BIOTROPIA Vol. 28 No. 1,2021: 46 - 53 DOI: 10.1 1598/btb.2021.28.1.1051 DETERMINING AN APPROPRIATE AGE FOR ESTIMATING SITE INDEX OF Acacia hybrid PLANTATIONS I N SOUTHEASTERN VIETNAM TRAN THI NGOANl AND NGUYEN TAN CHUNG2* 'Vietnam National University of Foresty, Dong N a i Province, Viet N a m ' F a c ~ ~ l g DfBiological Sciences, Nong Lam Uniuersio, Ho Chi Minh Ciiy, Ho Chi Minh, Viet Nam Received 5 April 2018/Accepted 21 January 2020 ABSTRACT This article documents the research results of a site index classification for Acacia hybrid plantations in Dong Nai Province. The objectives of this study were to 1) determine a baseline age for Acacia hybrid plantations to establish their site indices and 2) develop site index curves for Acacia hybrid plantations. Three standard plots were established for each age g o u p of 1-10 years with 111 trees per plot of which 108 trees were measured for the determination of tree growth criteria. The other three trees were cut for the tree truncation estimate, and were excluded in the computation of tree growth criteria. I n this study, the site index (SI) for Acacia hybrid plantations was divided into three levels based o n the mean total heights of the dominant trees; Levels I (24 m), I1 (20 m), and I11 (16 m). The heights of the 108 trees were used t o build the SI functions, and the three truncated trees were used to examine the possibilities of the SI functions. Research results showed that the appropriate baseline age of Acacia hybrid plantations at Dong Nai Province is 8 years. Moreover, in order to improve the effectiveness of Acacia hybrid plantation businesses, the owners should focus on growing plantations at site index levels of I or II. Keywords: Acacia hybrid plantations, curve of site index, dominant trees, site index INTRODUCTION Climate change has negatively affected human and environmental health, particularly forest ecosystem health (IPCC 2000). Tropical rainforest ecosystems are one of the most important carbon sinks on the earth, playing a very critical role in balancing the global carbon cycle and CO2 concentration (Chaiyo et al. 2011). As such, on forest biomass and carbon sequestration researches are very relevant in forest management and planning, as well as biomass energy use (Brown 2000 & 2002; Zianis et al 2005). Moreover, biomass changes are associated with increases in forest ecosystem growth and rates of carbon absorption and emission (FA0 2009). A forest population ratio of biomass to carbon intensity depends on an age and site index (Nguyen & Tran 2016). An *Corresponding author, email: ntchung@hcmuaf.edu.vn accurate estimation of forest biomass is an important factor in assessing the global carbon cycle (Chavt et al. 2005). Several studes are interested in determining the mechanisms for worldwide carbon sequestration in different environments (Bouman e t al. 1999). It is therefore, necessary to develop appropriate methods for surveying and evaluating forest biomass and carbon stocks (Chambers et al. 2001; Brown 2002, Chavi et al: 2005). Several studes also developed mathematical models for the estimation of forest productivity (Dong 1974; Nguyen & Dao 1999; Vu 2005 & 2012) and biomass functions (Vien 2008: Bao 2010; Vu & Vo 201 1; Nguyen 2012; Dang 2014; Nguyen & Tran 2016) for timber trees and different types of natural forests. Although a few studies have focused on the site index curves of Acacia mangizlm Wdld (Knsnawati etal. 2010; Lumbres e t al. 2018), very limited information are available on biomass estimation of Acacia hybrid Determining an appropriate age for estimating site index of Amcia &bed ... - T h i Ngoan and T a n Chung plantations at various tree ages and soil site Some researchers have studied the planting indices. Therefore, it is essential to predict Acacia hybrid plantation biomass to determine some functions based on different site indices. The forest soil site index or soil productivity is a criterion for evaluating the site suitability for forest productivity (Nguyen & Dao 1999). Soil site quality (or productivity