Aboveground Carbon Stocks Following Multi-Purpose Tree Species Enrichment in a Community-Based Forest Management Area in Central Lombok

Forests play a critical role in climate change mitigation through their capacity to sequester atmospheric carbon and store it in above- and belowground biomass [1]. Beyond carbon regulation, forests provide essential hydrological services by influencing interception, infiltration, runoff, and soil erosion processes [2, 3]. Forest landscapes also support rural livelihoods by supplying income and subsistence resources, particularly for communities living adjacent to forested areas [4-6]. Land-use change and forest degradation can significantly reduce biodiversity and ecosystem carbon storage, while forest restoration and sustainable land management practices can enhance biomass accumulation and ecological resilience [7-9]. However, forests located near settlements are highly vulnerable to degradation due to increasing human pressure and intensified resource extraction.

In response to these challenges, community-based forest management has been promoted as a strategy to address forest degradation, reduce deforestation, and mitigate conflicts over forest resource use [10, 11]. Empirical evidence indicates that community-managed forests under Indonesia's Social Forestry Program—including the community forest (Hutan Kemasyarakatan, HKm) and village forest (Hutan Desa, HD) schemes—can improve forest condition, optimize land use, enhance carbon uptake, and contribute to local livelihoods [12-14]. Several studies further demonstrate that community forestry initiatives can reduce deforestation, increase carbon sequestration, and simultaneously improve rural welfare and household income [10, 12-15]. Such management approaches are expected to support national climate commitments, including Indonesia’s Forestry and Other Land Use (FOLU) Net Sink 2030 target [16]. Nevertheless, community management does not automatically guarantee ecological sustainability. When short-term economic objectives dominate management decisions, ecological functions such as biomass accumulation and carbon storage may decline.

The selection of tree species and cultivation systems that align livelihood benefits with long-term ecological functions is, therefore, a key design challenge in HKm. Previous studies suggest that multi-strata agroforestry systems incorporating multi-purpose tree species (MPTS) can encourage community participation while maintaining tree cover and ecosystem services [17, 18]. Increasing the proportion of MPTS has also been proposed as an approach to reduce land tenure conflicts and improve management outcomes in socially managed forest landscapes [19]. Agroforestry systems integrating timber and non-timber tree species are widely recognized as effective restoration strategies that simultaneously support ecological recovery and livelihood diversification in degraded forest landscapes [18, 20-22]. Recent studies have highlighted the multifunctional role of agroforestry systems in enhancing biodiversity conservation, carbon sequestration, ecosystem services, and livelihood resilience across tropical landscapes [23, 24]. Species-diverse agroforestry systems can enhance ecosystem functioning by improving habitat quality, supporting biodiversity conservation, and providing a wider range of ecosystem services within agricultural landscapes [25, 26]. Consequently, the incorporation of MPTS into agroforestry systems can contribute to both environmental sustainability and rural livelihoods by enhancing ecosystem functions and generating a range of socioeconomic benefits for local communities [23, 27]. However, empirical evidence on the medium-term ecological outcomes of such enrichment interventions—particularly their effects on vegetation structure and carbon stocks—remains limited.

In Lombok, community participation in forest management has been extensive, with thousands of households involved in HKm schemes [28]. Lantan Village in Central Lombok represents one such case, where enrichment planting using MPTS was implemented in 2016–2017 within an HKm. The program introduced fruit-based tree species such as avocado (Persea americana), durian (Durio zibethinus), duku (Lansium domesticum), and mangosteen (Garcinia mangostana) as a management intervention intended to enhance land productivity while contributing to climate mitigation through increased biomass and soil carbon storage. However, the long-term performance and ecological implications of this intervention require systematic evaluation through post-enrichment monitoring.

Despite the increasing adoption of MPTS enrichment within HKm, important questions remain regarding its medium-term ecological effectiveness beyond the establishment phase. While enrichment planting is expected to increase vegetation diversity, promote biomass accumulation, and enhance carbon sequestration, the extent to which these outcomes are achieved under farmer-managed tropical forest conditions remains insufficiently documented. In many social forestry systems, management decisions are strongly influenced by livelihood considerations, often favoring fast-growing and economically valuable species over long-term ecological objectives. Consequently, understanding how MPTS enrichment affects vegetation composition, stand structure, and carbon storage is essential for evaluating whether community-based forest management can simultaneously achieve livelihood improvement and environmental sustainability. This study addresses this knowledge gap by examining post-enrichment vegetation structure, carbon stocks, and the growth performance of MPTS seven years after enrichment planting in an HKm landscape. Specifically, the study investigates whether MPTS enrichment contributes to improved vegetation structure and carbon storage while supporting the ecological sustainability objectives of HKm.

Accordingly, this study aims to (i) assess vegetation composition following MPTS enrichment in the HKm area of Lantan Village, (ii) quantify aboveground carbon stocks associated with enriched and existing vegetation, and (iii) examine the growth performance of enriched MPTS. The findings are expected to contribute to a better understanding of the ecological outcomes of MPTS enrichment in HKm and to inform management strategies that balance carbon sequestration, biodiversity conservation, and livelihood objectives in tropical forest landscapes.

