Nghiên cứu một số cơ sở khoa học chuyển hoá rừng trồng keo tai tượng (acacia mangium willd) kinh doanh gỗ nhỏ thành rừng kinh doanh gỗ lớn ở bắc giang tt tiếng anh

The thesis is completed at Vietnamese Academy of Forest Sciences MINISTRY OF EDUCATION ‘ MINISTRY OF AGRICULTURE AND TRAINING AND RURAL DEVELOPMENT VIETNAMESE ACADEMY OF FOREST SCIENCES ================== Supervisor: Dr Dang Thinh Trieu Dr.PHAM Dang VanQUOC Thuyet CHIEN ĐOÀN NGMAIỌC DAO Chairman: Reviewer 1: STUDY ON SCIENTIFIC BASES TO CHANGE THE SMALL Reviewer 2: TIMBER PLANTATION INTO LARGE TIMBER PLANTATION Reviewer 3: OF ACACIA MANGIUM WILLD IN BAC GIANG PROVINCE Specialty: Silviculture Code: 9620205 The thesis was approved by Dissertation Committee at academy level in Vietnamese Academy of Forest Science SUMMARY OF DOCTORAL THESIS On date , 2020 Thesis may be referred to in the National Library of Hanoi - 2020 Vietnam and Library of Vietnamese Academy of Forest Sciences INTRODUCTION Background According to the Vietnam Administration of Forestry (VNFOREST) (2019), by 2018, the area of Acacia plantations reached 1,515,898 hectares, accounting for 53.6% of the total plantation nationwide The area of monoculture stands of Acacia mangium accounted for about 50% of the total and the majority of this plantations have been planted in high density to provide small timber for pulpwood or woodchips The Ministry of Agriculture and Rural Development (MARD) has set a goal of establishing 201,220 of large timber plantations, of which 28,658 of small timber plantations are going to be changed into large timber area At present, the thinning activity for large timber plantations is of interest A number of researches on thinning have also been conducted, however researches on the changing of A mangium forests are still limited Acacia mangium Willd is one of the three large timber species of the Acacia genera which are widely grown in Vietnam but often grown in high density and short rotation for pulpwood or woodchips These forests are quite suitable for thinning to convert into large timber production However, further researches are needed on the scientific basis as well as the economic effects of the conversion prior to any wide adoption Given the aforementioned practical need, the topic "Study on scientific bases to change the small timber plantation into large timber plantations of Acacia mangium Willd in Bac Giang province" was conducted Meaning scientific and practical - Scientific significance Provide further scientific bases for the changing of small timber Acacia plantation into large timber plantation - Practical significance Identify productivity, quality and economic efficiency of Acacia mangium plantation in Yen The, Bac Giang after thinning Objectives + General objective Identify the scientific bases for the changing of small timber plantations of Acacia mangium into large timber plantations in Yen The, Bac Giang Determine the best density after thinning to ensure higher productivity, quality and economic efficiency than plantations un-thinning of Acacia mangium forests in Yen The, Bac Giang + Specific objectives Specific objectives of the research include (i) assesssing the current status of changing of small timber plantation into large timber plantations in Bac Giang; identifying the effects of thinning on the growth of Acacia mangium plantation; identifying on the effects of thinning on the quality of Acacia mangium timber and identifying the economic efficiency of Acacia mangium plantation after thinning New contribution of the thesis - Determined the effects of thinning on the growth parameters, productivity and quality of Acacia mangium plantation in Yen The, Bac Giang - Identified the economic efficiency of large timber plantations of Acacia mangium after thinning in Bac Giang Research scope - The research contents: (1) Assessment of the current situation of changing of small timber plantation into large timber plantations in Bac Giang As for this content, the thesis only focuses on assessing the area of Acacia mangium plantations, the changing of small into large timber plantations (time, scope, location, tree growth); advantages and disadvantages in the conversion of planted forests (2) Research on effects of