Growth and physiological responses to intensity and timing of thinning in short rotation tropical Acacia hybrid plantations in South Vietnam

Abstract

Acacia hybrid plantations are widely planted for pulpwood in Vietnam, but have the potential to be managed for high value saw-logs. This study examined the growth and physiological responses to thinning as a management option to increase the sawlog yield and value. Treatments were: unthinned (planting density 1111treesha−1), thinned to 833 or 600treesha−1 in a single thinning at either age 2 (early) or 3 (late)years, and progressively thinned from 1111 to 833treesha−1 at age 2years, and then to 600treesha−1 at age 3years. Three years after early thinning to 600stemsha−1, mean stem diameter was increased by 16.7% and the stand volume was reduced by 15.8%; thinning to 833stemsha−1 resulted in no significant loss in stand volume and increased mean diameter by 7.5%. The leaf area index, LAI of stands thinned to 600stemsha−1 recovered rapidly and there was no significant difference between unthinned and thinned treatments 1year after thinning; this recovery was also associated with a reduction in litterfall. Thinning to 600stemsha−1 increased A1500 of the lower crown by 23.5 and 17.4% at age 2 and 3yr, respectively; and this was associated with increased foliar phosphorus concentration. Thinning reduced leaf water stress during the dry season; leaf water potential and tree growth were significantly influenced by season. It was concluded that thinning to 600treesha−1 at age 2years or to 833treesha−1 at age 3years will produce wood with the highest diameter.