Improving the success of direct seeding through the application of seed briquettes, aquasorb, and sowing time: case studies on Ceiba pentandra, Enterolobium cyclocarpum, and Calophyllum inophyllum

Abstract

Direct seeding, a planting technique that has long been applied in restoring degraded land and forest, is often doubted to be successful due to many constrains, both biotic and abiotic. This study aims to increase the success of direct seeding through the application of seed briquettes, aquasorb treatment and determining the right sowing time for three forest tree species, i.e. Ceiba pentandra, Enterolobium cyclocarpum, Calophyllum inophyllum. This study used a randomized block design with two factorials (seed treatments and sowing dates) in each species. Seeds were sown in three blocks in the field according to the sowing time treatments. Each treatment consisted of 10 sowing plots in one block and 5 seeds or seed briquettes in each sowing plot. The results showed that direct seeding when the rain starts to stabilize (ST-2) supported by the use of seed briquettes and the addition of aquasorb was able to increase seedling survival, height and root collar diameter of all tested tree species. In general, seed briquettes and the addition of aquasorb provided better seedling survival and growth compared to direct seeding using untreated seeds. C. inophyllum gave higher seedling survival compared to the other two species indicating that this species is very prospective for direct seeding applications due to its high adaptability.

Keywords:

• Direct seeding
• forest tree
• seed treatment
• seedling growth

Introduction

Over-exploitation, illegal logging, mining, fires, and other changes in land use are the causes of deforestation in Indonesia. The Ministry of Environment and Forestry of the Republic of Indonesia (2019) stated that the marginal land and forest area in Indonesia reaches 14.01 million hectares with a degradation rate of 450,000 ha per year. Reforestation and land rehabilitation are priority programs to improve environmental conditions while reducing greenhouse gas emissions. In 2021, the Government of Indonesia pledged a commitment to restore a total of 14 million hectares (12 million hectares of degraded forest and land, and 2 million hectares of degraded peatland) by 2030 as a commitment to the United Nations Framework Convention on Climate Change through the Nationally Determined Contribution Program (MoEF 2021; Bosshard et al. 2021).

This ambitious plan to restore degraded forests and lands requires many efforts, both conventional planting and the application of other alternative reforestation technologies (Holbert et al. 2019). Although conventional planting generally results in a high success rate of reforestation, the technique requires high costs, starting from preparing the seedlings in the nursery, transporting them than planting them in the field, thus so costs are often a limiting factor for large-scale restoration programs (Meli et al. 2018; Sudrajat et al. 2018; Atondo-Bueno et al. 2018). Alternative reforestation methods such as direct seeding or aerial seeding may be an option because they can be carried out on a large-scale area when funds and facilities to support restoration programs are limited (Sudrajat et al. 2018; Perez et al. 2019; Raupp et al. 2020).

The success of direct seeding is influenced by several factors including the limited quality seeds availability, low germination, low seed survival, low young seedlings’ resistance to stress both abiotic stresses (temperature, light, and extreme sunlight), and biotic (pest and disease), as well as slow seedlings growth and development (Sovu et al. 2010; Palma and Laurance 2015; Sudrajat et al. 2018; Passaretti et al. 2020). To increase the success of direct seeding, some efforts have been made, such as selecting suitable species for direct seeding (Palma and Laurance 2015; Meli et al. 2018; Figueiredo et al. 2021a), seed treatments (Correia et al. 2022), prevention of seed predators and other disease pests (Birkedal 2010), and sowing technique and site engineering (Woods and Elliott 2004; Doust et al. 2006; Silva and Vieira 2017). Another alternative technique to increase the success of direct seeding is the use of seed briquettes, aquasorb, and seed sowing time.

