Temporary site index for two-invented teak clones with generative regeneration in the state forestland in East Java, Indonesia.
Site quality is an important factor which influences growth and it must be taken into account in determining spacing arrangement of teak plantation. It can be used to identify stand productivity as well as to provide benefit from planning and implementation of forest activities. A model is needed to predict height growth of dominant trees and site index of perhutani's teak plantation which will be used as reference to forest management plans. This research is aimed at defining model for predicting height growth of dominant trees andthe quality of site index of perhutani's teak plus plantation. Dominant height was defined as height average of 30 dominant perhutani's teak plus trees in a compartement. Compartements were evaluated based on the height average of 30 dominant trees in the age of 6 to 13 years.50% of data based on the values approaching the mean were used to predict the height growth of dominant heights The dominant heights were analyzed using three candidates of dominant height models namely Schumacher, Chapman Richards and Wolf von Wulving. The proposed model was selected through the highest coefficient of determination ([R.sup.2]).Schumacher equation model, in this case, was selected as the proposed model for predicting height growth of dominant trees and site index.. The site classes were divided into three classes namely good, medium and poor based on the age of 13 years used as the reference age. Data were divided into three classes based on the dominant height of the reference age, they were Site Index 12 as a poor class, Site Index 14 as a medium class and Site Index 16 as a good class. This temporary site index proposed to classify perhutani's teak plus plantation was in the age of 6 to 13 years in the research area.
KEYWORDS: perhutani's teak plus plantation, growth model, dominant height, Site index, Two-Invented teak clones
Teak is popular because of its attractiveness and durability. Sadono et al.  said that this wood has a very good quality with wonderful fiber and texture and has stable, strong, and durable characteristics, Therefore, it has been a target species in Plantation. In Java, Teak plantations cover 1.2 million hectares . Teak is characterized by slow growing species, that becomes an opportunity and a challenge for teak plantations management in order to make innovation to increase productivity.
Perhutani is a state owned enterprise whose duty is to manage forests around Java Island. In this case, they have developed perhutani's teak plus or Jati Plus Perhutani (JPP) in order to improve the teak productivity and the quality conducted through tree breeding programs. The tree breeding programs are vegetative propagation (cuttings and tissue cultures) and generative (seed orchard clones). The two invented teak clones are known as PHT I and PHT II where these have superior and stable growth. The JPP is adaptive in various places as it is a result in a very strict selection process. The JPP can grow faster than conventional teak, both in poor and good sites. The level of Perhutani's teak plus uniformity is high, cylindrical and having straight trunk .
The plantation of JPP from seed orchard clones was started in 2002 conducted in wide scale around Perhutani regions and last planted in 2007. The implementation was combined with manipulation of environmental factors through application of intensive sylviculture. The plantation of JPP was expected to realize effective and efficient management and to increase productivity within 20 year-rotation period. However, the different altitudes in compartment as become problem in JPP Plantation.Moreover, the determining factors were different fromsite quality, plant maintenance and imbalancing fertilizing. These factors were also found to influence the growth rate .
Site is the main factor which influences the growth athough treatment maintenance has been done similarly. The plantation of JPP in different sites will also produce different qualities. Harbagung  explained that information about site quality becomes important parameter in forest management in order to determine silvicultural treatments. Therefore, a model is needed to predict height growth of dominant trees and site index for perhutani's teak plus plantation which can be used as reference for forest management plans. The model is used toassess site productivity . Then, it can be used as reference for management planning of JPP.
This research is aimed to determine the prediction model of dominant tree height growth for perhutani's teak plus plantation, from which a site quality class model would be set. The model is expected to be applied on perhutani's teak plus plantation in Forest Planning Section (SPH) at Madiun, Saradan and Ngawi Forest Districts, East Java Perhutani Regional Division.
MATERIAL AND METHODS
This research was conducted in 2013, 2014 and 2015 in the Forest Planning Section (SPH) Madiun, Saradan and Ngawi Forest Districts, East Java Perhutani. Samples were selected by taking 30 dominant trees of Perhutani's teak plus with a good quality from clonal seed orchard in each compartment which was aged around 6 to 13 years. To meet the assumption of normal distribution, samples were taken from 30 dominant trees in each compartment.
