BIOTROPIA Vol. 30 No. 3, 2023: 308 - 317 DOI: 10.11598/btb.2023.30.3.1919 308 EFFECT OF THINNING ON GROWTH AND WOOD PRODUCTION OF NATURALLY REGENERATED 8-YEAR- OLD ACACIA MANGIUM WILLD. PLANTATION ON ABANDONED MINING AREA, SOUTHERN THAILAND JETSADA WONGPROM1*, SOMPORN MAELIM2, WASAN CHANDAENG1, SAKHAN TEEJUNTUK2, MONTATHIP SOMMEECHAI2 AND DECHA DUANGNAMON3 1Forestry Research Center, Faculty of Forestry, Kasetsart University, Bangkok 10900, Thailand 2Department of Silviculture, Faculty of Forestry, Kasetsart University, Bangkok 10900, Thailand 3Andaman Coastal Research Station for Development, Faculty of Fisheries, Kasetsart University, Bangkok 10900, Thailand Received 10 March 2023 / Revised 22 August 2023 / Accepted 22 August 2023 ABSTRACT Thinning is an important practice for promoting growth and maintaining forest plantation for wood production from the remaining trees. In this study, thinning was carried out in a naturally regenerated 8-year-old Acacia mangium plot in the Phangnga Forestry Research Station. Three thinning schemes, with 175 (T1), 300 (T2) and 600 (T3) remaining trees/ha, were compared with the control (no thinning) of 831 trees/ha. The diameter at breast height (DBH) and height (H) of the trees were measured. The differences in growth, current annual increment (CAI), aboveground biomass, and stem volume (V) were analyzed. We observed that the thinning of A. mangium increased the growth rate, with the DBH being clearly affected by thinning. CAIDBH increased significantly, with the DBH class of thinned A. mangium plots also improving after thinning. The stem volume and aboveground biomass of T3 plot was similar to the control plot after thinning. In addition, the number of large saw logs was the highest in T3 plot. The large saw logs can be used for multi-utilization and have a high value. These results suggest that thinning can promote stem growth, and increase the proportion of large saw logs in naturally regenerated A. mangium stands. Keywords: abandoned mining area, aboveground biomass, Acacia mangium, growth, merchantable volume, thinning INTRODUCTION Acacia mangium Willd. is a fast-growing multipurpose tree species and is usually found in tropical plantations (Hegde et al. 2013). It has been widely planted for soil improvement of degraded lands (Martpalakorn 1990, Majid et al. 1998), as it is a nitrogen fixing tree and can supply nutrients back into the forest floor via litter decomposition processes (Fisher & Binkley 2000). A. mangium has thus been widely planted in Southeast Asia in commercial plantations (Nambiar & Harwood 2014). A density of approximately 1,100 stem/ha or a spacing of 3 m x 3 m is commonly used while planting A. mangium. A high density is usually recommended for trees grown in short-rotation periods (Saharjo 2006), with the rotation for A. mangium being between 5-8 years to be used in wood chip and pulp production (Huong et al. 2020b). On the other hand, older large trees, usually around 15- year-old, are used in furniture making and to obtain sawn wood (Yahya 1993). Thinning is a silviculture practice to increase tree growth and stem volume of the remaining trees (Yahya et al. 2011; Beadle et al. 2013), as well as to improve the stem form and wood quality (Pérez & Kanninen 2005). It is commonly practiced in fast-growing trees species such as Eucalyptus, A. auriculiformis, and Acacia hybrid (Hung et al. 2019; Huong et al. 2020a). As a management practice in a plantation, thinning is used to reduce the number of trees in a stand, so as to increase the crown space between the remaining trees, to reduce the crown and root competition, and to increase growth. In addition, *Corresponding author, email: fforjdw@ku.ac.th https://www.researchgate.net/profile/Sakhan-Teejuntuk?