Quantitative Assessment of Root Anchorage and Binding Capacities of Teak and Pine for Shallow Landslide Mitigation in Sukabumi, Indonesia

Global climate change has emerged as a pressing concern, evidenced by rising global temperatures that affect all regions, including Indonesia and the island of Java. The average global temperature has increased by approximately 2℃ above pre-industrial levels. According to estimates, developing countries require an annual investment of approximately USD 6 billion solely for disaster risk management. Climate change adaptation forms a critical component of the 2030 Sustainable Development Agenda, particularly within Goal 13, which emphasises enhancing resilience and adaptive capacity to climate-induced hazards. Among the most significant natural disasters linked to climatic variability in Indonesia are landslides.

A lack of preparedness and adaptive measures toward natural hazards inevitably leads to heightened disaster risks. Environmental degradation—particularly deforestation—often intensifies community vulnerability rather than mitigating it. Likewise, unregulated urban expansion into hazardous zones, such as steep slopes or flood-prone lowlands, significantly increases risk exposure. Over time, populations tend to normalise slow-onset disasters, inadvertently accepting them as routine despite the escalating socioeconomic losses they incur [1].

Landslides are primarily triggered by rainfall [2-14], a finding consistently supported in numerous studies. Forested areas with sparse vegetation cover are particularly vulnerable to landslide impacts during intense precipitation [15]. In Indonesia, where the average annual rainfall ranges from 1,000 to 4,000 mm, regions with deforested or poorly managed forest areas face significantly elevated landslide risks.

West Java is one of Indonesia's provinces with the highest landslide disaster rates, with Sukabumi Regency being one of the most frequent. The National Disaster Management Agency (BNPB) reported 369 landslides in Sukabumi Regency between 2020 and 2025.

Forest ecosystems mitigate such hazards by intercepting rainfall, retaining water within the soil matrix, and gradually releasing it into downstream systems. This process significantly reduces surface runoff and improves slope stability [15-17].

In addition to climatic factors, variables such as land use intensity, vegetation type [18, 19], Human-induced activities—including deforestation—play a significant role in triggering landslides [2, 14, 19-21].

Landslides are also one of the consequences of deforestation [22]. Deforestation leads to increased landslide frequency due to the control of forest roots on slope stability. Numerous studies have highlighted how root systems enhance soil cohesion, particularly in sloped environments, making sustainable forestry practices integral to landslide mitigation strategies. Landslides are typically classified into deep-seated and shallow categories [10], with shallow landslides characterised by a failure depth of less than two meters.

Previous studies have highlighted the crucial role of vegetation in mitigating landslides and soil erosion through mechanical and hydrological mechanisms. Vegetation is pivotal in mitigating landslides and soil erosion by enhancing slope stability through both mechanical and hydrological effects [23]. One of the key mechanisms lies in the root systems, which significantly improve soil cohesion and shear strength; among various species, Pinus merkusii has demonstrated the highest soil-root cohesion [24]. Building on this, emphasised that vegetation with interlocking root systems can effectively reduce landslide risk in vulnerable areas [24]. Further supporting this, vegetation is particularly beneficial in lowering soil movement during the rainy season, owing to its dual mechanical and hydrological functions [25]. However, the overall effectiveness of vegetation in slope stabilisation is influenced by several factors, including root architecture, soil conditions, and plant species characteristics.

Vegetation is widely recognised as an eco-friendly and cost-effective approach to slope stabilisation, primarily through its mechanical and hydrological influences on soil strength. Root systems enhance soil cohesion and shear resistance, reducing the risk of shallow landslides. However, despite growing interest in nature-based solutions, challenges such as the time required for vegetation to establish and the need for continuous maintenance remain significant limitations. Moreover, while previous studies have demonstrated the general benefits of vegetation for slope stability, empirical data on the root reinforcement performance of specific tree species in particular soil types—such as latosols—are still limited [25]. Addressing this research gap, the present study quantitatively assesses root anchorage and binding capacities of Tectona grandis (teak) and Pinus merkusii (pine) in latosol soils. These two species are commonly used in reforestation programs across landslide-prone regions, including Sukabumi, Indonesia. The findings are expected to contribute to species-specific recommendations for shallow landslide mitigation and support adaptive land management strategies in response to increasing environmental challenges.