The material data were from dominant heights of perhutani's teak plus plantation from clonal seed orchard. The total numbers of compartment were 147. All compartments were established by selecting 30 dominant trees in each compartment which was aged around 6 to 13 years. The minimum number of compartment was three compartments in each age and each forest district. Table 1 shows research site for the year of 2013, 2014 and 2015.
Dominant height is usually defined as the average height of a specific number of the largest diameter or tallest trees for a unit area, it is useful to measure the site quality because it is little affected by variation in stand density or by most thinning regimes . However, Yevide et al.  used another definition, site index as the mean quadratic diameter at reference age wherethe guide curve method has been used. Dominant height in this research is defined as the average of 30 dominant trees of perhutani's teak plus plantation per compartment . The dominant height is characterized by trees that are healthy, straight and cylindrical. Height measurement used was haga hypsometer.Dominant height samples were taken by calculating the average of each compartment and age. Samples were analysed using non linear regression in SPSS 20 software. Normality distribution of dominant height was analysed using boxplot.To predict height growth ofdominant heights, 50% of data were used based on the values of approaching mean. It was expected that the result obtained would be more representative.
The three elements of modeling dominant height were selected as a modeling form, a modeling data structure, and statistical procedures used to estimate model parameters . Many growth model equations had been applied to construct site index model, such as Chapman Richards, Johnson Schumacher and log logistic models. Variations of those models have probably been the most commonly used for site index modeling [4,15]. These different methods can have important implications on the resulted site index equation [16,22].
The three non linear models were respectively adjusted by adopting the dominant height and the age as dependent and independent variables. Table 2 shows the dominant height models proposed for teak plus plantation. Guiding models had been generated since the dominant height was equal to the site index and the reference age. Reference age was 13 years as the oldest age of perhutani's teak plus plantation. The site class curve was drawn up using mean of the guiding model. This model was obtained using the model selected by the highest coefficient of determination ([R.sup.2]) as the statistical criterion.
Site index is commonly used to measure site quality, it is defined as dominant height of stand at site index age . Determination of the site index can be done in two ways, indirect and direct site evaluations [3,18]. Indirect site evaluation is determined by parameter of soil, climate, and vegetation types. While, direct site evaluation is determined by tree height, total production and standing stock. This research used direct site evaluation using dominant tree height as site index. The direct site evaluation is commonly used for site index modeling by considering efficiency. Dominant height has close correlation with forest yield that is related to volume [12,4,9,17]. It is also influenced by other factors such as stand density and thinning [3,18]. Site index classification is defined as dominant height because all of the environmental factors are being reflected by the height growth.
This research used direct site evaluation using dominant tree height as site index. In this case, the oldest age of perhutani's teak plus plantation was 13 years when data were collected from the rotation period set at 20 years. For that reason, the reference age used was 13 years. Dominant height model was used to divide data into three classes (good, medium and poor).Classification of the site index was based on the dominant height mean ofthe reference age. Classification value is based on the mean and it was arranged into site index table which attached the top height in certain age and site class . Based on the guiding model, construction site index curve and site classifications(good, medium and poor) would be set.
RESULTS AND DISCUSSION
Measurement of height refers to Laiho et al.  using the samples of 30 dominant trees in each compartment. Site index was constructed by collecting the sample of 30 dominant trees from each compartment in the age of 6 to 13 years. Data from 2013, 2014 and 2015 were compiled. Total compartment was 147 compartments. Samples were calculated by averaging the dominant heightsfrom each compartment and age. In order to find out the normality distribution and the outlier, through the use of SPSS 20, the samples were analyzed using descriptive analysis and boxplot. Figure 1 shows the result in boxplot and scatterplot.
[FIGURE 1 OMITTED]
Boxplot and scatterplot show dominant heights mainly close to normal distribution. Growth of dominant heights increased with age, except in the age of 6 years. Table 3 shows statistics of dominant height over the ages.
Dominant heights were selected by considering 50% of each age for analysis to which it was based on the values of approaching mean. It was expected that the result obtained would be more representative. Table 4 shows the statistical criterion of candidate models for the dominant height modeling based on the coefficient of determination ([R.sup.2]).
