Productivity of Clonal Teak Plantation Under Different Spacing and Thinning Intensity in Java Monsoon Forest

Intensive silviculture plays a crucial role in selecting superior clones with high productivity. The use of superior clones needs to be accompanied by appropriate environmental conditions. The arrangement of growing space is one way for teak clones to grow optimally and produce quality wood products. It can choose initial spacing in the early growth phase and thinning intensity in the later growth phase. This study aims to assess and determine the growth of superior teak clones with different thinning intensities at different planting distances to optimize the growing space. Optimizing growth space also has the potential for intercropping under the stand to produce more diverse products. The research design was a Split-plot design with three blocks as replications. The main plots had four spacings, including 3 m x 3 m, 6 m x 2 m, 8 m x 2 m, and 10 m x 2 m. Subplots were thinning intensity, including 0% thinning (control), 25% thinning (moderate thinning), and 50% thinning (heavy thinning). The results showed that spacing could significantly affect diameter growth. Silvicultural thinning showed significant differences in diameter growth, CADI, height, and volume per hectare. The diameter growth showed higher growth at larger spacing and higher thinning intensity, while the volume per hectare was low. The tree height parameter increased with the thinning intensity. The CADI after two years of thinning increased by 14–42% at 25% thinning intensity and increased by about 35–48% at 50% thinning intensity compared to no thinning at each spacing. Meanwhile, the volume per hectare was lower at wider spacing and decreased at a high thinning intensity. These results indicate an increase in spacing, and thinning accelerates diameter growth and provides better-growing space for intercrops.


Introduction
The lowland monsoon forest is one of the tropical forest types dominated by teak (Nair, 2004).Teak is one of the most important hardwood species in the world due to its durability, high economic value, and large-scale plantations (Kollert and Cherubini, 2012;Sreekanth et al., 2012;FAO, 2013).Teak forests in Java are the largest, 67% of the total production forest area in Java, and have experienced a 15-42% 2 decline in forest and land productivity (Perhutani, 2014;Perhutani, 2016).Rehabilitation efforts through superior clone selection are needed to enlarge productivity (Hardiwinoto et al., 2021).Teak breeding has produced superior teak clones with high growth (Budiadi et al., 2017).In addition to genetic quality, the arrangement of growing space is a crucial silvicultural consideration (Isaac-Renton et al., 2020).Space management can be accomplished by choosing the initial spacing at the beginning of planting and continuing thinning in the advanced phase.Cultivated target species in a suitable environment are prospected to promote faster growth (Soekotjo, 2009).Plant spacing affects the amount of light and the availability of nutrients (Faridah et al., 2009).
The initial spacing is one of the crucial silvicultural decisions in plantation establishment and affects competition between trees (Evans and Turnbull, 2004).Initially, most community teak forests were established with a spacing of 3m x 3m, following the planting pattern of Perum Perhutani, without thinning (Wahyudi et al., 2014).With the development of science, teak plantations began to be designed with larger spacing and thinning to increase productivity, one of which was the initial spacing of 6 m x 2 m.This spacing has also been successful in increasing teak productivity and accommodating agroforestry systems (Prehaten et al., 2021;Rahmawati et al., 2021;Maharani et al., 2022).In addition, the combination of plant genetic quality and initial spacing is a silvicultural consideration to increase forest productivity (Isaac-Renton et al., 2020).In addition, the rapid growth of teak clones causes the growing space to fill up quickly, which increases competition between trees (Sadono and Silalahi, 2010), making it necessary to intensify thinning.Silvicultural thinning is very important in teak because teak is very intolerant of competition (Ugalde, 2013).The silvicultural intervention of thinning is one of the keys to intensive stand management to increase productivity and produce high-quality timber (Nylan, 2002;Pachas et al., 2019).
On the other hand, the management pattern of teak plantation forests in Java is generally developed in intercropping by planting crops under teak stands (Pachas et al.2019).It is to meet the increasing demand for food and land as the population in Java.Java accounts for 50% of Indonesia's population (BPS 2018).Intercropping can increase land productivity and contribute to meeting food needs (Jamnadass et al., 2013;Coulibaly et al., 2017;Feliciano et al., 2018).Shade trees in agroforestry can also improve soil fertility and carbon sequestration (Teja, 2011).Intercropping was implemented on research plots in different studies.Studies on the productivity of teak clones with different thinnings at different spacings are limited.This study is needed to optimize teak forest management to provide timber and food for communities around the forest.