This study investigates explicitly shallow landslides and aims to evaluate the mechanical contribution of tree roots using the Root Anchorage Index (RAI) and Root Binding Index (RBI). These indices are applied to analyse root strength in teak (Tectona grandis) and pine (Pinus merkusii), the primary commercial forestry species managed by the state-owned enterprise Perhutani. Teak and pine have long served as the economic backbone of Indonesian forestry, not only due to their high commercial value in timber and non-timber products but also for their ecological services. Teak is cultivated predominantly for its high-density wood, while pine is managed for timber, non-timber, and resin derivatives such as gum rosin and turpentine. Additional species—cajuput (Melaleuca leucadendra), rubber (Hevea brasiliensis), and albizia (Paraserianthes falcataria)—were included in this study to provide a comparative framework for assessing root strength across species. The involvement of forestry enterprises was limited to logistical support and access to field sites, ensuring the objectivity of the study's scientific findings.

1.1 Study area

This study was conducted in the Sukabumi Forest Management Unit (Kesatuan Pemangkuan Hutan or KPH Sukabumi), one of the management units under the Perhutani Regional Division of West Java and Banten. KPH Sukabumi comprises six forest sections (BKPH): Bojong Lopang, Cikawung, Jampang Kulon, Lengkong, Pelabuhan Ratu, and Sagaranten, as shown in Figure 1. Administratively, KPH Sukabumi is located within Sukabumi Regency, West Java Province, Indonesia.

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Figure 1. Study area in KPH Sukabumi

According to the Forest Sustainability Management Plan KPH Sukabumi for 2023-2032, the research area comprises two dominant soil types: latosol and grumusol. Latosol soils are distributed across all Sub-Forest Management Units (BKPH), whereas grumusol soils are localised in the Bentang Barat Forest Subdivision of BKPH Sagaranten. All sampling locations for root index measurements in this study were on latosol soils. Latosol soils are typical of tropical rainforest ecosystems and are characterised by high porosity, generally between 40% and 60% [37], depending on site conditions and land management practices. This high porosity facilitates root penetration and gas exchange, promoting plant growth and overall soil ecosystem function. However, latosols are often nutrient-poor and require active soil management to sustain fertility and vegetation cover.

Additionally, latosol soils in the study area have a moderately acidic pH, typically ranging from 5.0 to 5.8, influencing nutrient availability and microbial activity essential for root development. The bulk density is relatively low, averaging around 0.9 to 1.1 g/cm³, indicating a loose soil structure that supports deeper root penetration and reduces mechanical resistance. Moreover, the shear strength of these soils is moderate, generally between 25 and 35 kPa, which affects the soil's ability to resist deformation and contributes to slope stability, an essential factor in assessing the anchorage capacity of tree roots in landslide-prone terrains.

The root system, a vital link between plants and soil, has been largely unexamined but is of immense practical importance. Plant roots are essential in strengthening the soil, preventing adverse soil movements, stabilising the plant itself, preventing soil erosion, and stabilising slopes. The mechanical stabilisation of soil slopes through tree roots depends mainly on the strength properties of the roots and their growth pattern in the soil. The effect of tree roots on slope stability can be considered in terms of their strength and distribution in the soil [14, 16, 17]. These findings have significant implications for soil management and conservation practices.

3.1 Standing conditions in KPH Sukabumi

The tree stands in the Sukabumi Forest Management Unit and represents a commercial species in the Indonesian timber trade. The existing species include Tectona grandis (teak), Pinus merkusii (pine), Melaleuca leucadendra (eucalyptus), Hevea brasiliensis (rubber), and Paraserianthes falcataria (albizia) with varying age classes and areas. Table 2 outlines the specific characteristics of the sampled areas, including species composition, age distribution, and site conditions.