_sg%5B0%5D=0q7r8kb8NERgtI99S50VgEJYB7-QeiPPLdJHVFfSjqIASeXygdDFaBrabD7DWeEBcgjIKNQ.tv_8L6j13rhLet-BfMLr6jvxvgxgTZC8KN8-AA9WYs9IsrdCkGEgOXNGRiLked6mKE4Cie0x5CF2AbuFDJQcQg&_sg%5B1%5D=53C3xrJFM1LU4E6xy8E2a6W-ZXXGv_0C563BLFRGh5CTSvchbEw_RCdicAqPwmmmbOaY1BY.3q9GWfjV2f_yfMlP5nQTnhZMC7GIn47HKF9V1GGUDO8OQj9tNIu4QJPB6hSc_0nzmYUaFQD_h4syBs4wu2kKnA https://sciprofiles.com/profile/347609 Effect of thinning on growth and wood production of Acacia mangium – Wongprom et al. 309 thinning can help to control pests and diseases, which in turn can increase the earned incomes before the final harvest (Onyekwelu et al. 2011). However, as a downside, the stem volume and aboveground biomass of thinned plantation can decrease. Past mining activity has had an extreme impact on the soil quality and has severely affected the adjoining ecosystems through the loss of soil structure and nutrient depletion (Thaiutsa & Rungruangsilp 1990; Maiti 2013). The Phangnga Forestry Research Station is located on one such abandoned mining area. The reclamation of mining area was undertaken by planting A. mangium, given its growth and high aboveground biomass compared to other fast- growing or native tree species (Martpalakorn 1990). Additionally, A. mangium has reportedly been recommended to rehabilitate degraded lands with a high survival rate (Majid et al. 1998) to restore the soil properties, forest structure, and nutrient cycling (Wongprom et al. 2020; Wongprom et al. 2022; Staporn et al. 2022). Seeds of A. mangium can accumulate in the soil and forest floor (Saharjo 2006), with the seedlings being highly dense after clear-cutting. A high tree density can result in the reduction of growth rate and yield, accompanied by high mortality due to competition. A. mangium plantations mainly focus on producing wood with a short-rotation period but the management of A. mangium for the production of large saw logs has been rarely studied. Timber production from natural forests is expected to decline, and as such, A. mangium plantations can play an important role in maintaining the commercial supply of wood. Thinning is recommended for trees used in timber and sawn wood production, with larger trees having the potential to increase the income earned (Onyekwelu et al. 2011). The objectives of this study were to identify the effects of thinning on the growth and wood production of a naturally regenerated A. mangium plantation on an abandoned mining area. Thinning was applied to stands which were established through natural regeneration after clear-cutting of the A. mangium plantation. These results can be used to manage A. mangium stand in the Phangnga Forestry Research Station and other such degraded sites located in southern Thailand. MATERIALS AND METHODS Study site The plantation is located on an abandoned area, previously under tin mining, at the Phangnga Forestry Research Station (8˚ 46' 5" N, 98˚ 16' 7" E), Takuapa district, Phangnga province, in southern Thailand (Fig. 1). Tin mining was done through the gravel pumping method. The post tin mining land forms could be mainly divided into sand, clay, and gravel areas. The soil nutrients and organic matter levels after tin mining were very low and the soil pH was strongly acidic (Anunsiriwat 1986). However, soil nutrients and organic matter content of this area were improved after the establishment of A. mangium, especially the topsoil level. The soil properties of this area are shown in Table 1 (Wachrinrat et al. 2002). The area receives an annual rainfall of 3,566 mm, with the rainy season spanning from April to October and dry season occurring during the months from November to March. The mean temperature was around 27.1°C and relative humidity around 83% (Wachrinrat et al. 2002). Figure 1 Location of the study sites in Phangnga province, southern Thailand BIOTROPIA Vol. 30 No. 3, 2023 310 Table 1 Soil properties of A. mangium plantation in clay area at Phangnga Forestry Research Station Soil depth (cm) Sand (%) Silt (%) Clay (%) Soil texture pH OM (%) Avai. P (ppm) Exchangeable bases (ppm) K Ca Mg 0-7 10.40 30.72 58.88 clay 5.0 3.84 1.01 62.2 