Research Site and Design
The research was conducted on a teak clone plantation in Perhutani Ngawi, East Java, Indonesia (7°22' S and 111°18' E).The dominant soil type in BKPH Kedunggalar is grumusol soil.The average annual rainfall is 1,172 mm, with around90%occuringbetween November and April (BPS Ngawi, 2018).The light intensity is around 16-30% at different spacings.The clonal teak plantation was established in 2011 with four different initial spacings.Thinning was carried out on the clonal teak plantation at eight years old.The research was conducted on an 8-year-old to 10-year-old for two years.The research design was a Split-plot design with 3 (three)blocks as replications (Figure 1).The main plot consisted of 4 planting distances, including 3 m x 3 m, 6 m x 2 m, 8 m x 2 m, and 10 m x 2 m.The sub-plot consisted of three thinning intensities.The second factor consisted of 3 thinning intensities, including 25% thinning (medium), 50% thinning (heavy), and 0% thinning (control).There were 12 (twelve) combinations of spacing and thinning treatments (Table 1).The thinning method was systematic thinning to provide trees space to grow optimally.The measurement of clonal teak productivity was as follows: a. Clonal teak spacing, trees density ha- Where CADI= current annual diameter increment (cm/year); DBH = diameter at breast height (cm); i = current year; 0 = previous year.The data obtained were analyzed using SPSS 18.0 software with analysis of variance (ANOVA).Differences between treatments were analyzed using DMRT (Duncan's Multiple Range Test) at the tα 0.05.In addition, canopy projection was performed using SExI-FS software to spatially characterize individuals in each plot (Vincent and Harja, 2008).

Effect of spacing and thinning on teak growth
The results showed that different spacings and thinnings affected the opening of the growing space and the growth of the teak clones (Table 2; Figure 2).Spacings affected DBH growth (P < 0.01) but did not affect height or volume growth (P > 0.05).Silvicultural thinning affected DBH growth (P < 0.01), height (P < 0.05), and volume per hectare (P < 0.01) of 8-year-old clonal teak (Table 2).In contrast, the interaction of spacing and thinning did not affect DBH growth, height, or volume per hectare of clonal teak stands (Table 2).).Thinning at 50% increased tree height growth but decreased volume per hectare (Figure 4).The 6 m x 2 m spacing had a faster growth response to thinning.That is due to the increased space for teak clones to grow.Increased space growth helps crown and root development due to reduced competition (Zahabu et al., 2015).Competition between trees is related to stand density, where the denser the stand, the more competition between trees will increase (Rahmawati, 2021).The diameter distribution showed larger in-diameter classes with larger plant spacing and higher thinning intensity (Figure 5).It indicates that wider plant spacing and higher thinning intensity can increase the diameter class, thus increasing the selling price of the timber.
Thinned stands also significantly influenced plant height growth and stand volume per ha (Figure 4).Height growth increased with thinning intensity (Table 3).Stand density also showed a statistically significant effect on height growth (Berrocal, 2020)

Effect of spacing and thinning on current annual diameter increment
Spacing and thinning significantly affected teak clone diameter increment growth (CADI) in years 1 and 2 after thinning (Table 4).Meanwhile, the interaction between spacing and thinning was not significantly different from the CAI of residual clonal teak stands (Table 4).The mean diameter increment increased with greater thinning intensity at a larger spacing in the first and second years after thinning (Figure 6; Figure 7).The percentage increase in diameter per year at each spacing showed different response.Diameter increment at 3 m x 3 m, 6 m x 2 m, 8 m x 2 m, and 10 m x 2 m spacing with thinned 25% increased by 16.8%, 42.3%, 28.3%, and 14.18%, respectively, while at thinned 50% it increased by 42.8%, 48.4%, 35.3%, and 37.3%, respectively, compared to no thinning (Figure 8).The diameter increment in 1 year was high with higher thinning intensity (Figure 6).It shows that thinning can significantly increase DBH growth.Diameter growth is positively affected in the first year after thinning (Sudomo, 2021).It indicates that the diameter increment in the no-thinning treatment decreases with the increasing age of the teak clones.Silvicultural thinning is a way to stimulate plant conditions to increase growth (Djati et al., 2014) so that the remaining individual teak trees can grow denser and larger (Seviset et al., 2017).Long-term thinning affects tree growth positively (Stefančík et al., 2018).
Heavy thinning (50%) increased DBH by 26% compared to un-thinned and was significantly different from moderate thinning (25%) and control (0%) (Budiadi et al., 2017).Intensive thinning increases tree growth to the desired proportion of DBH and total height (Perez and Kaninen, 2005).In this respect, thinning plays an essential role in optimizing teak growth.In a given period with the same spacing, teak growth will decline or not increase, thus requiring thinning to optimize teak growth (Seviset et al.,2017).