3.1 Species composition and vegetation structure

Field observations across the sampled plots recorded a total of 25 plant species, comprising 11 timber species, 10 MPTS, and 4 understory species (Table 1). The relatively high representation of MPTS, which accounted for a substantial proportion of all recorded species (40%), indicates that vegetation composition in the study area is strongly influenced by farmer management decisions rather than natural forest regeneration alone. This pattern reflects the emphasis placed on economically valuable species during enrichment activities and subsequent plot maintenance. Similar vegetation structures have been reported from other HKm in Lombok, where agroforestry-oriented species tend to dominate stand composition [38].

Table 1. Plant species recorded in the sample plots

Notes: MPTS: Multi-purpose tree species. A total of 25 plant species were recorded in the sampled plots, comprising 11 timber species, 10 MPTS, and 4 understory species.

Banana (Musa spp.) emerged as the most abundant species group overall, accounting for approximately 69% of all recorded individuals (Table 2 and Figure 2). Banana dominated the ≥20 cm, 10–<20 cm, and seedling class, although it was not the dominant group in the 2 cm ≤ D < 10 cm class. Such dominance is not unexpected in community agroforestry systems in Lombok, where banana is widely favored for its rapid growth and short-term economic returns. In addition, bananas propagate efficiently through vegetative suckers, enabling rapid clonal expansion and high stem densities within managed plots. Similar patterns have been documented in research [28], highlighting the combined influence of species growth characteristics and livelihood-oriented farmer management in shaping vegetation composition. This dominance may also limit the recruitment and development of long-lived woody species, thereby influencing the overall vegetation structure of the HKm system.

In contrast, timber species were sparsely represented, with densities of only 52 individuals ha⁻¹ in the ≥20 cm diameter class and 18 individuals ha⁻¹ in the 10–<20 cm class. Coffee (Coffea spp.) accounted for 60% of all individuals in the 2 cm ≤ D < 10 cm diameter class. The absence of timber species in this size class suggests limited recruitment of long-lived woody species, either through natural regeneration or active planting. Seedling composition further supports this interpretation, as it was largely dominated by MPTS and banana, indicating that farmer management practices continue to prioritize species with immediate or short-term economic value.

Table 2. Population density of vegetation groups across diameter classes in the sampled community forest (HKm) plots (individuals ha⁻¹)

Note: Dominant values within each diameter class and overall category are shown in bold. DBH: Diameter at breast height.

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Figure 2. Composition of timber species, coffee-based and non-coffee multi-purpose tree species (MPTS), and banana across diameter classes in the community forest (HKm) of Lantan Village

These findings underscore that vegetation structure in the HKm of Lantan is largely shaped by farmer management decisions, particularly the preference for fast-growing and economically valuable species. As a result, longer-lived timber and native tree species are comparatively underrepresented. When contrasted with natural forest vegetation in Lombok [39, 40], the studied HKm system exhibits lower structural complexity and a more simplified species assemblage, a pattern commonly associated with community-based agroforestry landscapes.

From an ecological perspective, the dominance of non-timber and short-rotation species has clear implications for long-term ecosystem services. Although this composition provides short-term economic benefits, it may constrain carbon storage, hydrological regulation, and biodiversity conservation, thereby limiting overall ecological resilience and long-term carbon sequestration potential of the site.

3.2 Carbon stocks

Aboveground carbon stocks for each vegetation group and diameter class are presented in Table 3 and Figure 3. Total aboveground carbon stock across the study site was 36.8 t ha⁻¹, derived from timber species, MPTS, banana, and litter. Timber species contributed the largest proportion at 50.6%, followed by banana (26.6%), MPTS (20.35%), and litter (2.45%). Among MPTS, enrichment-planted species accounted for 6.7% of total aboveground carbon, reflecting their relatively young age and smaller diameter classes.

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Figure 3. Composition of aboveground carbon stocks among timber species, multi-purpose tree species (MPTS), banana, and litter in the community forest (HKm) of Lantan Village

The variation in carbon stocks among vegetation groups was primarily influenced by differences in diameter distribution, growth form, and stand composition. Timber species contributed more than half of the total aboveground carbon stock despite their relatively low abundance because carbon accumulation increases disproportionately with tree diameter and biomass. In contrast, banana exhibited high population density but contributed a lower proportion of carbon relative to timber species due to its smaller stature and lower biomass per individual. Similarly, enrichment-planted MPTS were concentrated in the smaller diameter classes, limiting their current contribution to total carbon storage.

The observed carbon stock is relatively low when compared with natural forest systems in Lombok. Bae et al. [41] reported aboveground carbon stocks of 109.9 t ha⁻¹ for primary forest and 97.8 t ha⁻¹ for secondary forest in the West Rinjani Forest Management Unit, while shrub lands stored 26.5 t ha⁻¹. Thus, the carbon stock of HKm Lantan falls between degraded shrub systems and secondary forest, indicating partial recovery but still limited structural development.

Table 3. Aboveground carbon stocks by vegetation group and diameter class (t ha⁻¹)

Note: The highest value within each diameter class and the largest contribution to total carbon stock are shown in bold.

