Understanding growth and physiological responses to slash management, thinning and fertiliser application in short-rotation tropical acacia plantations

In Vietnam acacia plantations are an important resource for the production of pulpwood and sawn timber. However, research is needed to support sustainable and profitable production over successive rotations. This thesis reports on results from three field experiments which were conducted on Acacia auriculiformis and Acacia hybrid species in Vietnam. The experiments explored the growth and physiological responses of acacia to slash management, thinning and fertiliser application to test the hypothesis that constraints in nutrients and/or water and light resources were constraining the growth of acacias, both at establishment, and later in the rotation. The rationale for focusing on resource constraints is that they offer some opportunity for management intervention. The first study explored the impacts of contrasting slash and litter management techniques applied at the start of the second rotation and re-applied at the start of the third rotation with an additional phosphorus fertiliser treatment during the inter-rotation phase of A. auriculiformis plantations. Removal of slash and litter after harvesting the first rotation removed `20.2 Mg ha^-1` biomass, containing `169.6, 13.9, 76.3, and 25.1 kg ha^-1 of N, P, K, and Ca,` respectively, from the site. Greater amounts were removed after the second rotation commensurate with higher productivity and amounts of biomass produced. Growth of trees in the second rotation was significantly higher where slash and litter were retained compared to where they were removed. Soil organic carbon and nitrogen contents were greater (26% and 40% respectively) in treatments with slash and litter retained compared to initial levels before the treatments were applied. In the second rotation, there was no growth response to P fertiliser but extractable soil P declined during this period. In the third rotation there was a positive response to added P fertiliser. Slash and litter retention along with improvements in the genetics of planting stock, weed control and stocking with each rotation resulted in average growth rates increasing from `10.6 m^3 ha^-1 yr^-1` in the first rotation (age 7 yr) to `28.3 m^3 ha^-1 yr^-1` in the second rotation (age 6 yr) and to `33.9 m^3 ha^-1 yr^-1` at age 5 yr in the third rotation. Thus, the results indicated that there is an opportunity to increase and sustain production of A. auriculiformis over at least three rotations by integrated management practices promoting better stocking, planting of genetically improved stock, organic matter and nutrient conservation, P fertiliser addition and judicious weed management. The second experiment investigated how thinning, P fertiliser application and slash retention at age 4 yr interact to affect the physiology and growth of A. auriculiformis trees to age 7 yr. The photosynthetic rate (`A_(max`) in the thinned treatment was significantly higher than in the unthinned treatment one year after thinning. A combined treatment of thinning and phosphorus fertiliser (P) application increased `A_(max)`, but `A_(max)` in the unthinned treatment did not significantly increase when P was supplied. Foliar nitrogen and phosphorus concentrations were greater in thinned than in unthinned treatments. Tree diameter was significantly greater under thinning, but it was not influenced by the application of fertiliser (`50 kg P ha^-1`) and slash and litter retention. The recovery of larger diameter, sawlog sized, timber in thinned treatments was significantly higher than in unthinned treatments. At seven years, the total stand value of wood products in the thinned treatments (including the thinning harvested in year 4) was higher than (5 %) for the unthinned treatments. This experiment supports the practice of mid-rotation thinning of A. auriculiformis in these environments to increase the value of these plantations to acacia growers. Growth and physiological responses of the Acacia hybrid plantation to thinning treatments of different intensity at age two and three years were also examined in a field experiment. Three years, after intensive thinning at age two, stand volume was reduced by 15.8 % but average stem diameter was increased by 16.7 %. The moderate thinning regime resulted in no significant loss in stand volume and an increase in average diameter of 7.5 %. After thinning the LAI of the intensively thinned stand recovered rapidly and there was no significant difference between unthinned and thinned treatments one year after thinning. This was associated with decreased litterfall production. Intensive thinning increased photosynthetic rates of the lower crown by 30.4 % in association with increased phosphorus concentration in leaves of 37.5 %. Tree growth was significantly influenced by season and thinning reduced leaf water stress during the dry season. Thinning of Acacia hybrid at age 2 or 3 yr resulted in higher leaf-level photosynthesis, enhanced water relations, and improved foliar phosphorus relative to unthinned trees. This suggests that intensive thinning at age two years or moderate thinning at age three years are practices that are likely to confer greater benefit to acacia growers. The investment decision should also account for the market value of different log sizes, the costs associated with harvest of these logs, and the risks associated with managing plantations for sawlog production, including longer rotations. In conclusion, the productivity of commercial acacia plantations can be maintained and improved by adopting integrated management practices, especially by retaining slash and litter after harvesting to promote soil conservation during the inter-rotation phase. Applying thinning for short-rotation tropical acacia plantations can reduce intraspecific competition in stands for water and light resources and increase the benefits for acacia growers.