Seed briquettes are a technique for coating seeds with various organic materials to increase germination and early seedling growth (Sudrajat et al. 2018; Sudrajat and Rustam 2020) and also to increase drought resistance (Abusuwar and Eldin 2013). The addition of aquasorb at the time of planting reported can increase germination and seedling survival (Taban and Naeini 2006; Koupai et al. 2008; Tongo et al. 2014). Aquasorb is a hydrophilic polymer, that normally absorbs water hundreds of times its weight as a gel (Peterson 2002; Tongo et al. 2014). Aquasorb has been used to establish tree seedlings and out-planting to increase seedling survival (Save et al. 1995; Specht and Harvey-Jones 2000). The treatment can increase available water content to a maximum of about 2.3 times from the control (Koupai et al. 2008) which also functions as an additional water reservoir (Bharwaj et al. 2007) and reduce drought stress (Bouranis et al. 1995). Another factor that affects the germination and early growth of seedlings in tropical countries with two seasons (rainy and dry seasons) is the proper sowing of seeds (Sudrajat and Rustam 2020). The right time of seed sowing greatly affects seedling survival.

A combination of these factors above to increase the success of direct seeding has never been studied before. The purpose of the research was to identify the optimum sowing time and seed treatment using seed briquette and aquasorb on the direct seeding application of three important tree species, i.e. Ceiba pentandra (L.) Gaertn.), Enterolobium cyclocarpum (Jacq.) Griseb., and Calophyllum inophyllum L. The three tree species are considered multipurpose tree species. C. pentandra has pharmacological properties (e.g. anti-hyperglycemia or antidiabetic dominated mostly by its stem bark) as well as industrial potentials focused on biodiesel, bioethanol, absorbents, and absorbents production from different plant parts (Chan et al. 2023). E. cyclocarpum is one of the fast-growing species and adaptive for restoration in the ex-mining area (Sukariyan et al. 2021), while C. inophyllum is a potential biodiesel alternative as it grows well in harsh environmental conditions, produces non-edible seed oil, and has high amounts of kernel oil and fruits profusely (Leksono et al. 2021). The study can produce technology to increase the success of direct seeding as an alternative method for forest and landscape restoration in the tropics.

Materials and methods

Materials

Three species i.e. C. pentandra, E. cyclocarpum, and C. inophyllum, were used in this study. The seeds of C. pentandra and E. cyclocarpum were collected from Parungpanjang Forest Research Station, Bogor, West Java Province (06°20'42” S, 106°06'15” E, altitude of 52 m asl), while C. inophyllum seeds were collected from a natural population seed stand at Carita, Banten Province (6°11'37” S,105°50'32” E, altitude of 0-5 m asl). Seed processing and testing were carried out at the Laboratory of Forest Tree Seed Testing (Institute for Standard and Instrument Application of Environment and Forestry, Bogor). The seed germination test showed that the germination capacity of C. pentandra, E. cyclocarpum, and C. inophyllum was 78%, 80%, and 85%, respectively (Data not shown).

Seed briquette preparation

The briquettes were made manually using several ingredients with compositions: compost (40%), soil (10%), charcoal (30%), lime (10%), and tapioca (5%) (Sudrajat et al. 2018). All the ingredients were thoroughly mixed and tapioca flour was added as an adhesive with hot water to form the moulding dough. The dough was put into a mold measuring 5 cm in diameter and 3 cm thick with a hole for placing the seeds 2.5 cm in diameter and 2 cm deep. The briquettes were then dried for 2–3 days under the sunlight so they were stronger, easier to store, and protected from attacks by fungi and other pathogens. The seeds are put into briquettes and covered with the same material used for making briquettes, then dried again (at room temperature, 27° C) so that they are dry and can be stored before being sown in the field. The weight of one seed briquette is approximately 45–50 g. The briquettes had a relatively high nutrient content with 4.69% organic C, 0.56% total N, C/N ratio 8.37, P₂O₅ 2080.5 ppm, Ca 33.30 Cmol⁽⁺⁾ kg⁻¹, Mg 9.41 Cmol⁽⁺⁾ kg⁻¹, and 100% base saturation.