Table 2. Composition of plant species aged 4, 5, and 6 years

3.2 Root Anchorage Index (RAI) and Root Binding Index (RBI)

The results of the Root Anchorage Index (RAI) and Root Binding Index (RBI) analysis for five tree species of various ages are presented in Table 3. These metrics provide insights into the mechanical behaviour of root systems in stabilising soil under shallow landslide conditions. Our analysis revealed that RAI and RBI values for pine species generally increase with tree age, suggesting that older pine trees develop stronger anchoring and soil-binding capabilities.

However, this trend was not consistent across all species, primarily due to differences in the age composition of the sampled individuals.

This study analysed the RAI and RBI values of different tree species for soil stabilisation. Root samples from various tree species were collected, and their RAI and RBI values were measured. The results are presented in Table 3. We observed that the RAI and RBI values for pine species increase as the age increases. However, this is only sometimes true for other species since this was not done in the field due to differences in age composition for each species. Table 4 summarises each species' average RAI and RBI values and classifies them based on their stabilisation potential.

Table 3. RAI and RBI values at various types, ages, and diameters

Table 4. Mean values of RAI and RBI and their categories

On the other hand, the cajuput (Melaleuca Leucadendron) tree presents a more balanced performance between RAI and RBI. With its RAI slightly surpassing its RBI, at 0.131 and 0.137, respectively, it suggests that the cajuput species possesses a more evenly distributed ability to anchor its roots and bind the soil. This versatility makes it a potentially intriguing species for soil stabilisation in various environments.

Conversely, the Hevea brasiliensis (rubber tree) displays a more robust anchoring ability, with an RAI of 0.534. However, its RBI is notably lower at 0.076, indicating that while the rubber tree's root system is more effective at anchoring, it has a relatively low capacity to bind the soil. This finding suggests that further research and evaluation may be needed before considering the rubber tree for specific applications that demand strong absorption or binding properties.

Finally, albizia (Paraserianthes falcataria) and pine (Pinus merkusii) show the highest overall values in the chart. Paraserianthes falcataria has a balanced relationship between its RAI (0.408) and RBI (0.868), with a slightly more vital ability to bind soil than to anchor roots. Pine stands out with a firm root anchoring ability (RAI of 1.005) and a decent soil binding ability (RBI of 0.719). Pine is a particularly effective species for soil stability and erosion prevention.

The chart underlines the different strengths of these five species in soil stabilisation. While teak/JPP excels at binding the soil, species like pine and rubber show a more balanced or robust performance in root anchoring. The variation in RAI and RBI values underscores the importance of species selection for specific soil stabilisation needs, highlighting the significance of our work in this field.

A comparative analysis of RAI and RBI across the five species is shown in Figure 5. These indices are essential for understanding the ability of tree species to stabilise soil and prevent erosion. The RAI measures the strength of the root system when anchoring the tree, which is crucial for avoiding uprooting and maintaining stability. At the same time, the RBI assesses the root system's ability to bind the soil particles together, contributing to soil stability by preventing soil erosion. The chart shows a significant difference between the RAI and RBI values of the Tectona grandis (Teak) tree. Its RAI is relatively low at 0.086, suggesting that its root system is less effective in anchoring the tree. However, its RBI is very high at 14.635, indicating that while its root system does not anchor the tree firmly, it does bind the soil particles very effectively, which implies that teak may help stabilise soil in environments where soil binding is the primary concern.

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Figure 5. Comparison of RAI and RBI values

The analysis reveals that different species have distinct strengths, making them suitable for varying purposes. Pine and albizia demonstrate balanced performance, whereas Tectona grandis dominates binding efficiency. Conversely, species like rubber and cajuput may require further evaluation before being utilised for specific applications that demand strong absorption or binding properties. It highlights the importance of choosing suitable species for targeted ecological or industrial applications.