153.44 54.44 7-32 0.40 24.72 74.88 clay 4.9 0.91 0.03 38.4 11.68 67.34 Note: OM = organic matter, Avai. P = available phosphorus, K = potassium, Ca = calcium, Mg = magnesium. In 1987, A. mangium was planted at a spacing of 4 m x 4 m, to reclaim the clayey soil (Wachrinrat et al. 2002). However, a clear cutting of A. mangium plantation was done for wood utilization when the trees were 14-year-old. After the area was cleared by cutting, the site was prepared by burning the branches and other parts of the trees. A. mangium seedlings were then allowed to naturally regenerate in the area and this resulted in a high density stand. Methods The experiment plots were located in an 8- year-old naturally regenerated A. mangium plantation. We observed that the crown cover of A. mangium stand was closed leading to a strong crown competition, which meant that thinning was needed for plantation management. A randomized completely block design (RCBD) with three replications was used in a plot of size 20 m x 20 m. Three different thinning schemes were used and included 175 (T1), 300 (T2), and 600 (T3) remaining trees per ha which were compared with a control of 831 trees/ha. A low thinning method was used in T1, T2, and T3 plots. The low thinning scheme was used to remove the suppressed and poor crown trees (Hawley 1947). In this study, any small and irregular trees were removed in the thinned plots. In addition, a good stem form of the co-dominant and intermediate trees in the T1, T2, and T3 plots were also removed to determine the spacing and tree density. The diameter at breast height (DBH) and height (H) of remaining trees were measured for a period of three years. The DBH and H of each treatment was calculated as a mean among replications (n = 3). The current annual increment (CAI) of H and DBH were calculated using the equations: CAIDBH = (DBH2- DBH1)/ t2-t1 CAIH = (H2- H1)/ t2-t1 Where: H1 = tree heights (m) at times t1 H2 = tree heights (m) at times t2t DBH1 = diameters (cm) at times t1 DBH2 = diameters (cm) at times t2 t1 = beginning times of each period t2 = ending times of each period. Allometric equations of the 11-year-old A. mangium trees were developed to estimate the aboveground biomass and stem volume, as the estimation equation previously reported for woodchip was for trees within an age bracket of 4-5 years in Thailand (Peawsa-ad & Viriyabuncha 2002). In this study, trees sampled from seven different DBH classes (8.4 to 34.9 cm DBH) were cut and separated as logs according to their respective heights binned under 0-0.3 m, 0.3-1.3 m, 1.3-2.3 m, etc., i.e., at every 1.0 m increment from the bottom to top. The tree height and diameter of the logs were measured. The fresh weights of stem, branch, and leaf components were determined in the field and representative samples were taken from each tree to determine the dry weight. The stem, branch and leaf samples were oven-dried at 80°C for 48 h to obtain a constant weight. The aboveground biomass, i.e., stem (WS in kg), branches (WB in kg), and leaf (WL in kg), of A. mangium was estimated using the allometric equations derived using destructive sampling as follows: WS = 0.0199*(DBH2*H)0.9828, (R2 = 0.98) WB = 0.0001*(DBH2*H)1.3345, (R2 = 0.99) WL = 0.0009*(DBH2*H)0.9773, (R2 = 0.94) WT = WS+WB+WL, Where: WT = total aboveground biomass DBH = diameter at breast height (cm) H = total height (m) Effect of thinning on growth and wood production of Acacia mangium – Wongprom et al. 311 The volume of each log was calculated using the Smalian's formula: V = (BA1+BA2)/2 x L Where: V = log volume (m3) BA1 = upper cross section area of the log BA2 = lower cross section area of the log L = length of the log The total stem volume (VT) and merchantable volume (VM) was estimated using the equation derived from destructive sampling. The merchantable volume was set at a top end diameter > 10.0 cm, as determined by the local wood sawmill; VT = 0.0395*(DBH) - 