thinning on the growth of Acacia mangium plantations (including diameter at breast height, basal area; crown diameter, height, volume; leaf area index; canopy cover and photosynthesis) (3) Research on effects of thinning on the quality of Acacia mangium timber (including number of knots; rate of heartwood and sapwood; utilization rate of saw timber) (4) Assess the economic efficiency of Acacia mangium plantations after thinning in two scenariors including sale the timber at the harvest site and sale the lumber - The specific of research location: + Activities of the contents (1) were conducted in Son Dong, Luc Ngan, Luc Nam and Yen The districts, Bac Giang province + Activities of the contents (2), (3) and (4) were carried out in Yen The district - The research period: The research was carried out in years from 2013 to 2018 Thesis layout The thesis summary has25 pages, including 11 tables, 01 figures, and is structured into the following contents: Introduction: pages; Chapter 1: Literature review: pages ; Chapter 2: Contents and Methods: pages ; Chapter 3: Results and discussions: 11 pages ; Conclusions and recommendations: pages Chapter LITERATURE REVIEW 1.1 Situation of research in other countries Acacia mangium Willd is native to Australia, Papua New Guinea and Indonesia It is fast growing and adaptable on a variety of site conditions Acacia mangium therefore plays an important role in reforestation programs Different studies on this species have been conducted in many countries, from biological characteristics, variety selection to planting techniques as well as usability Thinning is usually carried out in a plantation after its canopy is closed with the aim of adjusting the density so that the stand always approaches the optimal density to promote the growth of diameter and height so that the highest productivity can be obtained from the stand (Rollinson, 1988b) Nicholas (1988) relied on canopy diameter criteria as a basis to determine the final density after thinning, taking advantage of the growth in diameter of crops in a 40 years production rotation in New Zealand Thinning affects the growth of trees, thereby affecting the leaf area index (LAI) LAI of the Acacia tortilis was influenced by nutrition and time of year (Elfeel and Abohassan 2016) In places where planting is thick, the trees grow well; the leaf area index can be up to 400-500% With low leaf area index, trees not make full use of light and space so economic productivity is not high In cased that the leaf area index is too high, the leaves cover each other's light and dispute the living space so there is no high economic productivity LAI can be used as a characteristic quantity to describe the canopy of an ecosystem Through this index we can predict primary production of photosynthesis, and evapotranspiration It is also a reference tool for plant growth After thinning, the light regimes of the remaining trees were the most obvious and significant changes (Wang et al 1995, Tang et al 1999) The photosynthesis capacity of leaves is enhanced due to the increased light in the canopy (Ginn et al 1991, Tang et al 1999) The photosynthesis effectiveness increase sharply, immediately after thinning, then gradually decreases when the canopy is closed (Gravatt et al 1997) The increased photosynthetic capacity (Amax) observed after thinning is mainly the change in photosynthesis capacity among mature leaves in the lower and middle canopy layer These increases, combined with improvements in the micro light environment, may be a key in strengthening plant growth after thinning The economic efficiency of thinning depends on the combination of the yield level, price, and the ability to stabilize the productivity of the plantation The thinning activity contributes to the sale increase but at the same time the plantation will bear the risks in places with unstable external conditions 1.2 Study in Vietnam Beadle et al (2013) studied the effects of thinning on 2.5 years old Acacia hybrid plantation in Dong Hoi (Quang Binh), after years of experiment, it is showed that thinning has markedly affected growth of these plantations (diameter; height; timber volume) Tran Lam Dong (2018), after years of thinning in Yen Bai and years in Nghe