The Effect of Various Media on Growth of Hopea odorata Seedlings Based on Seed Balls

Indonesia contains the world’s third-largest area of tropical forests, following Brazil and the Democratic Republic of the Congo [1]. These forests deliver essential ecosystem services by supporting livelihoods, economic resilience, and human well-being, while also sustaining high biodiversity and contributing to global sustainability [2].

Lowland dipterocarp forests, a major tropical rainforest type in Southeast Asia, are dominated by climax tree species of the family Dipterocarpaceae. Within this family, 510 species are distributed across 16 genera, of which 470 species (92.1%) occur in Asia [3]. Borneo serves as the centre of dipterocarp diversity, containing 13 genera and 269 species, whereas Sumatra hosts 12 genera and 113 species [4].

In Indonesia, dipterocarp species are also distributed across Papua, Maluku, Java, and Sulawesi [4]. Within lowland tropical rainforests, dipterocarps account for approximately 15.6–21.9% of total tree abundance [5], yet they dominate forest structure by contributing 41.7% of the basal area, 50–80% of emergent trees, and around 40% of understory individuals [3, 6].

Although deforestation rates in Indonesia have fluctuated over time, recent trends indicate a significant decline [7]. Annual net deforestation decreased from approximately 1.9 million hectares during 1990–1996 to about 121,000 hectares in 2022–2023 [7]. Nevertheless, forest degradation remains a critical issue, driven by factors such as illegal logging and forest fires [8]. This degradation results in substantial ecological consequences, including biodiversity loss and climate-related impacts such as increased CO₂ emissions, changes in temperature and humidity, reduced carbon sequestration, and lower evapotranspiration rates [9]. Consequently, strategies to mitigate the impacts of natural forest degradation must involve forest restoration with native species and the development of silvicultural techniques to enhance canopy cover and improve environmental conditions. Restoration aims to restore the structure of forest stands and enhance forest ecosystems to achieve ecological and socio-economic benefits [10]. Restoration with native species enhances ecosystem functions and biodiversity, improves resilience against environmental changes, and supports local livelihoods [11]. Native species are generally better adapted to local environmental conditions, resulting in higher survival and growth rates [12].

Selecting native species is crucial for restoring forest structure, function, and diversity to support accelerating forest restoration [13]. One Dipterocarpaceae species, Hopea, can be used for forest restoration in degraded tropical rainforests. Hopea, a genus with 104 species, is found worldwide. These plants are known for their high-quality wood, large size, and evergreen leaves, which are characteristics of tropical rainforest trees [14]. Hopea odorata is a protected species in the family Dipterocarpaceae and is classified as Vulnerable (VU) on the IUCN Red List because its population continues to decline globally [15]. Hopea odorata, a shade-tolerant dipterocarp, shows promising growth and physiological responses, indicating its potential as a native species for forest rehabilitation and restoration efforts in tropical environments [16].

However, the large area of degraded forest land requires technology that can be applied quickly. Additionally, some of the land replanted is hard to reach and cannot be planted using conventional methods. Conventional techniques require more time and money [17, 18]. One approach to restoration is seed ball technology integrated with drones. Seed balls are composed of seeds, clay, compost, and other natural ingredients. The clay shields the seeds from external stress and predation, while the compost offers critical nutrients for seed germination and early plant growth [19, 20]. Seed balls are inexpensive to make and can be disseminated manually or by aerial means, making them ideal for large-scale restoration [21].

Natural forest restoration using seed balls is an innovative technique that uses clay-coated seeds and combinations of them to improve germination and growth in degraded environments [19]. Natural regeneration through waiting for natural succession has a long-term constraint. Additionally, the condition of one of the factors of natural regeneration, the seed bank, is not necessarily the availability of seeds on the forest floor. Degradation significantly changes the composition and density of the seed bank [22]. Tropical forest rehabilitation requires human assistance to speed up recovery. Assisted regeneration enables greater coverage and is cost-effective [23].