of a soil site) reflects the productivity capacity of a forest, which might change due to the effects of over- exploitation, fire, ferulizer application and/or soil erosion (Clutter e t al. 1983). Site quality is determined through forest growth and yield, mean annual increment (MAI) and periodic annual increment (PAI) by site index (SI) and growth intercept (GI) methods (Clutter e t al. 1983; Larsen 1999). SI can be estimated dlrectly based on the average height of dominant trees (Ho) associated with a baseline age (Ao) (Monserud 1984). SI values also depend on tree species and is determined as functions of HO or G I (Monserud 1984; Larsen 1999; Vu 2005). The selection of a baseline age for a tree species is dependent on the life cycle (choosing a period at which population growth does not depend on species density) or business cycle (Monserud 1984; Larsen 1999). The site index is determined by three different methods (Monserud 1984; Larsen 1999). The first method involves the construction of a site index population curve, which has been commonly used since the 1940s. This method is based on the related function of H O = f(A0) to construct a site index curve. The second method is based on selecting pairs of H O and age and defining the function HO = f(A), where A is the age of a forest. Therefore, site index curves have different shapes by applying different functions. The third method involves truncation of individual trees, using site index curves based on results of tree truncation. This method was created in the 1980s and has been widely applied ever since. In general, each method gives a unique interpretation result (Monserud 1984; Larsen 1999; Vu 2005 & 2012). The SI curve converts H O at a b a s e h e age (Ao) to HO at a matured age of trees (A). SI curves are usually constructed as tree growth functions related to mature ages; in other words, they are based on data pairs of Ho/A. A suitable SI curve must be fitted by mathematical, statistical methods and must be chosen based on statistical tests (Larsen 1999; Vu, 2005 & 2012). - and breeding of AcaCid hybrid species in Vietnam (Le 2000). The mature age of Acacia hybrid plantations in southern Vietnam is eight years (Nguyen e t al 2006). Nguyen et al. (2020) records the total area of AcaCid hybrid plantations in Dong Nai Province as approximately 23,557 ha. Plantations are mainly distributed in the dstricts of Vinh Cuu, Xuan Loc, and Dinh Quan. As of writing this manuscript, very limited information is avdable on the classification of site indices for Acacia hybrid plantations in Dong Nai Province. The objectives of h s study, therefore were; (i) to determine a baseline age of Acacia hybrid plantations to establish their site indices; and (ii) to develop site index curves for AcaCid hybrid plantations in Dong Nai Province. Dong Nai Province was selected for the study site because many Acacia hybrid plantations have been planted in this area with different climatic conditions, topography and soil types. Currently, the total area of AcaCid hybrid plantations in Dong Nai province is 23,000 ha (Institute for Forest Ecology and Environment 2017). The results of this study will provide the scientific basis for the application of silvicultural methods to effectively manage and use Acacia hybrid plantations. MATERIALS AND METHODS Description of the Study Site This research was conducted on one to 10 year old Acacia hybrid (mention the hybrid type or parental species) plantations from in the districts of Vinh Cuu, Dlnh Quan, Xuan Loc, Tan Phu, Long Thanh, and Bien Hoa in Dong Nai Province. These sites have three main soil types formed on basalt, shale and silt soil foundations in Dong Nai and all located between 1 O0 30' 03"- 1 1 ' 34' 57" in the northern latitudes and 106' 45' 30"- 107' 35' 00" in the eastern longitudes, near the equator at 50-350 m above sea level. The rainy season occurs in May to October, and the dry season occurs in November to April. The average temperature, rainfall, and humidity are 22 "C, 2100 mm, and 80°/o, respectively. BIOTROPLA