Development of a Taper Equation for Teak (Tectona grandis L.f.) Growing in Western Thailand

Abstract

Teak is an important and valuable tropical hardwood species. In this study, we developed and evaluated suitable taper equations for teak growing in Western Thailand using a formulation of Goodwin cubic polynomial model combined with a bark thickness model. The best taper model calibration was selected based on goodness-of-fit and leave-one-out cross validation statistical testing. In total, 12 different model calibrations were tested, with Thong Pha Phum (TPP) 2 being the most suitable for teak in Western Thailand. The mean prediction error of three validation statistics: (prediction of diameter under bark given height; prediction of height given diameter under bark; and prediction of under bark volume given log length) were within 10% and the overall validation index was 5.454, which was the lowest when compared to other calibrations. A comparison of TPP 2 with a teak taper equation developed for Northern Thailand, using a graphical analysis of the stem shape and bark thickness, indicated that the teak trees growing in the two regions have similar stem shapes, but the trees in Western Thailand tend to have a thicker bark. These results will also help in further work as they indicate that bark thickness equations are particularly important.

Taper Equations for Evaluating Private Plantation Teak (<i>Tectona grandis</i>) in Thailand

“…To obtain a bark thickness equation based on our field data, we used the same power model (Eq. 5) as that of and Choochuen et al (2021) in Thailand.…”

“…where n is the number of data, y i is a measured value, ŷ i is its predicted value, and ȳ is the mean of y i . We used a mean prediction error (estimated/actual) of 10% for the model acceptability threshold to assess the suitability of a model (Choochuen et al 2021, Huang et al 2003. LOO cross-validation tested a model on three aspects: 1) prediction of d ob given h, 2) prediction of h given d ob , and 3) prediction of log volume overbark given two heights selected at random (v given hh), as per the process followed by .…”

“…Taper equations have been used in assessing various characteristics of teak plantations in some countries (Van Zyl 2005). In Thailand, two taper equations for teak plantations have been reportedly used: "FIO-teak1" in northern regions and "TPP2" in western regions developed by and Choochuen et al (2021), respectively. evaluated the variants of the polynomial model (Goodwin 2009) and the variable-exponent model (Kozak 2004).…”

“…They found that a suitable taper model for teak plantations in northern Thailand is a Goodwin model variant (called Goodwin-X3A), using which they established a taper equation termed "FIO-teak1." Choochuen et al (2021) suggested that for teak plantations in western Thailand, the best taper model among Goodwin model variants, was the same as that which outputted "FIO-teak1," and the taper model/equation was named "TPP2." However, these two studies focused only on teak plantations managed by the state-owned Forest Industry Organization (FIO) and not on privately managed ones, the latter being the focus of our study.…”

“…The FIO manages teak plantations under the following rules: a planting distance of 4 m × 4 m; regulated thinning at 10, 15, and 20 years of age; and a rotation length of up to 30 years (Himmapan & Noda 2012, Korwanich 1992. Additionally, Choochuen et al (2021) noted that the teak trees in plantations managed by the FIO in western and northern Thailand have similar stem forms because the seedlings are likely clones with the same genetic basis. In contrast, Thai private teak planters use seedlings with various genetic bases; select narrow planting distances, such as 2 m × 4 m (i.e., 1,250 trees/ha) or 2 m × 2 m due to requirements of the subsidy program (Furuya et al 2012b); and do not conduct regular thinning because of unfamiliarity with the technique (Furuya et al 2012a).…”