0.3369, (R2= 0.98) VM = 0.0403*(DBH) - 0.4030. (R2= 0.98) Allometric equations for DBH and stem-log types were developed from seven representative trees (with DBH between 8.4 to 34.9 cm). Sawlog types were categorized as either small (VSSL; diameter of log (D) 10.0 < D ≤ 15.0 cm and as a percentage of merchantable volume), medium (VMSL; 15.0 < D ≤ 20.0 cm), or large (VLSL; D > 20.0 cm). VSSL and VLSL were determined by establishing the respective allometric equations. VMSL was determined as the difference between the merchantable volume (100%) and the sum of VSSL and VLSL. The sum of VSSL, VMSL, and VLSL was equal to VM. The log components of each tree were estimated as a percentage of the merchantable volume (VM = 100%) according to the following equations: VSSL = 418.76e-0.137*(DBH), (R2= 0.97) VLSL = 8.9603e0.0677*(DBH), (R2= 0.77) VMSL = 100 – (VSSL + VLSL) VM = VSSL + VMSL + VLSL, where VM is the merchantable volume (m3). Data Analysis Growth performance, as indicated by DBH, H, CAIDBH and CAIH, the aboveground biomass of trees, stem volume, merchantable volume, and saw logs among the various treatments was compared using SPSS 16.0. A one-way analysis of variance (ANOVA) followed by Tukey HSD was used to determine the differences between means at a 5% probability level. RESULTS AND DISCUSSION Tree growth Thinning had a positive influence on the growth of A. mangium, although the DBH and H were not found to be significantly different (p > 0.05) during the initial stages of development. The parameters measured during the initial stages of growth and development after thinning are listed in Table 2. After thinning for one year, the DBH and H of trees in the T1 treatment increased rapidly compared with the control plot and were significantly different (p < 0.05). It has been previously reported that A. mangium grows well after an early thinning (Yahya 1993). Heavy thinning is an important factor influencing the DBH with its incremental change significantly and positively correlated with the thinning intensity (Mäkinen & Isomäki 2004; Juodvalkis et al. 2005). In this study, the small and suppressed trees were removed by thinning. This resulted in a structured stand, with a marked increase in measured DBH. The size distribution moved towards normality and then became positively skewed, and was affected by low intensity thinning. Larger DBH values were found in thinned plots, while trees with smaller DBH were frequently found in the unthinned plot (Fig. 2). CAIDBH was the highest in the T1 plot, followed by T2, T3, and control, respectively. The density of A. mangium significantly influenced the CAIDBH after thinning during the first and second years, but no significant difference was observed in the third year. CAIDBH of trees in the thinned plots peaked during the second year. However, CAIDBH tended to decrease and was not significantly different among treatments after thinning for three years. A rapid reduction in CAIDBH from 2.47 cm/cm/yr to 1.03 cm/cm/yr under thinning for 1-3 years in the T1 plot indicates a strong competition between the remaining trees. A dense A. mangium canopy was observed after thinning for three years. A crown competition within the stand led to the death of small and suppressed trees. A high relative mortality rate of 21.98% was observed in the unthinned plot. CAIH was significantly different after the thinning for one year, but was not significantly different after thinning for 2-3 years. This indicates that the increase in height was affected BIOTROPIA Vol. 30 No. 3, 2023 312 by thinning only during the early period. We observed that the thinning intensity only slightly affected the increase in height and is similar to results reported previously for many plantations (Wanthongchai & Sahunalu 2002; Mäkinen & Isomäki 2004; Cicek et al. 2013; Rytter 2013). Figure 2 Distribution of DBH classes of the thinned A. mangium during the initial stages of