Regarding the classification of RAI and RBI values [30], as shown in Table 4, rubber species with an average diameter of 26 cm have medium and low RBI values. Cajuput species with a diameter of 20.8 cm, RAI, and RBI values are included in the low criteria. Pine, with an average diameter of 13.1 cm, is categorised as a medium class (medium) with both RAI and RBI values. As for the albizia species, with an average diameter of 16.8 cm, the RAI value is in the low category, but the RBI value is in the medium category. Teak, with an average diameter of 13.3 cm, has a low RAI value but a high RBI value.

The very high RAI value for teak species is because the root samples studied for Teak Plus Perhutani (called JPP) species mostly do not have tap roots. Jati Plus Perhutani (JPP) is a superior variety of teak trees developed by Perhutani through vegetative propagation methods, primarily through shoot cuttings and tissue culture techniques. These two methods allow faster plant propagation and produce seedlings with the same genetic characteristics as the parent tree, so the quality and durability of JPP seedlings are more guaranteed.

Rubber species had the lowest Root Binding Index compared to the other species studied. It is thought to be because rubber species have a root system consisting of a taproot, lateral roots attached to the taproot, and fibrous roots. In 3-year-old plants, the depth of the taproot has reached 1.5m. The taproot length of 3-year-old and 8-year-old rubber trees was about 1.5 meters and 2.5 meters, respectively. When the plant is 7 years old, the taproot has reached a depth of more than 2.5 meters. It was found that 3-year-old trees have lateral roots that extend up to 6-9 meters, and 7-8-year-old trees have lateral roots that are more than 9 meters long [10-13].

Rubber plants are dicotyledonous plants with a root system consisting of taproots, lateral roots attached to the taproot, and root fibres. The taproot strengthens the plant so that it does not collapse quickly, and the taproot is firmly stuck into the soil, while the lateral roots and root fibres attached to the taproot absorb water and minerals from the soil.

Pine species, although the types and methods of research are different, in China's Beigou forest (800-1600 m a.s.l.), through root removal tests, Pinus tabulaeformis (25 years old) was the least effective in strengthening/anchoring the soil compared to other species (Betula platyphylla, Quercus mongolica, Larix gmelinii, and Ulmus pumila) [10]. Ferera, in 2009 [38-42], states that trees with extensive root index values are needed to maintain cliff stability. The data above shows that pine species, although categorised as a medium for RAI and RBI values, have a higher average root index than albizia, rubber, and cajuput. Pine's root index has a higher RAI value than teak, but the RBI value remains below. Table 3 also shows that the older the plantation, the higher the RAI and RBI values, thus maintaining slope stability. However, increased RAI is only sometimes followed by increased RBI [43].

Dewi et al.'s [44] research in 2015 showed an RAI value of 0.94 and an RBI value of 0.51. They mentioned that pine trees can anchor themselves firmly to the ground and grip the soil quite well, even better than jackfruit (Artocarpus heterophyllus) and melinjo (Gnetum gnemon) species for RBI values.

Teak species had the lowest RAI value compared to other species (0.086) but the highest RBI value. It is likely because the teak species planted is Perhutani Teak Plus, which results from tissue culture propagation, so it has a firm grip but relatively no tap roots. In teak (Tectona grandis), the phenomenon of high Root Binding Index (RBI) but low Root Anchorage Index (RAI) is closely related to the configuration of the root system. Teak with high RBI usually has a large root biomass, especially in well-developed lateral roots, while taproots are underdeveloped or weak. This condition causes the total mass of roots that bind the soil to be quite large, so the RBI is high, but the structure and depth of the roots that support the tree mechanically are not optimal, so the RAI is low. Such root systems tend to produce dense root networks at the soil surface, are practical at binding soil and preventing erosion, but are less effective at providing mechanical stability against physical disturbances such as strong winds. In other words, the dominance of large lateral roots without the support of a strong taproot results in a heavy but less architecturally complex root system, resulting in low tree anchoring ability (RAI) despite high soil binding ability (RBI) [45].