Vol. 28 No. 1,2021 Experimental Design and Sample Collection The site index (SI) for Acacia hybrid (Acacia aaticahyormis or Acacia mangk) plantations was determined based on the dominant tree height (Ho) at a baseline age (Ao). Based on a preliminary survey of the age dstribution of Acacia hybrid plantations in the study locations, standard sampling plots were established to measure growth criteria for each age group. Based on secondary data, three standard plots were established for each of the Acacia hybrid plantation age group, and each of those plots represented a typical kind of soil, as mentioned above. The areas of each standard plot varied due to plantation density at each study location; however, the number of trees per plot was consistently 39. Within each plot, three sample trees representing different growth status (Vietnam Standard 11 567-1:2016, 201 6) were cut for tree truncation; one tree of good growing status, one of medum growing status, and one of poor growing status. The remaining thirty-six trees were used to measure tree growth criteria. Thus, a total of 30 standard plots were established, in which 108 trees were measured to estimate the growth criteria and nine trees were used to measure truncation for each Acacid hybrid plantation age group. The truncated trees were not included in the estimation of tree growth criteria. An average height (Ho) for each age group (from 1 to 10 years) was determined by the tree truncation method. The trees used for truncation were cut at 10 cm above the ground, and the diameter at breast height (DBH or D) and stem height (Ho) were determined before carrying out the tree truncation steps. Each trunk was cut into segments 1.0 m in length, except for the top trunk section, which was 0.5 m. Annual rings were counted at 0.0 m, 1.0 m, 1.3 m, 2.0 m, 3.0 m, 4.0 m and so on. At each truncated dameter of the tree, annual rings of the tree were counted to determine a tree age corresponding to the tree height reached within the truncated section. The rings within each tree were assembled and then labelled accrodingly for each truncated tree. The statistical characteristics described for Ho, such as the mean height (Ho), standard deviation (SD), coefficient of variation (CV), minimum height (Ho, -), maximum height (Ho, ,,,) and dfference of Ho,,,, - Ho,-, were calculated by using results of statistical analyses. Determination of a Site Index Function The site index for Acacia hybrid plantations was constructed based on functions described by Schumacher (1 939). The observed empirical data of the 108 sample trees in each age group were used to fit the functions. The consistency of each fitted function (equation (3)) was evaluated by considering statistical factors, such as coefficient of determination (R2), standard deviation (SD), mean absolute error (MAE) and mean absolute percent error (MAPE). Ho = a*exp(-b/AAc) (1) or Ln(H0) = Ln(a) - b/AAc (2) Ln(H0) = b o + b l / A A c (3) Ho = exp(bo + bl/AAc) (4) bo = Ln(H0) - bl/AAc (5) where: Ln(a) = bo and b = b~ From equations (4) and (5), a function of site index, SI = f (A), was defined by equation (6), and parameters of SI function, such as boy were defined by equation (7). Ln(SI) = bo + bl/AoAc (6) bo = Ln(SI) - bl/AoAc (7) By substituting equation (7) into equation (3), equations (8-10) were obtained. Equation (10) is described as the function of site index, with HO at a certain baseline age (Ao). Ln(Ho) = Ln(S1) - bl/AoAc + bl/AAc (8) Ln(S1) = Ln(H0) - bl/AoAc + bl/AAc (9) SI = exp(Ln(Ho) - bl(1 /AAc -1 /AoAc)) (10) Determination of A Baseline Age (&) and Parameters of Site Index Curve An appropriate baseline age (Ao) was chosen at the time when SI was used to convert H O at A0 to HO at a certain age (A) with the smallest regression sum of squares (SSR-). The appropriate baseline age was tested for only Acacia hybrid plantations with age groups of 6- 10 years (A = 6-10 years). The SI value was defined based on the fluctuation of H O at Ao (HO,~,, - Ho,,,~,). The slope (parameter bl) remained the same