Implication for sustainable clonal teak plantation and sustainable agroforestry in Java
The largest teak forest in Indonesia is on Java Island, with 67% of Java's production forest area (Perhutani, 2014).Increasing the productivity of teak clone stands is necessary to increase teak wood production with good quantity and wood quality.Intensive silviculture with a selection of superior clones and environmental manipulations such as spacing and thinning can increase productivity and optimal quality of forest products (Soekotjo, 2009).Superior clones can be harvested in the short term to maintain timber supply.At the same age, the clonal teak diameter is larger than the diameter of conventional teak (Basri and Wahyudi, 2012).An increase in the productivity of the forest through the use of superior teak clones is expected to fulfill the high market demand for teak wood (Keogh, 2009).Plant spacing and thinning are critical to achieve desired wood properties and optimize growth (Pfister et al., 2006).Wide spacing and high thinning intensity can increase the diameter class.An increase in teak diameter will increase the selling price of teak in the market (Budiadi et al., 2017;Pachas et al., 2019;Seta et al., 2021).Clonal teak plantations were developed to produce timber for construction, with high diameter growth as one of the requirements.Wide spacing increases diameter growth and increases the heartwood percentage, although not significantly (Rahmawati et al., 2022).However, wide spacing reduces the bole height, so it needs to be accompanied by pruning maintenance (Rahmawati et al., 2021).Intensive silviculture, such as growth space management, is required to increase productivity and quality.For this reason, intensive silvicultural measures by selecting superior clones and environmental manipulation by thinning can increase the productivity of clonal teak stands.
The reasons for intensive silviculture include the increasing needs of forest communities (Soekotjo, 2014).Intensive silviculture can be combined with intercropping patterns to obtain food reserves (Na'iem, 2014).Intercropping is an effort to utilize the space between the trees.Post-thinning intercropping can provide space for the remaining stands and intercropping plants (Sudomo et al., 2021).Thus, Superior teak clones with intensive silviculture combined with intercropping can increase the productivity of teak forests.The intercropping period is usually 2-3 years after the initial planting of the teak.However, crops can be extended under teak plantations at a distance of more than 6 m to 10 m (Pachas et al., 2019).Teak plantations in the early growth phase managed with intercropping patterns are combined with crops and herbal plants in the advanced phase of teak growth (Sabarnurdin et al., 2011).Thus, in the early or advanced stages of clonal teak stands, intercropping can be implemented under the trees to optimize the growth of clonal teak and sustainable agroforestry products.

Conclusion
Stand management practices such as plant spacing and thinning affect DBH growth significantly.Wide spacing and high thinning intensity can increase the diameter class and, therefore, the commercial value of the timber.The thinning of superior teak clones is crucial to optimizing the growth of advanced teak clones.Thinning enlarged in diameter, CADI, and height but significantly decreased in volume per hectare two years after thinning.The diameter increased by 14-42% at 25% thinning intensity and by approximately 35-48% at 50% thinning intensity compared to no thinning at each spacing after two years of thinning.Treatment of spacing and thinning also provides growing space for the remaining stands and the potential for intercropping under the trees.However, maintenance, such as pruning, needs to accompany this treatment.It optimizes the productivity of the clonal teak plantations and sustainable agroforestry under the plantations.

Figure 1 .
Figure 1.Research designs of clonal teak at different spacing and thinning2.2.Data Collection and AnalysisThe tree growth parameters observed in this study are diameter at breast height (DBH), total height, total volume, and current annual diameter increment (CADI).The diameter tape measured DBH at the height of 1.3 m, and the hagameter measured the total height of the teak.The total volume and current annual diameter increment were calculated as follows: V= (-0.0884+0.0297*DBH)2(1)where V = volume (m3 ha−1) and DBH = diameter at breast height (cm).The volume (V, m3) of individual trees was calculated using the formula proposed byPerez & Kanninen (2003) CADIi = DBHi -DBH0(2)

Figure 2 .
Figure 2. Crown projection of superior teak clones before thinning (top) and after thinning (bottom) at different spacing Mean DBH increased with increasing plant spacing and thinning intensity (Figure3).Increasing plant spacing affects diameter and crown area(Rahmawati et al., 2021) and suppresses diameter growth and smaller log sizes(Charlton et al., 2020).It confirms that teak is an intolerant species of competition between trees(Pachas et al., 2019).Wide plant spacing provides adequate space for plant growth to use sunlight, water, and minerals(Faridah et al., 2009) but reduces bole height and increases branching angle(Rahmawati et al.,2021).

Figure 3 .
Figure 3. Mean DBH in teak clones with different spacing (A) and thinning (B).