Field experimental design

The field trial of direct seeding was conducted in Parungpanjang Forest Research Station, Bogor, West Java, Indonesia, and geographically lay on 06°20'42” S and 106°06'15” E, with an altitude of 52 m asl. The area has an annual rainfall of 2440 mm with an average temperature of 28° C (Sudrajat et al. 2016). The research site has low fertility. The soil is categorized as podzolic haplic with C-organic 1.20–2.31%, N 0.22–0.27%, P 10.88–13.75 ppm, K 0.13–0.15 cmol⁽⁺⁾ kg⁻¹, Ca 2.30–4.98 cmol⁽⁺⁾ kg⁻¹, Mg 1.80–3.48 cmol⁽⁺⁾ kg⁻¹, CEC 24.42–37.54 cmol⁽⁺⁾ kg⁻¹, base saturation 18.05–25.59%, and pH 4.4–4.6. Soil texture is composed of 15.42–36.70% sand, 60.75–78.79% clay, and 2.55-15.62% silt (Anna et al. 2020). The site was previously a scrub area dominated by Melastoma malabathricum, Imperata cylindrica, and Schima wallichii coppices, which were then cleared manually.

The sowing seed design used a randomized block design with two factorial treatments (seed treatment and sowing times) for each species. There were four seed treatments, namely untreated seeds (SBA0), seeds with aquasorb (SBA1), seed briquette (SBA2), seed briquette with aquasorb (SBA3), while the treatment for sowing time (four treatments) was direct sowing at the beginning of the rainy season in November 2020 (ST-1), when the rain starts to stabilize in December 2020 (ST-2), when the daily precipitation intensity is relatively stable in February 2021 (ST-3), and toward the end of the rainy season in March 2021 (ST-4) (Figure 1). The experimental design layout is presented in Figure 2. Microsite engineering was carried out by cleaning grass and scrub debris in the form of a circle with a diameter of 50 cm so that the micro-environment can support seed germination (Doust et al. 2006; Nurhasybi and Sudrajat 2017; Sudrajat et al. 2018) and reduces weed competition in early seedling growth (Sovu et al. 2010; Nurhasybi and Sudrajat 2013). The aquasorb used is a product of super-absorbent anionic polyacrylamide. This product is a crosslinked copolymer of acrylamide and potassium acrylate. As much as 5 g of aquasorb was immersed around the sown seeds and seed briquettes. Seeds from the three species were sown in 3 blocks in the field according to the planting time. Each treatment consisted of 10 sowing plots in one block and 5 seeds or seed briquettes were sown in each sowing plot. The total number of seeds per treatment at each sowing date was 150 seeds or seed briquettes.

Figure 1. Precipitation pattern and sowing time of direct seeding of Ceiba pentandra, Enterolobium cyclocarpum, and Calophyllum inophyllum.

Figure 2. Layout of the experimental design of direct seeding based on the sowing time and seed treatment. Notes: layout shows the design of seed sowing for 1 species in a block,

= sowing plot containing 5 seeds or seed briquettes, ST-1 = the begining of the rainy season in November 2020, ST-2 = when the rain starts to stabilize in December 2020, ST-3 = when the daily precipitation is relatively stable in February 2021, ST-4 = toward the end of the rainy season in March 2021, SBA0 = untreated seed SBA1= seed with aquasorb, SBA2 = seed briquette, SBA3 = seed briquette with aquasorb.

Sowing of seeds was done by placing seed briquettes on the soil surface, whereas for seeds without briquettes, sowing of seeds was done by planting seeds to a depth of 0.5-1 cm, depending on the size of the seed. Seeds were buried or covered by topsoil to avoid risk of predation and seed desiccation (Sovu et al. 2010; Silva and Vieira 2017; Alem 2020; Figueiredo et al. 2021b), and to retain seed moisture to stimulate germination (Becerra et al. 2022). Weed control was carried out after 6 months from seeds sowing to reduce competition and ensure the seedlings grow well. Seedling survival, height, and root collar diameter were evaluated for all seedlings grown at 12 months after sowing. The height of the seedlings was measured using a ruler, while the root collar diameter was measured using a digital caliper. The measurements were carried out according to the implementation of the sowing time so that to obtain a 12-month-old seedling, the measurement time is different for each sowing time. Data on height and root collar diameter of the seedlings were averaged for each sowing plot, then data analysis was performed.

Data analysis

Data on seedling survival and growth parameters (seedling height and root collar diameter) were analyzed for variance (ANOVA) according to a randomized block design with factorial treatments using SAS^® software version 9.1. If there is a difference, the analysis will be continued with Duncan’s Multiple Range Test to determine the seedling survival and growth differences between treatments.