across the site index functions. To obtain an SI curve, a value of bl from equation (4) was first calculated and then substituted into equation (10) along with the value of Ho at Ao. The SI curves were validated by using empirical data measured from the truncated trees. Determining an appropriate age for estimating site index of Amcia &bed ... - Thi Ngoan and Tan Chung RESULTS AND DISCUSSION of Acacid hybrid plantations for age groups from 1 to 10 years, was defined (Equation 11). Statistical Characteristics of Acacia Hybrid Plantation The average HO values of the dominant trees varied from 2.6 m (year 1) to 22.4 m (year 10) (Table 1). The different ranges between Ho, ,,, and Ho, ,,, were from 2.1 to 3.3 m (year 1) and from 15.7 to 29.0 m (year 10). Coefficients of variation (CV) fluctuated from the highest 23.9% (year 2) to the lowest 14.7% (year 10). In general, Ho values exhibited large variations with age and site con&tions. Therefore, a categorization of Acacia hybrid plantations in Dong Nai Province into different site levels is necessary. Selection of A Site Index Function HO = exp (3,65344 - 2,76734/AA0,70746) (1 1) with 2 = 83.4%; MAE = 2.2; MAPE = 16.2%. Ho values were fitted by substituting the values of age groups into equation 10 (Table 2). The annual periodc growth of height (ZHo) increased gradually for the first year (2.4 m/ year), reached a peak at 4.7 m/year in year 2, and then gradually decreased unul age 10 (0.9 m/year). The annual average height growth (AHo) also increased gradually starting at year 1 (2.4 m/year), reached the highest at year 3 (3.6 m/year), and then gradually decreased unul age 10 (2.2 m/year). Height growth rate (Pho) decreased rapidly from 100% in year 1 to 14.1% Based on the result of the regression and in year 5 and decreased again to 4.1% in year 10. correlation analyses, the function HO = f (A) that Thus, at age 2 the Acacia hybrid plantations expresses a relationship between height and age transitioned from rapid to slow growth. Table 1 Statistical characteristics of the tree heights in different age groups (n = 108) A (year) Ho, observed (m) cv ("/o) Ho, min (m) Ho, max (m) Ho, max - Ho, min (m) 1 2.6 15.4 2.1 3.3 1.2 2 6.7 23.9 4.0 9.1 5.1 3 11.0 17.3 7.1 14.2 7.1 4 14.0 17.1 9.4 17.9 8.5 5 15.9 18.2 10.5 20.4 9.9 6 18.0 15.0 12.2 22.7 10.5 7 18.5 20.0 12.3 24.4 12.1 8 20.3 18.7 13.6 26.8 13.2 9 21.9 15.1 14.8 27.6 12.8 10 22.4 14.7 15.7 29.0 13.3 Notes: A = age; Ho = height of dominant tree; CV = coefficient of variation; Ho, fin = minimum height; and Ho, ,,, = maximum height. Table 2 Fitted values of periodic annual and average annual height growths and growth rate Notes: ZHo = periodic annual height growth; A H 0 = average annual height growth; and Pho = height growth rate. BIOTROPIA Vol. 28 No. 1,2021 - ZHO (m) ------ AH0 (m) Figure 1 Periodic annual and average annual height growth of Acacia hybrid plantation by age Baseline Ages (&) of Acacia Hybrid Plantations Baseline ages (Ao) of the Acacia hybrid plantations were chosen at a year when the functions SI = f(A) were fitted with the smallest regression sum of scpares (SSR,i,). In this study, the ages of Acacia hybrid plantations ranged from 1 to 10 years; therefore, determination of an appropriate baseline age was tested at ages 6, 7, 8, 9 and 10 years. Predicted HO values based on equation (10) and differences between observed Ho and predicted Ho values are presented in Table 3. The highest and lowest values of SSR were 2.87 and 0.86 at the ages of 7 and 8, respectively. Thus, the appropriate baseline age used for constructing SI curves was 8 years. Age 8 is consistent with the mature age for Acacia hybrid plantations in southern Vietnam (Nguyen e t al. 2006). Table 3 Predicted Ho at Ao and SSR associated with Ho for ages 6-10 Predicted Ho (m) at A. (year) SSR values associated with the predicted Ho Total sum of SSR 1.14 2.87 0.86 1.84 0.98 Determining an appropriate age for estimating site index of Amcia &bed ... - Thi Ngoan and Tan Chung Levels of Site Index at the Selected Baseline Age The