development (A) and after three years of thinning (B) at the Phangnga Forestry Research Station Table 2 Diameter at breast height (DBH), height (H), and current annual increment (CAI) of the thinned A. mangium plots after thinning for one, two, and three years at the Phangnga Forestry Research Station Treatment DBH (cm) H (m) CAI DBH (cm) H (m) Before thinning T1 21.94 ± 1.95 24.18 ± 0.53 - - T2 19.91 ± 1.20 23.56 ± 0.76 - - T3 19.47 ± 1.57 23.17 ± 0.71 - - control 17.24 ± 2.23 21.79 ± 2.59 - - F- value 2.58ns 3.40ns - - After thinning for one year T1 25.14 ± 1.96b 26.22 ± 0.72b 2.47 ± 0.26c 1.20 ± 0.04c T2 22.34 ± 1.16ab 24.84 ± 0.47b 1.80 ± 0.24b 0.87 ± 0.13b T3 21.22 ± 1.49ab 24.38 ± 0.64b 1.40 ± 0.27b 0.67 ± 0.14b control 17.56 ± 3.03a 22.29 ± 1.18a 0.64 ± 0.25a 0.54 ± 0.12a F- value 7.13* 12.57** 26.74** 19.14** After thinning for two years T1 27.65 ± 2.52c 27.19 ± 1.00b 2.51 ± 0.74c 0.96 ± 0.33 T2 24.30 ± 1.25bc 25.69 ± 0.50b 1.96 ± 0.13bc 0.85 ± 0.10 T3 22.85 ± 1.39b 25.10 ± 0.56b 1.63 ± 0.11b 0.72 ± 0.10 control 18.33 ± 3.19a 22.94 ± 1.74a 0.77 ± 0.27a 0.65 ± 0.62 F- value 8.98** 8.15** 9.90** 0.47ns After thinning for three years T1 28.65 ± 2.55b 27.58 ± 0.99b 1.03 ± 0.13 0.39 ± 0.10 T2 25.28 ± 1.15b 25.99 ± 0.40b 0.98 ± 0.14 0.38 ± 0.15 T3 23.71 ± 1.12ab 25.48 ± 0.44ab 0.86 ± 0.30 0.29 ± 0.14 control 18.93 ± 3.36a 23.08 ± 1.70a 0.60 ± 0.22 0.14 ± 0.10 F- value 9.62** 9.82** 2.36ns 3.64ns Notes: * = significantly different at p < 0.05; ** = significantly different at p < 0.01; ns = not significant at p > 0.05 and letters a, b and c in the same column indicate significant differences at p < 0.05 and p < 0.01, as determined by Tukey HSD. A B Effect of thinning on growth and wood production of Acacia mangium – Wongprom et al. 313 Aboveground biomass The aboveground biomass among treatments was significantly different after thinning for years 1-3 (Table 3). In the thinned plots, the aboveground biomass from all parts of a tree was the highest for T3 plot, while that for T1 plot was the lowest. However, the aboveground biomass of the T3 and control plots was similar. Differences in density can significantly affect the production of plantation. This is evident from our observation that the aboveground biomass estimated for the T3 (169.89 t/ha) and control (185.74 t/ha) plots was relatively high, but was lower than that of a 10-year-old A. mangium plantation (1,050 stem/ha) planted on degraded lands in Indonesia, with value of 241.10 t/ha (Hardiyanto et al. 2004). Stem biomass contributed the most to the main productivity, accounting for approximately 81-84% of the total aboveground biomass. On the other hand, the contribution of leaf component to the biomass pool was the lowest (4%). Leaf production of A. mangium is not the main objective of a commercial plantation. However, the contributions of leaf biomass and leaf litter were reported to be the most significant in terms of nutrient return to the forest floor (Nambiar & Hardwood 2014; Wongprom et al. 2022), as A. mangium leaf is rich in nutrient concentration, especially nitrogen. It therefore plays an important role in increasing the soil nutrients, improving the soil properties (Wongprom et al. 2020), and promoting nutrient supply for stand growth (Hardiyanto & Nambiar 2014). Previously, it has been reported that the wood production in an Acacia plantation is strongly correlated with the soil nutrients (Harwood et al. 2017). This observation was similar to Huong et al. (2020b), who found that four rotations in A. auriculiformis plantation resulted in high growth and production compared to the first rotation. Table 3 Aboveground biomass of the 8-year-old A. mangium under various thinning durations and intensities Treatment Aboveground biomass (ton/ha) WS WB WL WT After thinning for one year T1 52.02 ± 8.53a 8.16 ± 1.94a 2.65 ± 0.47a 62.83 ± 10.94a T2 69.03 ± 6.17a 10.34 ± 1.79ab 3.49 ± 0.35a 82.86 ± 8.33a T3 119.95 ± 21.04c 16.34 ± 4.33 ab 5.99 ± 1.13b 142.28 ± 24.84b control 140.77 ± 10.70c 17.22 ± 0.11ab 5.03 ± 0.43b 163.02 ± 11.05b F- value 33.68** 8.49* 28.03** 30.62** After thinning for two years T1 61.99 ± 12.05a 10.60 ± 2.99a 3.21 ± 0.67a 75.80 ± 15.71a T2 83.16 ± 7.08a 13.67 ± 2.40ab 4.27 ± 0.41a 101.10 ± 9.77a T3 141.67 ± 22.63b 21.05 ± 5.16b 7.17 ± 1.24b 169.89 ± 27.01b control 156.38 ± 7.16b 20.61 ± 0.91b 7.75 ± 0.25b 185.74 ± 6.54b F- value 33.38** 7.05* 26.51** 30.78** After thinning for three years T1 66.65 ± 13.09a 11.77 ± 3.41a 3.47 ± 0.73a 81.89 ± 17.24a T2 90.42 ± 5.14a 15.05 ± 2.57ab 4.64 ± 0.34a 110.11 ± 7.91a T3 152.53 ± 19.82b 23.27 ± 4.66b 7.75 ± 1.09b 183.55 ± 25.54b control 161.46 ± 6.61b 21.79 ± 1.00b 7.92 ± 0.28b 191.17 ± 6.15b F- value 42.41** 8.01* 30.89** 37.63** Notes: * = indicates a significant difference at p < 0.05; ** = significantly different at p < 0.01; letters a, b, and c in the same column indicate significant difference at p < 0.05, as determined by Tukey HSD. WS, WB, WL, and WT are the stem, branches, leaf biomass, and total aboveground biomass, respectively. BIOTROPIA Vol. 30 No. 3, 2023 314 Stem volume, merchantable volume, and sawlogs After thinning for three years, the stem and merchantable volumes of the thinned and control A. mangium plots were significantly different (p<0.01) (Table 4). In addition, the production of small, medium, and large saw logs was also significantly different among the treatments. However, stem and merchantable volumes of the T3 and control plots were similar. Thinning intervention positively affected the diameter of logs. Heavy thinning in the T1 plot resulted in a high proportion of large saw logs (67% of merchantable volume). The proportion of large saw logs in the thinned plots was higher than both the medium and small sized saw logs. In contrast, the proportion of small saw logs was high in the unthinned plot (25%), while those in the T1, T2, and T3 was 9, 14 and 16%, respectively. This indicates that thinning increased the merchantable volume and the number of large saw logs. For 9-year-old A. auriculiformis plantation (1,333 stem/ha), the medium sized saw logs formed the major proportion of saw logs obtained from the unthinned plot (Huong et al. 2020a). Thinning is important for improving the DBH class of a stand as well as the merchantable volume and is usually used as a plantation management strategy to increase growth and utilization. For 9 and 10 year plantations, the current annual increment of volume (CAIV) was found to be significantly different (p<0.01), with the T3 plot having the highest CAIV. However, CAIV of the T3 and unthinned plots was similar for the 11- year-old plantation. Low thinning resulted in a significant increase of stand stem volume (Fig. 3). After thinning for one and two years, CAIV of the T1 and T2 plots increased, while the values were not different for the unthinned plot. Heavy and moderate thinning intensity had a significant influence on the increment in stem volume of the A. mangium plot, given that the trees had a positive response to thinning. Table 4 Stem volume, merchantable volume, and saw logs of the thinned 11-year-old Acacia mangium stand under various thinning intensities Treatment Stem volume (m3/ha) Merchantable volume (m3/ha) Sawlogs (m3/ha) Small Medium Large T1 135.83 ± 16.79a 127.92 ± 17.37a 11.25 ± 1.75a (9%) 31.25 ± 6.75a (24%) 85.42 ± 21.11a (67%) T2 187.92 ± 6.71b 180.99 ± 13.65b 25.75 ± 5.88b (14%) 47.83 ± 10.07a (27%) 107.41 ± 23.46a (59%) T3 353.83 ± 23.76c 326.19 ± 23.73c 51.00 ± 3.54c (16%) 101.08 ± 7.44b (31%) 174.16 ± 24.56b (53%) control 351.10 ± 22.33c 298.44 ± 15.27c 74.60 ± 10.69d (25%) 95.49 ± 7.07b (32%) 128.33 ± 12.57a (43%) F-value 145.57** 92.44** 165.70** 51.05** 7.54* Notes: * = indicates a significant difference at p < 0.05; ** = significantly different at p < 0.01; letters a and b in the same column indicate significant difference at p < 0.05, as determined