Result

The interaction between sowing time and seed treatment (seed briquette and aquasorb) significantly affected on seedling survival of all species (C. pentandra, E. cyclocarpum, and C. inophyllum). Similar results also occurred for the height and root collar diameter of the seedlings, the interaction between sowing time and seed treatment had a significant effect on the seedling height and root collar diameter of all the species tested (Table 1).

Table 1. Summary of analysis of variance of sowing time and seed treatment on seedling survival, height, and root collar diameter of direct seeding of Ceiba pentandra, Enterolobium cyclocarpum, and Calophyllum inophyllum at 12 months after seed sowing.

Parameter	Source of variation	Degrees of freedom	F-test
			Ceiba pentandra	Enterolobium cyclocarpum	Calophyllum inophyllum
Seedling survival	Block	2	2.614*	0.874*	0.176^ns
	Sowing time	3	4.644**	1.277*	5.472**
	Seed treatment	3	1.905*	1.529*	4.537**
	Interaction of sowing time and seed treatment	9	2.557**	1.153*	1.143*
Seedling height	Block	2	0.902*	0.439^ns	0.141^ns
	Sowing time	3	4.884**	1.435*	10.171**
	Seed treatment	3	2.832**	1.194*	4.206**
	Interaction of sowing time and seed treatment	9	1.618*	1.796*	1.025*
Root collar diameter	Block	2	0.390^ns	0.520^ns	3.535**
	Sowing time	3	4.954**	2.530**	4.954**
	Seed treatment	3	2.914**	1.985*	5.191**
	Interaction of sowing time and seed treatment	9	2.441**	1.294*	1.755**

Notes: ** = significant at 99% confident level, * = significant at 95% confident level, ns = not significant at 95% confident level.

The sowing time and seed treatment gave significantly different survival rates for C. pentandra and E. cyclocarpum, while for C. inophyllum there were no significant differences found (Table 2). The best seedling survival as indicated by the survival rates of C. pentandra, E. cyclocarpum, and C. inophyllum, i.e. 23.9%, 55.0%, and 68.7%, respectively, were found in the sowing date of SD-2 (when the rain starts to stabilize in December) and seed treatment (seed briquette and applications of aquasorb). C. inophyllum showed higher seedling survival compared to the other two species.

Table 2. Seedling survival of Ceiba pentandra, Enterolobium cyclocarpum, and Calophyllum inophyllum on the direct seeding using the different sowing time and seed treatment at 12 months after seed sowing.

Species	Seed treatments	Sowing time
		ST-1 (%)	ST-2 (%)	ST-3 (%)	ST-4 (%)
Ceiba pentandra	Untreated seed	4.0 fg	21.3 ab	6.0 efg	3.0 g
	Seed with aquasorb	9.3 de	20.7 ab	6.7efg	2.7 g
	Briquette	13.3 cd	16.7 bc	8.3 ef	2.3 g
	Briquette with aquasorb	15.7 c	23.7 a	8.7 def	3.7 fg
Enterolobium cyclocarpum	Untreated seed	13.3 fg	30.7 bcdef	39.3 abc	23.3 cdef
	Seed with aquasorb	32.3 bcde	17.7 defg	33.3 bcde	15.0 efg
	Briquette	12.0 g	42.3 ab	23.3 cdefg	21.7 cdefg
	Briquette with aquasorb	34.3 bcd	55.0 a	31.7 bcde	30.0 cdefg
Calophyllum inophyllum	Untreated seed	16.7 cde	33.3 bcde	31.7 bcde	10.0 e
	Seed with aquasorb	20.0 cde	31.3 bcde	38.3 bcd	13.3 de
	Briquette	36.7 bcd	53.3 ab	40.0 bc	10.0 e
	Briquette with aquasorb	53.3 ab	68.7 a	33.3 bcde	16.7 cde

Notes: ST-1 = the beginning of the rainy season in November 2020, ST-2 = when the rain starts to stabilize in December 2020, ST-3 = when the daily precipitation is relatively stable in February 2021, and ST-4 = toward the end of the rainy season in March 2021. Different letters (a to g) in the same species indicate significant differences at p ≤ 0.05 between treatments.