Trees that allocate more biomass to lateral roots than taproots generally have a large total root biomass (high RBI). However, because the taproot is underdeveloped or weak, the root system is less complex and has low mechanical strength (low RAI). This explains why teak trees, predominantly lateral roots with a poorly developed taproot, exhibit high RBI but low RAI.

A high RAI indicates a solid and stable root system that supports the tree to prevent it from collapsing, while a low value suggests potential instability. Meanwhile, a high RBI indicates the ability of the roots to bind the soil well, which is vital for preventing erosion and strengthening soil stability. However, it does not necessarily correspond to the tree's mechanical resistance to wind.

3.3 Organic matter content

To complement the mechanical root index analysis, laboratory assessments were conducted to evaluate the quality of organic matter in teak and pine roots. Parameters analysed included total nitrogen (N), carbon-to-nitrogen (C/N) ratio, lignin content, and polyphenol concentration—factors that influence root decomposition and mechanical resistance. Differences in test results for teak and pine roots are shown in Table 5.

Table 5. Comparison of organic materials between teak roots and pine roots

According to established criteria [46], high-quality organic matter should possess N content > 2.5%, C/N ratio < 20, lignin < 20%, and polyphenols < 2%. Based on these benchmarks, both teak and pine roots exceed acceptable lignin and polyphenol thresholds, suggesting that their root tissues are not readily decomposable. This structural resistance contributes to their functional longevity in providing soil cohesion and mechanical support, even under high-moisture conditions. The high lignin content observed in teak (28.91%) further reinforces its role in long-term slope stability through persistent soil binding despite its low RAI.

Conversely, the higher RAI observed in pine, combined with a lignin content within the moderately decomposable range, makes it suitable for applications requiring anchorage and gradual nutrient cycling.

These findings confirm that both teak and pine species exhibit root traits effective in shallow landslide mitigation, albeit via distinct mechanisms—binding versus anchoring.

3.4 Diagram of a typical root system

A general overview of teak and pine root systems typically shows a prominent taproot accompanied by an extensive network of lateral roots. The differences between the two types of roots (Table 6). In both teak and pine, root development begins with a robust taproot that anchors the tree and reaches deep for nutrients, complemented by lateral roots that expand outward to enhance stability and nutrient absorption. Teak roots are primarily distinguished by their considerable depth and broad horizontal reach. In contrast, the balance between the taproot and lateral roots in pine can vary based on the soil type.

Table 6. Different types of roots

3.5 Vegetation selection for recommendations

For landslide-prone areas, it is recommended to use vegetation with root systems that are able to provide mechanical stability as well as strong soil binding, through a combination of Root Anchorage Index (RAI) and Root Binding Index (RBI) values. Plants with high RAI usually have deep and sturdy taproots, which can resist shear forces and prevent trees from falling or mechanical landslides. On the other hand, plants with a high RBI have a large root biomass with dense lateral roots, which are effective in binding soil grains and increasing soil cohesion, reducing the risk of erosion and surface landslides.

The results show that the ideal root system to reduce landslide risk is one that combines strong deep roots (increasing RAI) with abundant and dense lateral roots (increasing RBI). Deep anchoring roots provide high mechanical stability, while lateral roots strengthen soil structure horizontally and help improve soil hydrology by accelerating the post-rainfall drying process, thereby reducing water saturation that can trigger landslides [47].

Therefore, tree vegetation with deep taproots and good lateral root development is highly recommended to be planted in landslide-prone areas, as it can provide optimal soil reinforcement and slope stabilisation. In addition, roots with good mechanical properties (such as flexural and tensile strength) as well as complex root architecture (e.g., degree of branching and root area ratio) also contribute greatly to reducing landslide susceptibility [48].

Thus, vegetation selection in landslide-prone areas should consider these two parameters (RAI and RBI), so that the plants are not only effective in binding the soil but also mechanically strong enough to resist forces that can trigger landslides.