An, most thinning formulas yielded lower or equal reserves than the un-thinning treatments, so there was no value in case of selling small timber price However, in the current production, the price of wood increases with diamter size and depends on the degree of the suitability required as raw materials for woodchip, peeled or sawn timber The thinning formulas all showed higher values than the control treatments, except for the conversion forest at age in Nghe An, the difference was not significant In Yen Bai, the total value reached the highest when thinning was applied, leaving a density of 1,000 trees -1, reaching VND 193.6 million (fertilizer application ) and VND 184.8 million VND (without fertilizer application), compared with un-thinning (VND 147.1 million dong and VND 165.2 million, respectively), in which, the value of wood with D1.3> 15cm is VND130.5 million (without fertilizer application) and VND 118.6 million (fertilizer application), compared with without thinning (VND 67.5 million and VND78.8 million, respectively) Thus, previous studies indicated that the growth of tree in thinning tratments was much better than those from the trees in un-thinning treatment 1.3 Comment From the overview of domestic and foreign researches, it shows that A mangium plays a relatively important role in afforestation in Vietnam as well as in many other countries Regarding researches on thinning techniques for nurturing planted forests, , the researches in the world on planting large timber forests of Acacia and Eucalyptus species have been carried out quite comprehensively, especially on silviculture techniques of thinning in plantation forests These studies have created a scientific basis for intensive afforestation, conversion of small timber production forests to the supply of large timber of acacia and eucalyptus species; contributing to providing raw materials for processing in countries around the world in recent years In Vietnam, domestic researches mainly focus on species of Acacia, Eucalyptus, Bodhi, and Cunninghamia (Sa Mộc) with the main purpose of providing small timber Recently, there have been many studies on planting large timber forests, thinning techniques to transform small into large timber production plantations of some species such as Acacia and Eucalyptus However, these studies still have some shortcomings, such as researches on the conversion of small into large timber production forest conversion were carried out on a narrow scale; monitoring and evaluation time is not long; there have been very few studies on the effects of thinning on leaf area index, photosynthesis intensity as well as sawn timber quality Especially in Bac Giang in particular and in the Northern Uplands in general, there are no in-depth studies on thinning effects on some of the above indicators Given the above-mentioned practices, the implementation of "SStudy on scientific bases to change the small timber plantation into large timber plantations of Acacia mangium Willd in Bac Giang province" is necessary and meaningful in terms of scientific and practical aspects Chapter CONTENTS, MATERIALS AND METHODS 2.1 Contents In order to achieve the thesis’ objectives, following activities have been carried out: - To assess the current situation of the changing of small timber plantation into large timber plantation in Bac Giang province - To study the effects of thinning on the growth of Acacia mangium, which include effects of thinning on the survival rate; LAI and photosynthesis; diameter and basal area; tree height; crown diameter; diameter distribution and timber volume - To study the effects of thinning on the quality of timber of Acacia mangium plantation, which include effects of thinning to quantity and size of knot; to saw log utilization rate and volume of chipwood - To assess the economic efficiency of Acacia mangium after thinning, including economic efficiency when selling timber at harvesting site; and the economic efficiency when selling lumber products 2.3 Methods 2.3.1 Methods to assess the situation of changing of small timber plantation into large timber plantations in Bac Giang The interview method combined with survey were applied in this study Total of interviewer were 12 households