The use of a seed briquette and the addition of aquasorb gave higher growth in height and root collar diameter at each stage of the sowing date. Generally for all species, sowing on ST-2 using a seed briquette and adding aquasorb gave the highest seedling height and root collar diameter, followed by sowing on ST-1 (Tables 3 and 4). The seedling's performance from direct seeding using a seed briquette and the addition of 5 g aquasorb for C. pentandra, E. cyclocarpum, and C. inophyllum can be seen in Figure 3.

Figure 3. Seedling performance of Ceiba pentandra (a), Enterolobium cyclocarpum (b), and Calophyllum inophyllum (c) of direct seedling using seed briquette with the addition of 5 g aquasorb at 6 months after sowing.

Table 3. Seedling height of Ceiba pentandra, Enterolobium cyclocarpum, and Calophyllum inophyllum on the direct seeding using the different sowing times and seed treatments at 12 months after seed sowing.

Species	Seed treatments	Sowing time (ST)
		ST-1 (cm)	ST-2 (cm)	ST-3 (cm)	ST-4 (cm)
Ceiba pentandra	Seed	22.3 efgh	33.1 bcdef	19.2 h	19.6 gh
	Seed with aquasorb	19.8 fgh	37.1 abcd	21.1 fgh	26.0 cdefgh
	Seed briquette	33.4 bcde	43.7 ab	23.2 e-h	31.7 cdef
	Seed briquette with aquasorb	42.8 abc	47.3 a	22.1 e-h	29.9 defgh
Enterolobium cyclocarpum	Seed	34.5 cd	30.6 f	38.9 bcde	38.7 bcde
	Seed with aquasorb	41.6 bcde	47.9 bcd	39.6 bcde	36.1 cde
	Seed briquette	36.4 cde	50.3 b	36.4 cde	49.5 bc
	Seed briquette with aquasorb	35.4 de	64.4 a	39.0 bcde	42.9 bcde
Calophyllum inophyllum	Seed	27.9 c	35.9 bc	33.9 bc	38.1 bc
	Seed with aquasorb	30.7 c	45.0 ab	37.7 bc	35.9 bc
	Seed briquette	30.7 c	49.9 a	37.5 bc	44.8 ab
	Seed briquette with aquasorb	38.1 bc	51.4 a	35.5 bc	38.2 bc

Different letters (a to h) in the same species indicate significant differences at p ≤ 0.05 between treatments.

Table 4. Root collar diameter of Ceiba pentandra, Enterolobium cyclocarpum, and Calophyllum inophyllum seedlings on the direct seeding using the different sowing time and seed treatment at 12 months after seed sowing.

Species	Seed treatments	Sowing time
		ST-1 (mm)	ST-2 (mm)	ST-3 (mm)	ST-4 (mm)
Ceiba pentandra	Untreated seed	6.4 bcdef	5.2 cdef	3.7 f	4.6 def
	Seed with aquasorb	4.3 ef	6.4 bcdef	4.4 ef	6.9 abcde
	Seed briquette	7.2 abcd	8.0 ab	4.3 ef	7.3 abc
	Seed briquette with aquasorb	9.7 a	8.5 ab	4.5 ef	7.9 abc
Enterolobium cyclocarpum	Untreated seed	6.2 bcde	4.4 e	4.7 e	7.4 abcd
	Seed with aquasorb	6.8 abcde	8.3 ab	4.9 de	7.3 abcd
	Seed briquette	5.4 cde	7.8 abc	4.9 de	8.3 ab
	Seed briquette with aquasorb	8.4 ab	8.9 a	5.5 cde	7.6 abc
Calophyllum inophyllum	Untreated seed	6.0 abcd	5.5 cd	5.3 d	6.0 abcd
	Seed with aquasorb	6.6 abcd	7.9 ab	5.9 abcd	7.4 abcd
	Seed briquette	7.3 abcd	7.6 abc	5.9 abcd	8.0 a
	Seed briquette with aquasorb	7.3 abcd	7.3 abcd	5.7 abcd	7.7 abc

Different letters (a to f) in the same species indicate significant differences at p ≤ 0.05 between treatments.