Several studies on the use of seed balls—such as in India with Leucaena leucocephala and in East Africa with Sesbania sesban—have shown that they enhance growth and improve the success of land rehabilitation efforts [20, 24]. The use of seed ball technology in peatland rehabilitation with jelutung (Dyera polyphylla) has affected the germination percentage [25]. Research on seed balls with various media compositions has been conducted on various adaptive plants, such as Artocarpus heterophyllus and Intsia bijuga [26, 27]. However, research on a combination of seed ball media and H. odorata plants as native species on land intended for restoration has not been widely studied. Therefore, research is needed to prepare the formula of seed ball media and the site of planting to find the best media composition and site for planting to support the growth of H. odorata.

3.1 Effect of media on Hopea odorata growth

Table 2. Analysis of variance (ANOVA) for the effects of growth media and seed balls on the growth of H. odorata

The best planting media have a loose, porous soil structure [32], providing optimal conditions for seedling growth. In general, the use of soil significantly increases the root growth of H. odorata seedlings (Figures 2 and 3). In soil media (M2), the average root length was approximately 15.74 cm ± 1.23, which was about 31% higher than in sand media. Soil media also significantly increased the diameter, height, and the increment in diameter and height (Figure 4). The diameter growth in soil media (M2) was around 4.33 ± 0.40 mm, and the diameter increment was 2.84 ± 0.14 mm (Figure 4(a)). In addition, the height growth was around 25.89 ± 1.83 cm, and the increment was around 18.38 ± 3.33 cm (Figure 4(b)). This growth in diameter and height was 48% and 58% higher, respectively, than that observed in sand media. Soil provides good aeration, which improves root development and increases water absorption [33]. It improved nutrient adsorption and water use efficiency, facilitating better water and nutrient uptake by roots during the experiment. This is consistent with previous research indicating that soil media is more effective than sand media for seedling growth [34]. Therefore, it has the potential to promote plant growth.

Figure 3. Root growth of H. odorata seedlings in (a) sand media and (b) soil media

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In contrast, H. odorata seedlings grown in sand media exhibited lower growth. Sand is highly porous, allowing nutrients, aeration, and drainage to pass easily. It has large pores, so adding organic matter is necessary to retain water and nutrients [32]. Each growing medium has its own advantages and disadvantages, particularly regarding the naturally occurring nutrients available in the media. The structure of the media and their nutrient content are important factors to consider when choosing the right medium for each plant type [35].

3.2 Effect of seed ball media on Hopea odorata growth

The results showed that the type of seed ball media significantly affected the survival ability (p < 0.05), diameter (p < 0.01), diameter increment (p < 0.01), and height (p < 0.01) of H. odorata seedlings (Table 1). Seed ball media containing a mixture of clay, compost, and hydrogel (S3) exhibited the most significant growth (Figure 5, Figure 6). Seed ball media containing clay, compost, and hydrogel (S3) increase survival ability, height, diameter, and diameter increment of H. odorata seedlings (Figure 5, Figure 6). The survival rate of H. odorata seedlings in the S3 seed ball treatment was around 55 ± 5.00%, which was higher than in treatment S1 (around 36 ± 6.15%) and treatment S2 (around 25 ± 6.32%) (Figure 5). Seed balls protect seeds from biotic and abiotic stressors, increasing their survival and establishment rates [20]. The highest growth of H. odorata seedlings in height, diameter, and diameter increment in treatment S3 was 20.9 ± 2.68 cm, 3.6 ± 0.88 mm, and 2.08 ± 0.21 mm, respectively, during the observation period (Figure 6(a, b, c)). This is due to the presence of organic materials, such as compost, which contain the essential nutrients that plants need [36]. Providing compost in the planting media provides additional nutrients to support seedling growth [27, 37].