who changed their small timber plantations into large timber plantations On the plantation area of each households, three plot with an area of 500 m2 were established to measure the DBH (D1.3), total height and crown diameter The total number of the plots was 36 The survey was conducted from November to December, 2018 2.3.2 Experiment and data collection 2.3.2.1 Experiment Different thinning regimes were establsihed for (planted in 2010), (planted in 2009) and (planted in 2008) years old plantatations such as: - Three years old plantation: CT1 (un-thinning); CT2 (thinning to 1100 trees ha-1); CT3 (thinning to 800 trees ha-1) and CT4 (thinning to 600 trees ha-1) - Four years old plantations: CT5 (un-thinning); CT6 (thinning to 800 trees ha-1) and CT7 (thinning to 600 trees ha-1) - Five years old plantations: CT8 (un-thinning); CT9 (thinning to 800 trees ha-1) and CT10 (thinning to 600 trees ha-1) The treatments were layouted as rondomized complete design with three replications of 0.12 giving total of 3.6 Experiments were conducted from 2013-2018 2.3.2.2 Data collection - Growth parameters: In each replication, a plot with 500 m were establish to measure the survival, D1.3; total height and crown diameterand - Leaf area index and canopy: In each replication, five hemisphere photos were taken using digital camera (Nikon Coolpix 8400) with fisheye lens (Fisheye FC-E92 180o) The leaf area index data was collected once in April 2019 The total number of photos taken was 50, of which the experiment carrying out thinning at 3-year plantation took 20 photos As for experiments with and 5-year plantations, taking 15 photos for each The photosynthesis data was also collected in April 2019 The Licor6800 was used to measure photosynthesis of leaves in the chamber with light intensity of 1,500 μmolm-2s-1 (A 1,500 μmolm-2s-1), the concentration CO2 is 400 μmol liters of air -1, air humidity 70% and air temperature 25oC At each iteration, select a tree of medium diameter to get branches for experiments In each tree, choose branches with a diameter of 1.5 - 2.0 cm A branch at position of ¼ (upper layer-Tr) and a branch at position ½ (lower layer-D) of the canopy from top to bottom are selected to measure photosynthesis The branch chosen to measure was the leafy branches that receive full sunlight Medium-old leaves were used for this measuring The measurement was carried out times per day at 5:00; 9:00; 13:00 and 17:00 forests of about 1,000 will have been implemented; 2,300 poor natural forests to plant production forests rehabilitated; 29,000 of concentrated forests planted The average productivity of plantation will increase to 20 m3/ha/year 7200 of large timber plantation will have been established, accounting for 10% of production plantation area This is the basis for implementing the conversion of planted forests in general and Acacia mangium in particular in order to quickly improve the productivity of planted forests in the province At present, the area which is feasible to produce large timber in Bac Giang province as follows: plantation forest: 17,998 including 4,609 of Acacia hybrid plantations and 13,390 of Pine and Acacia plantatations - Natural regeneration and additional planting of large timber tree: 1,836 It is expected that by 2020 the whole Bac Giang province will continue to their target of planting 15,000 of large timber plantation in four key districts named Son Dong, Luc Ngan, Luc Nam and Yen The A intensive planting model of large timber tree species with Acacia mangium was carried out in three years (2014, 2015, 2016) The total area of the model in Bac Giang is 120ha (Bac Giang Forest Protection Department, Review report on forestry extension models) Table 3.3: Area of large timber plantation in Bac Giang Year Area (ha) Location No Of participating households 2014 20 Luc Ngan, Son Dong 22 2015 50 Luc Nam, Luc Ngan, Son Dong, Yen The 28 13 2016 50 Son Đong, Luc Ngan Total 120 3.2 Effects of thinning to the growth of Acacia mangium 3.2.1 Survival rate 20 70 Five years after experiment, the mortality rates among the treatments were different between treatments In general, the treatment of un-thinning had a higher mortality rate than that in the thinning treatment (Table 3.5) In