dfference between Ho, ,, and Ho, ,,in (Ho, ,,, - Ho, mi, = 26.8 m - 13.6 m) at the selected baseline age of 8 is 13.2 m (rounded to 13.0 m) (Table 1). At the age of 8, the average height Ho value for Acacia hybrid plantations was 20.4 m (rounded to 20.0 m). The measured error of height was usually from k0.5 to kl.O m. If the Ho value at the age of 8 (13.0 m) is divided into three levels, a range of each level is equal to 4.3 m (rounded to 4.0 m). The &stance between two levels of adjacent site indices (4.0 m) is four to eight times higher than the measured error of height. Therefore, the Acacia hybrid plantation was divided into three levels of site in&ces (I, 11, 111) based on the Ho values in which the distance between two levels of adjacent site indices was 4.0 m. The SI values of the three levels are; 24 m (Level I), 20 m (Level 11) and 16 m (Level 111) at the baseline age of 8. The SI values midway between levels I and I1 and midway between levels I1 and I11 were 22 m and 18 m, respectively. Similarly, the SI value at the lower margin of level 111 was 14 m, while the upper margin of level I was 26 m. Table 4 Functions of site index (SI) at selected levels. Selected Site Index Level Curves In this study, the slope @I) of SI curves was the same for all three SI levels. The results of the regression analysis showed that the slope value (bl) was 2.76734 and l/AoAc was equal to 1/8"0.70746 = 0.22967. The values of these parameters were substituted into equation 9, and the SI curves were fitted (Table 4). There were no significant differences in the growth intercepts between functions of SI = f (A) among the three site index levels and functions of HO = f (A), with p-values of 0.239, 0.285 and 0.261 for Levels I, I1 and 111, respectively. Similarly, the slopes of the site index functions were not sigmficantly different from those of the functions of HO (p-value = 0.570 for level I, p-value = 0.61 1 for level I1 and p-value = 0.380 for level 111). These results proved that functions 11 to 17 can be used to construct site index curves for Acacia hybrid plantations as represented by the predicted values of site indices for each age group (Table 5) and the fitted site index curves of height values by years of AcaCid hybrid plantations at levels of IuPp,,, I, 1-11, 11, 11-111, 111, and 111~0,,, (Fig. 2). SI levels Fitted functions of SI Iupper SI = exp (Ln (26) - 2.76734*(1/AA0.70746 - 0.22967)) I SI = exp (Ln (24) - 2.76734*(1 /AA0.70746 - 0.22967)) I1 - I SI = exp (Ln (22) - 2.76734*(1 /AA0.70746 - 0.22967)) I1 SI = exp Q n (20) - 2.76734*(1 /AA0.70746 - 0.22967)) I1 - I11 SI = exp (Ln (18) - 2.76734*(1 /AA0.70746 - 0.22967)) I11 SI = exp (Ln (1 6) - 2.76734*(1 /AA0.70746 - 0.22967)) IIIlowcr SI = exp (Ln (14) - 2.76734*(1 /AA0.70746 - 0.22967)) Table 5 Predicted values of site index levels for age groups from 1 to 10 years Predicted Ho (m) at different site index levels A (year) I,,,, I I - I1 I1 I1 - I11 I11 IIIlo,,, BIOTROPIA Vol. 28 No. 1,2021 SI curve 30 5 6 A (year) Figure 2 Curves of siteindices associated with Ho at different age groups CONCLUSION Brown S. 2002. Measuring carbon in forests: current status and future challenges. Journal of Environmental Pollution 116:363-72. Acacid hybrid plantations in Dong Nai province could be divided into three site index Chaiyo U, Garivait S, Wanthongchai K. 2011. Carbon storage in above-ground biomass of tropical levels namely; Level 1 (24 m height), I1 (20 m) deciduous forest in ratchaburi province, Thailand. and I11 (16 m). For these three site index levels World Academy of Science, Engineering and - - the appropriate baseline age of the plantations Technology 5(10):495-500. was at age 8. The site index curves for each level Chambers JQ, Santos JS, Ribeko RJ, Hlguchi N. 2001. of the Acacia hybrid plantation sites were Tree damage, dometric relationships, and above- established according to the baseline age. Hence, ground net primary production in central Amazon to improve the efficiency of plantation forest. 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