in this study. However, it is clear that the decomposition of harvest residues and the associated rate of nutrient release can supply a significant amount of nutrients required for stand development in the next rotation. The variation in standing volume, crown health and soil properties between slope positions suggest that factors driving any correlation of tree productivity with slope, for example surface run-off and soil erosion will need careful management to arrest potential yield decline on steeply sloping sites. Thus harvest residue retention with adequate weed and termite control may be preferential to burning on a steep slope because the residue not only can provide nutrients but reduce water run-off and soil erosion