Figure 4 .
Figure 4. Mean height (A) and volume per hectare (B) in teak clones with different thinning intensitiesIn addition, high thinning intensity resulted in significant diameter growth(Budiadi et al., 2017;Seta et al., 2021).Thinning at 50% increased tree height growth but decreased volume per hectare (Figure4).The 6 m x 2 m spacing had a faster growth response to thinning.That is due to the increased space for teak clones to grow.Increased space growth helps crown and root development due to reduced competition(Zahabu et al., 2015).Competition between trees is related to stand density, where the denser the stand, the more competition between trees will increase(Rahmawati, 2021).The diameter distribution showed larger in-diameter classes with larger plant spacing and higher thinning intensity (Figure5).It indicates that wider plant spacing and higher thinning intensity can increase the diameter class, thus increasing the selling price of the timber.Thinned stands also significantly influenced plant height growth and stand volume per ha (Figure4).Height growth increased with thinning intensity (Table3).Stand density also showed a statistically significant effect on height growth(Berrocal, 2020).On the other hand, high thinning intensity reduced the volume per unit area.It is due to high thinning intensity that can reduce the number of trees per hectare.Stand density affects diameter growth and volume produced per unit area(Vigulu et al., 2019;Rahmawati et al., 2022).Thinning at 25% intensity significantly increased diameter growth while providing a high volume per hectare.Thinning is the principal aspect of silviculture to produce better-quality logs at the end of the cycle(Stewart et al., 2020;Seta et al., 2021).However, high thinning intensity can increase both branch diameter and the number of knots (Isaac-Renton et al., 2020).Straight stems and low branching are crucial factors(Seviset et al., 2017).For this reason, increasing the DBH of the plant can be done by increasing the maintenance of the plant in the form of pruning.Pruning maintenance at the right time Figure 4. Mean height (A) and volume per hectare (B) in teak clones with different thinning intensitiesIn addition, high thinning intensity resulted in significant diameter growth(Budiadi et al., 2017;Seta et al., 2021).Thinning at 50% increased tree height growth but decreased volume per hectare (Figure4).The 6 m x 2 m spacing had a faster growth response to thinning.That is due to the increased space for teak clones to grow.Increased space growth helps crown and root development due to reduced competition(Zahabu et al., 2015).Competition between trees is related to stand density, where the denser the stand, the more competition between trees will increase(Rahmawati, 2021).The diameter distribution showed larger in-diameter classes with larger plant spacing and higher thinning intensity (Figure5).It indicates that wider plant spacing and higher thinning intensity can increase the diameter class, thus increasing the selling price of the timber.Thinned stands also significantly influenced plant height growth and stand volume per ha (Figure4).Height growth increased with thinning intensity (Table3).Stand density also showed a statistically significant effect on height growth(Berrocal, 2020).On the other hand, high thinning intensity reduced the volume per unit area.It is due to high thinning intensity that can reduce the number of trees per hectare.Stand density affects diameter growth and volume produced per unit area(Vigulu et al., 2019;Rahmawati et al., 2022).Thinning at 25% intensity significantly increased diameter growth while providing a high volume per hectare.Thinning is the principal aspect of silviculture to produce better-quality logs at the end of the cycle(Stewart et al., 2020;Seta et al., 2021).However, high thinning intensity can increase both branch diameter and the number of knots (Isaac-Renton et al., 2020).Straight stems and low branching are crucial factors(Seviset et al., 2017).For this reason, increasing the DBH of the plant can be done by increasing the maintenance of the plant in the form of pruning.Pruning maintenance at the right time

Figure 5 .
Figure 5. Diameter distribution at 3 m x 3 m (A) 6 m x 2 m (B) 8 m x 2 m (C) and 10 m x 2 m (D) with different thinning intensities.

Figure 6 .
Figure 6.Mean current annual diameter increment in the first year after thinning at different spacing (A) and thinning intensities (B).

Figure 7 .
Figure 7. Mean current annual diameter increment in the second year after thinning at different spacing (A) and thinning intensities (B).The diameter increment in 1 year was high with higher thinning intensity (Figure6).It shows that thinning can significantly increase DBH growth.Diameter growth is positively affected in the first year after thinning(Sudomo, 2021).It indicates that the diameter increment in the no-thinning treatment decreases with the increasing age of the teak clones.Silvicultural thinning is a way to stimulate plant conditions to increase growth(Djati et al., 2014) so that the remaining individual teak trees can grow

Figure 8 .
Figure 8. Growth of current annual diameter increment in years 1 and 2 (cm/year) of superior teak clones after thinning.

Table 1 .
Combinations of Spacing and Thinning Treatments Codes of Clonal Teak

Table 2 .
Results of variance analysis of treatments effect on clonal teak growth

Table 3 .
Mean values of DBH, height, and volume of teak clones with different spacing and thinning after two years of thinning

Table 4 .
Treatment effects on Current Annual Diameter Increment (CADI) at 1 and 2 years after thinning of clonal teak Note: df=Degree of freedom; MS= Mean Square; * significant at p ≤ 0.05, ** significant at p ≤ 0.01, ns: non-significant different at tα 0.05