the treatment of thinning to 600 trees /ha, the mortality were 0.4%, 2.0% and 0.9% for 3, and years old plantation experiments, respectively 14 Table 5:Survival rate of Acacia mangium after years of the experiment Survival rate of Acacia mangium (%) Treat ment Average mortality (%/year) During Prior to After Nov./20 Nov./20 Nov./20 Nov./20 Nov./20 Final the the thinni 14 15 16 17 18 density experime experim ng -nt -ent CT1 100 96.6 93.2 87.3 81.7 74.5 1.170 5.1 2.0 CT2 100 100.0 98.2 95.8 92.1 85.5 940 2.9 2.0 CT3 100 100.0 98.3 95.8 94.2 91.7 733 1.7 2.0 CT4 100 100.0 100.0 100.0 97.8 97.8 587 0.4 2.0 CT5 100 97.5 93.8 90.1 87.0 84.6 914 3.1 4.75 CT6 100 100.0 97.5 95.0 92.5 90.0 720 2.0 4.75 CT7 100 100.0 97.8 96.7 95.6 94.4 567 1.1 4.75 CT8 100 97.5 95.0 90.6 86.2 83.1 886 3.4 4.0 CT9 100 100.0 98.3 97.5 94.2 92.5 740 1.5 4.0 CT10 100 100.0 98.9 97.8 96.7 95.6 573 0.9 4.0 15 3.2.2 Leaf area and photosynthesis indices 3.2.2.1 Leaf area and canopy cover index Leaf area index (LAI) and canopy cover of Acacia mangium plantation years after the thinning is presented in Figure 3.1 - Leaf area index (LAI): Thinning has a remarkable effect on leaf area index The thinning to 600 trees/ha has the lowest leaf area index, respective followed by un-thinning and the thinning to 800 trees / ha, respectively - Canopy cover index: The thinning to 800 trees/ha has the highest canopy cover index, followed by the treatment of un-thinning and thinning to 600 trees/ 2.5 79 1.5 76 73 Độ tàn che (%) LAI (m2m-2) 0.5 70 LAI Độ tàn che Figure 1: Leaf area and canopy cover index of Acacia mangium 3.2.2.2 Photosynthesis Table 1: Photosynthesis of Acacia mangium in different times in a day Positi Treatm on of ent leaf cut Tr CT1 D CT2 Tr A (μmol m-2 second-1) 5:00 9:00 (μmol 13:00 17:00 m-2 s-1) 1.82 2.31 2.24 7.24 8.44 6.28 6.45 7.54 7.33 16 5.23 4.23 3.12 5.19 5.63 4.74 (μmol m-2 s-1) 5.41 5.52 Treatm Positi A (μmol m-2 second-1) (μmol (μmol ent on of 5:00 9:00 13:00 17:00 m-2 s-1) m-2 s-1) D 3.07 8.56 8.32 5.21 6.29 Tr 2.36 8.92 5.24 6.43 5.74 CT3 5.59 D 2.45 9.43 5.32 4.54 5.44 Tr 3.05 7.56 5.48 4.23 5.08 5.38 CT4 D 2.15 8.87 6.21 5.51 5.69 Tr 2.27 7.75 6.38 4.06 5.11 CT5 5.18 D 2.17 8.36 7.33 3.15 5.25 Tr 2.13 7.92 5.92 3.63 4.90 CT6 5.13 D 2.23 9.38 5.66 4.14 5.35 Tr 1.96 9.25 7.37 4.78 5.84 CT7 6.44 D 2.01 10.25 9.03 6.87 7.04 Tr 3.54 7.65 4.79 3.39 4.84 CT8 5.86 D 4.25 9.22 8.63 5.43 6.88 Tr 3.57 8.12 6.76 4.45 5.73 CT9 6.08 D 2.93 9.67 7.32 5.85 6.44 Tr 3.14 7.98 6.56 3.23 5.23 CT10 5.57 D 2.36 8.35 7.37 5.56 5.91 Table 3.1 showed that photosynthesis of Acacia mangium were lowest at am., highest at am and then decreases until pm The treatment of thinning to 600 trees ha-1 has the highest photosynthesis in a day and is significantly different from the remaining treatmens.3.2.3 Diameter at breast height and basal area The diameter of trees in thinning treatment was significantly larger than those from un-thinning treatment because after thinning the remaining trees have more space for growth Table 3.2: Diameter of Acacia mangium years after thinning Treatment CT1 CT2 CT3 Diameter growth by time (cm) From From From From From Total 11/2013- 11/2014- 11/2015- 11/2016- 11/2017- after 11/2014 11/2015 11/2016 11/2017 11/2018 years 0.91 0.87 1.07 0.97 0.89 4.71 0.90 0.85 1.16 1.05 0.96 4.91 1.11 1.28 1.01 1.10 1.11 5.60 17 D (cm/year ) 0.94 0.98 1.12 Treatment CT4 CT5 CT6 CT7 CT8 CT9 CT10 Diameter growth by time (cm) From From From From From Total 11/2013- 11/2014- 11/2015- 11/2016- 11/2017- after 11/2014 11/2015 11/2016 11/2017 11/2018 years 1.36 1.35 1.20 1.09 1.47 6.47 0.94 0.66 0.54 0.62 0.48 3.23 0.90 0.89 0.77 0.70 0.61 3.87 1.09 0.93 0.95 0.83 0.80 4.61 0.89 0.78 0.70 0.66 0.47 3.49 1.10 0.79 0.74 0.66 0.50 3.79 1.27 0.95 0.72 0.73 0.58 4.26 D (cm/year ) 1.29 0.65 0.77 0.92 0.70 0.76 0.85 At the end of the experiment, there was no treatment satisfying the standards for large timber in accordance with the Circular no 29/2018/TTBNNPTNT According to this document, large timber must have at least 70% trees having diameter at 1.3m position from 20 cm and above on an area unit for fast growing trees at their main harvesting Specifically, on November 2018, in the experiment of thinning year plantation, the