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Burning harvest residues during site preparation can compromise the soil-nutrient stock in short-rotation plantations, but this practice remains common in northern Vietnam. This study compared the ... more...

Forest plantation growers in Vietnam commonly burn residues after harvesting and often apply suboptimal amounts of nutrients during plantation establishment. We examined whether the retention of forest residue, and application of phosphorus fertiliser at higher rates, can increase rates of growth. A factorial combination of residue management (burning vs retention) and phosphorus fertiliser application at planting (15 vs 100 kg ha-1) treatments were applied at a steeply sloping site in northern Vietnam. Two adjacent experiments were established, one with Acacia mangium and the other with a Eucalyptus hybrid (Eucalyptus urophylla × Eucalyptus pellita). Standing volume and leaf area index 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 Eucalyptus hybrid, diameter and height responses to the higher rate of fertiliser were observed at age 6 and 12 months, but not beyond. High phosphorus application also led to higher CDI. Standard fertiliser treatment, applied in amounts equivalent to 17, 15 and 8 kg ha-1 of nitrogen, phosphorus and potassium, respectively, was adequate to meet the early growth requirement of eucalypt and acacia plantations at this site. The relatively low amounts of harvest residue and high fertility levels at the site may have masked more significant responses of trees to the silvicultural treatments applied in this study. On steep slopes, especially if soil is poorly fertile, harvest residue retention with adequate weed and termite control may be preferential to burning as it is closely correlated with reducing factors that negatively impact productivity, i.e. water run-off and soil erosion. more...

Background: Young saplings of Hopea odorata, a native dipterocarp in Vietnam, require shading to prevent photoinhibition but they must avoid competition that stagnates their growth.Aims: To develop a silvicultural regime by examining how the biomass production of H. odorata changes along a light gradient in gaps within Acacia hybrid plantations.Methods: Hopea odorata saplings were planted in 22-m-diameter circular gaps within a 3-year-old Acacia hybrid plantation and in 5-m and 7.5-m-wide strip gaps within a 2.5-year-old Acacia hybrid plantation.Results: In the circular gap, biomass growth increased nearly tenfold from the gap perimeter (GP) to about 9 m from the perimeter, and then decreased. This was associated with a fivefold increase in the absorption of photosynthetically active radiation (APAR) and a nearly twofold increase in light-use efficiency (LUE). The increasing APAR was primarily related to increasing H. odorata crown sizes and a reduction in shading from the Acacia hybrid. In the strip gaps... more...

Background: Mixtures of tropical acacia nurse crops and understorey native species have been established to aid forest restoration in Vietnam, but with partial success. Knowledge of physiological mechanisms underlying competitive interactions remains limited.Aims: To examine growth and physiological responses of Hopea odorata, a shade-tolerant dipterocarp, established within an Acacia hybrid (Acacia mangium × Acacia auriculiformis) nurse-crop plantation.Methods: H. odorata seedlings were planted within three 22-m diameter gaps in a 3-year-old Acacia hybrid plantation in Central Vietnam. Growth and physiology responses to an environmental gradient in gaps were examined over 2 years.Results: Growth rate and maximum rates of photosynthesis and stomatal conductance of H. odorata saplings increased significantly with increases in relative daily incident photosynthetically active radiation from 24% at the gap perimeter (GP) to 61% at the gap centre. Leaf N, P, and chlorophyll concentration were unaffected by po... more...

Acacia hybrid (A. mangiumA. auriculiformis) is the main species planted for short-rotation forestry in Vietnam. In this study, the effect of these plantations on some key properties of degraded gravelly soils in Central Vietnam was assessed. Soil samples were collected from second- or third-rotation plantations representative of five age classes (0.5-5 years old), and in adjacent abandoned lands as controls. Compared with abandoned land, stock of total soil carbon (C) was significantly higher at ages 0.5, 1.5, 2.5 and 5 years (18.4-19.5 v. 13.0Mgha -1 ), total nitrogen (N) at 0.5 and 1.5 years (1.5-1.7 v. 1.0Mgha -1 ), exchangeable calcium at 0.5, 1.5 and 2.5 years (215-294 v. 42Mgha -1 ), magnesium at 0.5, 1.5, 2.5 and 3.5 years (39-48 v. 19Mgha -1 ), and sodium at all ages (46-59 v.5Mgha -1 ). Electrical conductivity was significantly higher at all ages (58.5-69.4 v. 32.7 mScm -1 ). Differences in extractable phosphorus and exchangeable potassium were not significantly different between plantations and abandoned land. Bulk density was significantly lower in plantations than abandoned land at all ages (1.36-1.42 v. 1.55Mgha -1 ), pHCaCl2 at 0.5 and 5 years (3.78-3.84 v. 3.98), and pHH2O at 5 years (4.30 v. 4.52). Because the soils were gravelly, differences in concentration of total C and nutrients between abandoned land and plantations were not the same as those for stocks after correction for gravel content and bulk density. Within a rotation, most soil properties did not change significantly with plantation age, although they appeared to decrease during the first 3 years; total C then recovered to initial levels, but total N and exchangeable cations remained lower. Some soil properties were strongly related to gravel content and elevation, but not to growth rate. We conclude that consecutive plantings of short-rotation Acacia hybrid on degraded and abandoned land can lead to changes in some soil properties. Additional keywords: degraded land, gravelly soil, nitrogen fixation, nutrient stock, soil amelioration. more...

Background Forest ecosystem functioning is strongly influenced by the absorption of photosynthetically active radiation (APAR), and therefore, accurate predictions of APAR are critical for many process-based forest growth models. The Lambert-Beer law can be applied to estimate APAR for simple homogeneous canopies composed of one layer, one species, and no canopy gaps. However, the vertical and horizontal structure of forest canopies is rarely homogeneous. Detailed tree-level models can account for this heterogeneity but these often have high input and computational demands and work on finer temporal and spatial resolutions than required by stand-level growth models. The aim of this study was to test a stand-level light absorption model that can estimate APAR by individual species in mixed-species and multi-layered stands with any degree of canopy openness including open-grown trees to closed canopies. Methods The stand-level model was compared with a detailed tree-level model that has already been tested in mixed-species stands using empirical data. Both models were parameterised for five different forests, including a wide range of species compositions, species proportions, stand densities, crown architectures and canopy structures. Results The stand-level model performed well in all stands except in the stand where extinction coefficients were unusually variable and it appears unlikely that APAR could be predicted in such stands using (tree- or stand-level) models that do not allow individuals of a given species to have different extinction coefficients, leaf-area density or analogous parameters. Conclusion This model is parameterised with species-specific information about extinction coefficients and mean crown length, diameter, height and leaf area. It could be used to examine light dynamics in complex canopies and in stand-level growth models. peerReviewed more...