Similar findings have been reported in other community-managed or agroforestry landscapes. Carbon stocks in agroforestry or degraded forest systems may vary widely depending on vegetation structure and land-use history. For example, Sufardi et al. [42] reported aboveground carbon stocks of 69.9 t ha⁻¹ in shrub forests and 24.5 t ha⁻¹ in shrub lands in sub-optimal dry lands of Aceh Besar, Indonesia. Idris et al. [43] documented an average of 83.7 t ha⁻¹ in agroforestry systems in KHDTK Senaru, while Siarudin and Indrajaya [44] found 60 t ha⁻¹ of aboveground carbon in Gmelina arborea agroforestry systems in West Java. Compared with these systems, the HKm site shows lower carbon storage, largely due to the dominance of short-rotation, low-biomass species such as banana.

The predominance of MPTS and banana in the smaller diameter classes (10–<20 cm and 2–<10 cm) explains the limited carbon accumulation in enriched plots. Most enrichment-planted species have not yet reached substantial diameter growth, resulting in modest contributions to total carbon stocks. In contrast, the larger carbon contributions from timber species originate from individuals that were already present prior to the MCAI enrichment program.

Although the current contribution of enrichment-planted MPTS to aboveground carbon storage remains relatively small, this pattern is closely linked to the young age and limited diameter growth of these trees. Species such as durian (Durio zibethinus), avocado (Persea americana), and mangosteen (Garcinia mangostana) are known to accumulate biomass gradually over long growth periods, suggesting that their contribution to carbon stocks is likely to increase as stands mature. However, the realization of this potential will depend on sustained management practices that allow trees to reach larger diameter classes.

From an ecological perspective, species-diverse agroforestry systems are generally expected to support greater carbon accumulation than simplified systems dominated by a single crop species. This pattern is commonly explained by ecological mechanisms such as functional diversity and niche complementarity, whereby species with different growth forms, canopy structures, rooting depths, and resource-use strategies utilize environmental resources more efficiently than monocultures [25, 26]. As a result, multi-species systems often produce greater total biomass, enhance structural complexity, and maintain more stable ecosystem functioning over time [24, 26]. The integration of MPTS within HKm therefore has the potential to improve long-term carbon sequestration and other ecosystem services as trees mature and occupy different ecological niches within the vegetation structure [23, 24].

Overall, the results indicate that the current vegetation structure constrains carbon storage capacity due to the limited representation of large-diameter woody species. From a management standpoint, promoting the recruitment and retention of long-lived tree species could substantially improve future carbon stocks and strengthen ecosystem resilience within the HKm landscape [18].

3.3 Growth performance of enrichment-planted multi-purpose tree species

Three enrichment-planted MPTS were identified in the study area, namely durian (Durio zibethinus), avocado (Persea americana), and mangosteen (Garcinia mangostana). Measurements of diameter and height for these species are summarized in Table 4. Durian and avocado displayed comparable growth performance, with mean diameters of 11.6 cm and 12.4 cm and mean heights of 8.1 m and 9.5 m, respectively. In contrast, mangosteen exhibited considerably smaller dimensions, reaching a mean diameter of only 5.0 cm and a height of 4.3 m. This difference reflects the slower early growth characteristics of mangosteen relative to other enrichment-planted species.

Table 4. Mean diameter, height, abundance, and density of enrichment-planted multi-purpose tree species (MPTS)

The observed diameter and height values indicate moderate early growth performance for durian and avocado under the HKm management system, whereas mangosteen exhibited slower growth, consistent with its physiological characteristics as a late-maturing species. Durian was the most frequently planted enrichment species, with 84 individuals recorded across 33 plots, suggesting strong farmer preference as well as relatively higher establishment and survival rates compared with other species.

Comparisons with previous studies provide additional context for interpreting these patterns. Haba et al. [45] reported that six-year-old mangosteen grown under irrigated lowland conditions attained mean diameters of 7.12 cm and heights of 4.2 m, while individuals growing on forest soils reached 5.45 cm in diameter and 2.8 m in height. Growth values observed in the present study fall within this reported range, indicating that mangosteen performance under mixed agroforestry management in HKm Lantan remains within expected limits.

A similar correspondence is observed for avocado. Biazin et al. [46] documented avocado heights ranging from 10.1 to 17.6 m across different age classes and land-use types in coffee-based agroforestry systems in southern Ethiopia. The mean height recorded in HKm Lantan (9.5 m) is slightly lower but remains consistent with early- to mid-stage growth trajectories in comparable agroforestry environments.

Taken together, these results suggest that enrichment-planted durian and avocado are establishing satisfactorily, although their current contribution to carbon accumulation remains limited by age and size. As trees progress into larger diameter classes, their role in carbon storage is expected to increase, provided that management practices allow long-term retention. Mangosteen, despite slower early growth, may still offer long-term ecological and economic benefits characteristic of high-value tropical fruit trees. Continued monitoring will therefore be necessary to evaluate survival, growth dynamics, and the cumulative contribution of enrichment-planted species to ecosystem carbon stocks over time.