The growth of teak (Tectona grandis L. f.) seedlings in sandy soil in northeast Thailand is suppressed by infertility and drought stress. In a preliminary field pot experiment, we confirmed that bentonite was useful for increasing soil water availability. To monitor early growth characteristics of teak seedlings in sandy soil, we curried out an experiment of teak seedlings using bentonite and fertilizer in the field from July 2014 to November 2015. We then compared the growth, biomass, photosynthetic rate, leaf water potential, and concentration of elements in the plant organs among the four treatments. Bentonite increased both soil pH and Ca during the experimental period, and retained P in fertilizer. The dry mass of teak seedlings was markedly increased by concurrent use of bentonite and fertilizer. However, the use of bentonite alone showed little increase of dry mass of teak seedlings. Moreover, application of bentonite was mitigated drought stress in dry season, and photosynthetic rate showed high value by high concentration of chlorophyll. The concentration of K in root of teak seedlings was increased by application of bentonite, and its value showed positive correlation with the relative growth rate of teak seedlings. We concluded that bentonite was useful for the uptake of K and retention of P in fertilizer, as a result, growth of teak seedlings was accelerated.

When teak (Tectona grandis L. f.) is planted on acrisols in Northeast Thailand, its growth is suppressed by low pH, infertility, and low water holding capacity. To examine materials capable of increasing water holding capacity in soil and improving teak growth, we conducted an experiment with teak seedlings. We selected bentonite, charcoal, and corncob and added these materials at a rate of 4% to sandy soil from northeast Thailand. Teak seedlings were potted on these soils and raised from July 2013 to July 2014. We compared growth, photosynthetic rates, leaf water potential, and concentrations of elements in plant organs among bentonite, charcoal, corncob, and no addition (control) treatments. Water content in the soils was increased for the bentonite and charcoal treatments. Teak seedlings potted in these two conditions did not suffer from drought stress. Comparing the growth traits of the teak seedlings, the charcoal treatment produced larger root growth and promoted the uptake of phosphorus and potassium, whereas the bentonite treatment did not show positive effects on growth or nutrient acquisition. In contrast, the corncob treatment decreased water content in the soil, and teak seedling growth was suppressed. We concluded that charcoal was a useful material to improve teak growth in sandy soils.

We applied data from 407 trees (5-33 years old) from private teak (Tectona grandis) plantations to three typical taper models, including the Goodwin cubic polynomial model comprising hyperbolic and parabolic terms and the Kozak variable-exponent taper model. On the basis of the three models, 18 variants were fitted using nonlinear regression analysis. All models were defined to predict stem diameters overbark using diameter at breast height overbark. A bark thickness equation was prepared to convert overbark diameters to ones excluding the bark. Goodness-of-fit and leave-one-out cross-validation appraisals were used to select the best model. A variant of the Kozak model (Model K8) performed the best across three prediction tests: diameter given height, height given diameter, and log volume given two heights. Taper equation K8, derived from Model K8, provided actual values within the 10% mean error and was sufficiently accurate and precise at the valuable bole part. Teak trees in our study were different in stem form and slender (a high value in slenderness coefficient) compared to those in the state-owned Forest Industry Organization (FIO) teak plantations, and the use of the FIO taper model for slender stems was challenging. Trees in the private plantations generally had thicker barks than those in the FIO plantations. We concluded that equation K8 is recommended for private teak plantations in this study area. These results will contribute to studies on teak taper equations and bark thickness in Thailand.