Effects of tree density and size symmetry of competition on diameter growth in the early stages of growth in planted teak (Tectona grandis) trees in northern Thailand

Teak (Tectona grandis) is among the most valuable tropical hardwoods, but silvicultural guidelines are needed to improve the growth and quality of trees in teak plantations, particularly those owned by smallholders in Asia. We analyzed the diameter growth of individual trees to determine the effects of density control in the early stages of growth in a teak plantation in northern Thailand. Site water conditions estimated using the topographic index (TI) affected initial tree size at 10 years after planting, before the first thinning was conducted. Thinning comprised three levels: high, low, and none (control). Five years after thinning, we assessed the effects of thinning, diameter at thinning, TI, and neighborhood competition on tree diameter growth using generalized linear mixed models. We found two significant factors on diameter growth: negative effect of neighborhood competition and positive effect of initial diameter at thinning. We used two indices of neighborhood competition: size-asymmetric (one-sided) and symmetric (two-sided) competition. The two-sided competition model predicted diameter growth more accurately. The results indicate that inter-tree competition in even-aged teak plantations is size-symmetric or two-sided. In addition, the presence of size-symmetric competition implies that teak trees compete not only for light but also for below-ground resources, such as water and nutrients.