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(b)

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Organic fertilizers, such as compost, increase water absorption and storage capacity, boost organic matter levels, and improve soil physical properties [38]. Furthermore, adding hydrogel helps absorb and store water for optimal growth. Hydrogels help maintain water availability for plants over a longer period [39]. Furthermore, when combined with compost, they can enhance nutrient availability [40], thereby promoting plant growth. Due to its organic matter content and crumbly structure, it is ideal for plant growth as it fulfils their nutrient needs. The integration of these three media types into seed balls is the most effective for increasing height, diameter, and overall growth. The ideal seed ball formulation varies according to the target crop and environmental conditions. To ensure maximum efficiency, seed ball compositions must be adapted to individual environmental conditions. Soil type, moisture availability, and local climate are all important considerations [41]. The right combination of planting media can provide the necessary nutrients for optimal growth, development, and reproduction [32].

The seed ball media contains nutrients that support seed growth and the success of planting [20, 42]. It entails encapsulating seeds in a mixture of locally accessible materials to generate seed balls that may easily disseminate in varied conditions [43]. A combination of seeds, clay, and compost creates a protective, nutrient-rich environment for germination [19]. However, the success of seed balls depends on soil quality, climatic conditions, and appropriate dispersal methods, such as drone technology [44]. Additionally, they create conditions conducive to germination and survival until the seeds germinate [45]. A limitation of this study is the absence of additional blank controls, such as untreated seeds or no seed ball treatments. The clay-only treatment was used as the experimental control to evaluate the relative effects of seed ball amendments within the clay matrix. Consequently, the results should be interpreted as comparisons among seed ball formulations rather than absolute effects relative to untreated conditions.

3.3 Opportunity for the seed ball for restoration in the degraded tropical rainforest

The rate of forest degradation in Indonesia is high due to deforestation, forest fires, and land use change [46]. The net deforestation in Indonesia remains hundreds of thousands of hectares per year, while the accumulated deforestation over recent years has reached millions of hectares [7]. Deforestation has a direct impact on declining biodiversity, increased carbon emissions, and disrupted ecosystem stability [47, 48]. Restoring degraded forests is crucial as a climate change mitigation strategy, as forests serve as vital carbon sinks that absorb greenhouse gases [49].

Additionally, forest restoration using native species is an essential measure to address the biodiversity crisis [50]. Such large-scale forest damage requires effective, efficient, and adaptive rehabilitation efforts that take into account various biophysical conditions in the field. Conventional rehabilitation approaches through manual seedling planting have proven to have limitations, including high labour requirements, difficult land access, high planting costs, and limited coverage in remote areas [51]. Furthermore, technological innovation in enrichment planting is necessary to accelerate forest restoration.

One restoration effort involves seed ball technology. Seed balls are a vegetation restoration technology that encases seeds in a soil mixture and protects them from extreme environmental conditions, and increases their chances of germination [20, 26]. Seed ball technology is a cost-effective reforestation approach that is suitable for degraded land and large-scale applications [21]. Using seed balls with native species increases seedling survival by 50-60% [52]. Using seed balls can increase seed mass. This way, they can fall on the target restoration site. Tropical forest restoration efforts can be supported by using drones, which can increase the number of native species individuals. Seed balls can be disseminated manually or by drones to efficiently cover large, inaccessible areas [42, 53]. Drones offer flexibility, low cost, and improved efficiency for seed ball dispersal in large-scale restoration projects [54, 55]. However, obstacles include high seed predation rates, uneven effectiveness, and a paucity of success statistics [44].

The combination of drone technology and seed balls offers an innovative, efficient approach to forest rehabilitation that has the potential to be applied on a large scale. Drones can deliver seeds to targeted microsites with precision, potentially boosting success rates [44]. Meanwhile, seed balls can increase seed viability after dispersal [56]. The integration of drones and seed balls enables accelerated forest restoration at a lower operational cost than traditional methods, while also improving planting accuracy in priority restoration areas [57]. This approach is expected to be a strategic alternative for restoring resilient, sustainable forest ecosystems aligned with national climate change mitigation and biodiversity conservation targets. The novelty of this research lies in the information on the best seed ball media for the native species Hopea odorata. The implications and benefits of this study's results include selecting the best seed ball media for native species, which can contribute to the success of natural forest restoration and large-scale plantations. It is hoped that further research can be carried out directly in degraded tropical forest areas to determine the germination and growth of Hopea seed balls in actual environmental conditions.