Compared to Chapman Richards and Wolf von Wulfing models, Schumacher model presents the proposed model based onthe highest coefficient of determination ([R.sup.2]). The Schumacher model was selected for drawing the site index curve. This curve of dominant heights is shown in Figure 2.
Schumacher model is the proposed model used in this research in order to predict the dominant height growth of Perhutani's teak plus plantation. It is similar with the result in Friday's research, which used Schumacher and Log-log models for teak in the Limestone hill, Poerto Rico. The Schumacher model has constant approach for height with increasing age, while the log-log model does not approach the limit. In contrast, Sajjaduzzaman et al. suggested that the Chapman Richards model was found to be the most suitable model for teak in Bangladesh. Furthermore, Tewari et al.  roposed equation from Korf model to predict the dominant height of teak in India. Anwar also  invented equation model for predicting teak bonita in Central Java from Wolf von Wulfingtable , but the results show that the model performance have the lowest coefficient of determination ([R.sup.2]) for predicting dominant height of Perhutani's teak plus plantation. The different objects of height probability influence the result. Wolf von Wulfing  used height of conventional teak characterized by slow growing species and this research used height of Perhutani's teak plus characterized by fast growing species.
[FIGURE 2 OMITTED]
Figure 2 shows the guiding curve that was drawn using Schumacher model. Dominant height scattered is mainly close to the guiding curve site index. On the contrary, for the age of 6 years, most of the dominant heights are scattered below the guiding curve site index. Based on Schumacher site index equation model, the dominant heights were divided into three classes that belong to the age of 13 years as the reference age. The use of reference age should be the same as the rotation period . However, there was not yet plantation in the age of 20 years that could be used as the rotation period. Hence, the selected reference age in this research was 13 years as the oldest age. Graph site index curve class is shown in Figure 3. Dominant heights for the age of 13 years were 11 m as the minimum height and 17 m as maximum height. Furthermore, mean of the dominant height for the age of 13 years was 14 m. Dominant heights were divided into three classes based on the reference age. The limit of the dominant height is 14 m. The equation for the class division is as follow:
[FIGURE 3 OMITTED]
Based on the guiding curve as the limit, the site class was divided into three parts (Site Index 16 as a good class, Site Index 14 as a medium class and Site Index 12 as a poor class).Site Index 16 was the dominant heights that were located above the curve of Site Index 14. Site Index 13 was dominant heights that were located under the curve of Site Index 14. Table 5 presents the values of site class limit of Schumacher guiding model.
Table 5 shows that the class limit increased with age. However, for the age of 7 and 8 years, they are different, the class limit for the age of 7 years is higher than the age of 8 years and the class limit for the age of 8 years is lower than the age of 8 years.
Reliable growth model is essential for effective long term planning and decision making. This is important in intensively managed forest plantations and is important to evaluate alternative planting densities, thinning regimes and rotation lengths . The forest height growth is directly related to the site characteristics and the forest productivity, as well as the improvement of stand growth that represents better productivity estimation .
Three candidate models were analysed using non linear regression and fitted to the data in dominant height growth. Eventhough all models presented similar performance, the highest coefficient of determination ([R.sup.2]) went to Schumacher model. Schumacher model was the proposed modelused for predicting the dominant height growth and the site index forPerhutani's teak plus plantation in the Forest Planning Section (SPH) at Madiun, Saradan and Ngawi Forest Districts, East Java Perhutani. Based on the data, Schumacher site index equation model has produced site index class for perhutani's teak plus plantation for the age of 6 to 13 years. The site index class was divided into three classes, they were Site Index 16 as a good class, Site Index 14 as a medium class and Site Index 12 as a poor class.
The authors are grateful to Gadjah Mada University for financial support and to Perum Perhutani East Java Region Division for permission to conduct this research. Next acknowledgement is addressed to the management and the staff of Madiun, Saradan, and Ngawi Forest Districts that facilitated this research with the measurement in the field. Thank to Wieke Herningtyas for preparing the manuscript and helping in the aspect of data analysis, and to the measurement team for good cooperation and hard work during data collection period in the field as well as for anonymous reviewers for the valuable comment and suggestions to help to improve this manuscript.
