Effects of Thinning Intensity on Production and Stem Form of Teak (Tectona grandis L.f.): A Case Study of Private Plantations in Mueang District, Uttaradit Province

Abstract

Thinning is one of the silvicultural methods used to increase the productivity of a forest plantation that often involves selecting trees to be removed based on their canopy class or spacing. This study investigated the effects of different thinning intensities on the productivity and stem form of teak (Tectona grandis) spaced at 4 m x 4 m in a private forest located in Mueang district, Uttaradit province, Thailand. A completely randomized block design was applied with 3 experimental treatments: moderate thinning (40% basal area removal), heavy thinning (60% basal area removal), and unthinned. Data collection was conducted 9 years after thinning. It was found that the stand density in the unthinned plot was significantly higher than in the thinned plots. The average diameter at breast height over bark (DBH) and the total tree volume (m3) were significantly higher in the thinned plots than in the unthinned plot. However, tree height was not affected by thinning. The mean annual increment (MAI) based on the DBH for all trees was significantly higher in the heavy-thinned plot compared to the unthinned plot. Calculation of the timber volume indicated there were significant differences in the stand volume, volume increment, and total volume among the plots. However, more trees in the unthinned plot were lost due to natural mortality than in the thinned plots. Considering stem form, the live crown ratio was higher in the thinned plots than the unthinned plot, whereas the slenderness ratio and artificial form factor were significantly lower in the thinned plots. In conclusion, thinning and its intensity positively affected tree growth and improved tree stem form.