number of trees having D1.3 >20 cm was 4.4%, 10.4%, 15.7% and 22.1% for treatments CT1, CT2, CT3 and CT4, respectively In the experiment of thinning year-plantation, the number of trees having D 1.3 >20 cm was 8.9%, 20.6% and 30.5% for CT5; CT6 and CT7, respectively In the experiment of thinning year plantation, the number of trees having D 1.3 >20 cm was 19.1%, 27.7% and 33.4% for treatments CT8, CT9 and CT10 3.2.4 Height of Acacia mangium Table 3: The height of Acacia mangium years after thinning Height growth (m) Treatment CT1 CT2 CT3 CT4 CT5 From From From From From 11/2013- 11/2014- 11/2015- 11/2016- 11/201711/2014 11/2015 11/2016 11/2017 11/2018 2.5 2.7 2.6 2.4 2.5 2.2 2.3 2.4 2.2 2.0 2.0 2.3 2.2 2.0 1.7 18 1.8 1.7 1.6 1.8 1.8 1.5 1.2 1.4 1.6 1.4 Hvn years (m) 10.0 10.2 10.2 9.9 9.4  Hvn (m/ year) 2.0 2.0 2.0 2.0 1.9 CT6 CT7 CT8 CT9 CT10 2.3 2.3 2.6 2.5 2.4 2.4 2.1 2.4 2.2 2.1 2.0 2.0 2.0 1.9 2.1 1.9 1.9 1.6 1.4 1.5 1.3 1.4 1.3 1.2 1.2 9.9 9.7 9.9 9.2 9.3 2.0 1.9 2.0 1.8 1.9 From results in Table 3.3, statistical analysis shows that thinning did not affect tree height at the end of the experiment 3.2.5 Crown diameter of Acacia mangium Crown diameter is significantly affected by thinning where the higher the thinning intensity is, the faster the crown growth, which is showed by larger diameter (Table 3.4) Table 3.4: Crown diameter of Acacia mangium years after thinning Treatm ent CT1 CT2 CT3 CT4 CT5 CT6 CT7 CT8 CT9 CT10 Crown diameter (m) Nov./20 Nov./20 Nov./20 Nov./20 Nov./20 13 14 15 16 17 2,4 2,7 3,0 3,3 3,6 2,3 2,8 3,4 3,9 4,3 2,3 2,8 3,4 4,0 4,5 2,4 2,9 3,5 4,2 4,7 2,9 3,3 3,8 4,1 4,3 3,0 3,8 4,2 4,6 4,8 3,1 3,9 4,4 4,8 5,2 3,2 3,6 3,9 4,1 4,3 3,3 3,7 4,1 4,5 4,8 3,3 3,8 4,2 4,7 5,1 Nov./20 18 3,7 4,5 4,8 5,1 4,4 5,0 5,4 4,4 5,0 5,3 3.2.6 Timber volume of Acacia mangium plantation The timber volumes the control treatments was higher than those from thinning treatments Table 5: Ratio of timber between thinning and un-thinning treatment and after the experiment Treatmen t M1 (before thinning M2 (right after M3 (when Ratio comparing the with nonthinning experimen treatment (%) Right When 19 M1-M2 (%) M3-M1 (%) CT1 CT2 CT3 CT4 CT5 CT6 CT7 CT8 CT9 CT10 47,01 48,74 46,55 47,57 67,00 68,56 68,85 87,95 88,52 88,19 47,01 39,46 29,58 23,08 71,00 55,77 45,45 87,95 73,05 56,96 176,50 163,05 141,76 117,20 182,03 163,69 144,90 203,80 186,85 161,10 after thinnin g (%) the exp ended (%) 83,9 62,9 49,1 92,4 80,3 66,4 16,1 37,1 50,9 8,4 17,4 17,3 78,6 64,0 89,9 79,6 21,4 36,0 11,4 15,6 83,1 64,8 89,5 77,2 16,9 35,2 6,4 12,4 3.3 Effects of thinning to quality of timber Thinning improves ratio of large timber The research shows that the older the plantation, the higher the ratio of large timber is (Table 3.6) Table 6: Timber volume in thinning experiment Treatment CT1 CT2 CT3 CT4 CT5 CT6 CT7 CT8 CT9 CT10 Volume of log with D ≥10cm (m3/ha) 132.3 119.3 111.3 103.2 131.2 129.9 122.6 149.3 154.2 137.4 Volume of log with D ≥15cm (m3/ha) 53.5 58.3 66.5 69.4 68.4 85.9 87.9 99.0 121.2 114.7 Ratio of log with D>10 comparing to plantation reserves (%) 74.9 73.2 78.5 88.0 72.1 79.4 84.6 73.3 82.5 85.3 Ratio of log with D> comparing to plantation reserves (%) 30.3 35.7 46.9 59.2 37.6 52.5 60.7 48.6 64.9 71.2 If considering the sawn timber in accordance with the standard of the Yen The Two-Member Forestry Company, in the control treatment of unthinning in the thining experiment of and years old plantations, the timber volume is higher than those of thinning treatments For the thinning 20 experiment of years old plantation, the thinning treatment to 800 trees has the highest timber volume If considering the sawn timber in accordance with the standard of the TCVN 11567-1: 2016, timber volume in the un-thinning treatments was the lowest The treatment to 600 trees ha-1 showed the highest volume for and years old plantations experiments; the thinning treatment to 800 trees ha-1 provided the highest timber volume for the thinning experiment of years old plantation 3.3.4 Number of knots Thinning reduced the dead knots and increase live knots Statistical analysis indicates that the number of dead knots in thinning treatments to 600 trees ha-1 and 800 trees ha-1 is obviously lower than un-thinning treatment (P