Productivity and sustainability of acacia plantations in Vietnam

Clonal Acacia hybrid (A. mangium x A. auriculiformis) is widely planted across Vietnam due to its fast growth and adaptation to a wide range of site conditions. But there is significant variability in Acacia hybrid plantation productivity across Vietnam, most likely explained by marked regional differences in soils, climate and management practices. There is insufficient knowledge to allow the matching of management practices to site potential. While the environmental benefits of Acacia hybrid plantations can be assumed to be similar to other tree plantings, there is a paucity of specific knowledge about the impact of Acacia hybrid plantations as a land use in Vietnam. My research used empirical mensuration and modelling approaches to broaden the knowledge of Acacia hybrid's response to its environment and its impact on this environment. Three studies were undertaken (1) an estimation of Acacia hybrid productivity for a range of climate and soils, (2) an examination of the impact of alternative silvicultural management practices on sawlog production from Acacia hybrid plantations (3) an investigation of the impact of Acacia hybrid plantations on soil properties in comparison to fallow land within a shifting cultivation system. For the first study we calibrated the 3-PG growth model using ten permanent sample plots located in stands aged 1, 3 and 6 years. The model was then validated using 55 additional permanent plots from 12 plantations growing in four regions of Vietnam that support plantation forestry. The model performed well for most of the validation sites; model efficiencies (EF) were ‚Äö¬©v¶0.76. The model was more accurate in predicting the productivity of plantations in the North and North Central Coast than in the South and South Central Coast regions. Growth was most affected by soil water deficit in this wet/dry tropical environment, than by temperature, particularly in the north. Soil fertility was best predicted by a relationship with soil organic carbon and the base cations Ca\\(^{2+}\\) and K\\(^+\\). Across regions, the mean current monthly increment of stand volume for a 15-yr rotation was 3.21 and 1.97 m\\(^3\\) ha\\(^{-1}\\) month\\(^{-1}\\) for the wet and dry seasons, respectively. Sensitivity analysis indicated how much the model parameters affect the main outputs and how this changes with stand age. In the second study the growth responses of Acacia hybrid plantations to a range of thinning and fertiliser-at-thinning treatments at six experimental trials in North, South Central Coast and South Vietnam were quantified using empirical data and compared to predictive data derived from the 3-PG process-based model. Different experimental thinning regimes reducing initial stockings of 2000, 1667 and 1111 trees ha\\(^{-1}\\) to 1333, 1000, 900, 667, 600 or 450 trees ha\\(^{-1}\\) at ages varying from 2 to 5.6 years were applied. Tree diameter (DBH) responses to thinning and stand volume (SV) after thinning were greater in south than in North and South Central Coast Vietnam. Application of fertiliser at thinning increased DBH and SV, compared with the non-fertiliser control. Early thinning to 450 or 600 trees ha\\(^{-1}\\) resulted in the greatest DBH for all diameter classes and was the most successful treatment for sawlog production compared to other thinning regimes, with a substantial increase of sawlog volume and the proportion of larger diameter logs. Lighter thinning to 900 or 1000 trees ha\\(^{-1}\\) resulted in comparatively low diameter increments but higher yields of small sawlogs and total SV than higher intensity thinnings. The 3-PG process-based model was more accurate in predicting DBH and SV for all silvicultural treatments at sites in North and South Central Coast Vietnam than in the South where productivity was highest. Predictions suggested that extending the rotation length to at least 5 ‚Äö- 7 years in the South and South Central Coast and 6 ‚Äö- 10 years in North Vietnam in Acacia hybrid plantations managed for large sawlog production would best optimise the production of this category of sawlogs and thus benefit growers. There have only been a few studies that have examined the effect of land use on soil properties including total soil carbon (TC), total soil nitrogen (TN) and soil pH in Vietnam. Soil properties under tropical Acacia hybrid plantations (AH) and fallow land within a shifting cultivation system (FSC) were compared. We investigated various soil properties for the two land uses including TC, TN, pH, bulk densities and particle-size distribution in 10 cm increments down to 30 cm for 25 paired sites in northern Vietnam. The results show that TN and TC concentrations in AH were significantly higher at all 10 cm depth increments when compared to FSC. While both TC and TN decreased significantly with depth under both land uses, the C/N ratio only decreased in AH and not the FSC. However, there was a significant decrease in soil pH in AH at all depths (>0.4 pH units) and this may potentially cause acid infertility issues. In summary, I parameterised, calibrated and validated the 3-PG process-based model for Acacia hybrid. The model provided an accurate description of the potential productivity of Acacia hybrid plantations across a wide range of climates and soils in Vietnam. Acacia hybrid plantations appear to improve TC and TN more than fallow land within a shifting cultivation system and thus have a demonstrated and positive impact on environmental sustainability. The 3-PG model can be used to predict stand growth of Acacia hybrid plantations managed for sawlogs, under different silvicultural regimes, with acceptable accuracy and could be a useful tool for strategic planning of thinning and fertilization. Overall, my thesis provides valuable information to enable sustainable forest management of Acacia hybrid plantations across Vietnam, the determination of potential productivity and the selection of those management practices which will optimise sawlog production.