by Tukey HSD. The numbers in brackets are percentage of saw log types estimated for each treatment. Figure 3 The current annual increment of stem volume (CAIV) of the thinned 9 to 11-year-old A. mangium plots under various thinning durations Effect of thinning on growth and wood production of Acacia mangium – Wongprom et al. 315 In southern Thailand, the local sawmills have a high demand for large saw logs to make wood furniture, as these products fetch a higher value compared to woodchips. In Thailand, during the year 2021, the timber imported was approximately 220,000 m3 (Royal Forestry Department 2021). As such, fast-growing tree plantations can reduce the import bill related to import of timber wood, while also reducing the illegal cutting of natural forests. Thinning can be a valuable management strategy in plantations to increase the domestic timber production. A short rotation period (4-7 years) is generally used for the production of fuelwood and woodchips from A. mangium plantations. The growth, aboveground biomass, and stem volume estimates reported in this study are based on a plantation established on poor soil conditions in an abandoned mining area where soil nutrients and organic matter were very low (Anunsiriwat 1986; Thaiutsa & Rungruangsilp 1990). However, A. mangium was able to grow well in this area as the site receives a high amount of rainfall (more than 3,500 mm/yr), with a rainfall of more than 2,500 mm/yr being suitable for the optimum growth of A. mangium (National Research Council 1983). The DBH and H of the T2 plot (24.30 cm and 25.69 m, respectively) in the 10- year-old A. mangium plantation was similar to that of A. mangium planted in West Java, Indonesia, with a DBH, H, and density of 27.81 cm, 25.12 m, and 225 stem/ha, respectively (Heriansyah et al. 2007). In addition, the estimated total aboveground biomass and stem volume in this study was not different from that of other areas with a similar stand density. For example, the aboveground biomass estimated for the lightly thinned 11-year-old A. mangium plantation, with a density of 600 stem/ha was similar to that of an A. mangium plantation in southeastern, Vietnam (Cuong et al. 2020). CONCLUSION In this study, we presented the results of thinning on the growth of A. mangium trees in a naturally regenerated 8-year-old plantation in the Phangnga Forestry Research Station, with three different thinning schemes, which were compared with the control (unthinned plot). The growth and production of the remaining trees after thinning were observed to be affected by thinning. After thinning, the DBH and CAIDBH of trees in the thinned plots were found to be significantly higher than those in the control plot. However, tree height was only slightly affected by thinning. A reduction in CAIDBH was observed after thinning for three years, possibly resulting from a stronger competition for resources. The total aboveground biomass and stem volume in the lightly thinned T3 plot was similar to that of the control plot. The thinning intensity significantly affected the growth and productivity of the A. mangium stand. CAIV of the T3 plot was relatively higher than that of the T1 and T2 plots. However, the large saw logs obtained from the T3 and unthinned plots was significantly different, suggesting that thinning should be done for promoting stem growth and to obtain large sized timber wood. ACKNOWLEDGEMENTS This work was partially funded by the National Research Council of Thailand through the Knowledge Hub for Integrated Economic Trees: Plantation Establishment, Management, Utilization and Industry. We thank Dr. Atsushi Sakai and Asst. Prof. Dr. Kobsak Wanthongchai for their comments and help in improving this paper. We are grateful to the staff at the Phangnga Forestry Research Station for their assistance during the field work. REFERENCES Anunsiriwat A. 1986. 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