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Gadjah Mada University, Department of Forest Management, Faculty of Forestry
Address For Correspondence:
Ronggo Sadono, Universitas Gadjah Mada, Department of Forest Management, Faculty of Forestry. Jl. Agro No. 1, Bulaksumur, Yogyakarta, 55281, Indonesia
E-mail: firstname.lastname@example.org, phone and fax: +62 274 548815
Received 8 January 2017; Accepted 28 April 2017; Available online 24 May 2017
Table 1: Perhutani's teak plus compartments for study site measured in 2013, 2014 and 2015 Year of Age (years) Number Compartment in Forest District of measurement Madiun Saradan 2013 6 83a, 92, 112a 121, 104g, 136c 7 76d, 92b 50a, 165a, 136a 8 65b, 89b, 34a 133a, 152g, 6b 9 118d, 114f 92b, 2b 10 62e, 103g, 76a 77c 11 118h, 121d, 62b 123a 12 - - 13 - - 2014 6 - - 7 112a, 60c, 59a 152g, 98b, 100a 8 72c, 70d, 92b 106b, 152a, 160g 9 34a, 72b, 74d 152b, 152c, 102a 10 100k, 118d, 114c 2b, 93b 11 123b, 76a, 74c 77c 12 117d, 121f, 117k 123a 13 - - 2015 6 69a, 67d, 103c 16d, 67a 7 26, 2b, 3a 121c, 6b 8 228a,145b, 79a, 69e, 100a 100a, 89b, 104g 9 92a, 78c, 152b 160b, 114d, 81a 10 34a, 74d, 80a 94b, 6c, 96d 11 118d, 296b, 90g 2b, 93c, 84a 12 76a, 123b, 82a 71c, 93d, 96a 13 121f, 101i, 117k 16a, 91a, 123a Year Number Compartment in Forest District of measurement Ngawi 2013 53b, 54c, 56k 44d, 44a, 47g 103a 151g, 3i 151h - - - 2014 - 51d, 3c, 50b 4h, 4b, 7b - 151g, 3i 151h 156b - 2015 - 73c, 174k 45b, 48f, 64g 47h, 49d, 65g, 65h2 22h, 175g, 143a, 135e 2b 167e 45a, 47a, 52d, 72b Table 2: Candidates Model of Dominant height growth for teak plus plantation Name of candidate Equation Regression model coefficient Schumacher in H = [ae.sup.b/t] a,b Clutter et al. Chapman-Richards H = a[(1 - [e.sup.-kt]).sup.(1/1-b)] a,k,b in Clutter et al.  Chairil Anwar  Ln [??] a,b Where: H = dominant height t = age a, b, k = regression coefficient Table 3: Descriptive Statistics of dominant height over ages Age (years) Number of Mean of dominant height Std. Deviation compartments (m) 6 14 9.642 2.307 7 24 11.689 1.212 8 27 12.074 1.517 9 22 12.318 1.460 10 22 13.000 1.772 11 16 13.125 1.707 12 12 13.583 1.443 13 10 14.000 2.160 Age (years) Minimum Maximum 6 8.310 10.975 7 11.177 12.201 8 11.473 12.674 9 11.670 12.965 10 12.213 13.786 11 12.214 14.035 12 12.666 14.500 13 12.454 15.545 Table 4: Paramaterization of candidate models for dominant height growth model name equation Number compartments Schumacher H = [ae.sup.b/t] 74 Chapman-Richards H = a[(1 - [e.sup.-kt]).sup.(1/(1-b)] 74 Chairil Anwar Ln [??] 74 model name of [R.sup.2] parameter value Schumacher 0.532 a 16.524 b 2.649 Chapman-Richards 0.530 a 70.011 k 0.149 b -0.573 Chairil Anwar 0.529 a 3.509 b 1.941 Table 5: Dominant height limits of each site class for perhutani's teak plus Age (years) dominant height (m) for site quality class SI 12 SI 14 SI 16 6 >9.460 9.460-12.614 <12.614 7 >10.076 10.076-13.086 <13.435 8 >10.565 10.565-14.086 <14.086 9 >10.960 10.960-14.614 <14.614 10 >11.288 11.288-15.051 <15.051 11 >11.563 11.563-15.418 <15.418 12 >11.797 11.797-15.730 <15.730 13 >12.000 12.000-14.000 <14.000
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|Publication:||Advances in Environmental Biology|
|Date:||May 1, 2017|
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