Discussion

Based on our results the planting time and seed treatment affected the success of early growth in the field for the three species tested, i.e. C. pentandra, E. cyclocarpum, and C. inophyllum which were planted on marginal land in Parungpanjang. In general, we found an interaction effect of sowing time and seed treatment (seed briquette and aquasorb applications) on seedling survival and early plant growth. As known, seed germination and early seedling growth are uncertain stages in the plant life cycle (which are the main obstacle in the application of direct seeding (Vieira and Scariot 2006). Death at this stage can be caused by various factors, such as seed predation, competition, and abiotic factors (extreme temperatures, frost, drought, and sunburn). Seeding time is one important factor influencing abiotic factors for optimal germination (Hypponen and Halikainen 2011; Grossnickle and Ivetić 2017).

Treatment of sowing time ST-2 (when the rain starts to stabilize in December) was the best seedling survival for the three species tested. Relatively stable rainfall with high intensity (rain almost every day) can improve seed germination and seedling survival. Seeding should be carried out when site environmental conditions are least stressful (Grossnickle and Ivetić 2017). The best time for seeding is when they have the best chance of germinating, meaning plenty of moisture, optimum temperatures, minimal weed competition, and a potentially favorable growing season before exposure to stressful environmental conditions (Schmidt 2008). These statements were supported by several previous studies which shown that different sowing times result in differences in germination and seedling survival, such as in Acacia pycnantha, A. acinacea, and Eucalyptus macrocarpa (Carr et al. 2007), Pinus palustris (Barnett, 2014), Quercus spp. (Sánchez-González et al. 2016), and Gmelina arborea (Sudrajat and Rustam 2020). Direct seed seeding determined by the time of year provides the best chance of consistently maintaining optimal environmental conditions so that once the seeds germinate, the young seedlings escape planting pressure and become established. In dry tropical forests planting seeds when the soil is sufficiently moist during the rainy season can increase seedling establishment (Govinden-Soulange and Levantard 2008). As with planted seedlings, young seedlings from direct seeding need to grow their root systems into the soil to achieve the proper water balance as they are integrated with the hydrologic cycles of the planting site (Margolis and Brand 1990; Grossnickle 2005).

Our study showed that the appropriate sowing time supported by the use of seed briquettes and the addition of aquasorb provided better seedling survival compared to the others. In general, the use of seed briquettes and the addition of aquasorb significantly increased seedling survival, height and root collar diameter of all species. The briquettes can increase seed germination and provide nutritional reserves for the early growth of seedlings because these briquettes have a relatively high nutrient content. Several previous studies reported that seed briquettes can improve the percentage and speed of germination (Choong et al. 2006; Holbert et al. 2019; Sudrajat and Rustam 2020) and the effect was similar to priming treatment (seed invigoration) (Govinden-Soulange and Levantard 2008).

Seed briquettes also improve biological control capacity, protection from biotic and abiotic stress, moisture attraction, supply of growth regulating nutrients, and microenvironmental influences (Choong et al. 2006; Ryu et al. 2006; Jyoti and Bhandari, 2006). The addition of aquasorb was also able to improve the germination environment and the initial growth of seedlings because aquasorb can bind water and maintain the environmental humidity (water availability) needed during the germination process. Seed briquettes and the addition of aquasorb were able to increase the height and root collar diameter growth of the three species tested at each stage at each sowing time (Table 3 and Table 4). Most of the ingredients used to make seed briquette are organic matter plus lime. Organic materials will increase the availability of nutrients in the briquette, while lime will increase the pH of the briquettes. Application of lime to Arachis hypogaea seed pellets increased seedling emergence, reduced seedling mortality, and enhanced plant growth (Murata et al. 2008). Thus, besides being able to increase germination caused by effects similar to priming treatment (Govinden-Soulange and Levantard, 2008), seed briquettes also have other benefits, such as moisture attraction, seed protection from abiotic or biotic stresses, microenvironmental influences, and supply of growth regulator nutrients (Jyoti and Bhandari 2006), to increase the adaptation and growth of forest trees seedlings in direct seeding. The addition of aquasorb also provides benefits for the initial growth of the seedlings. Aquasorb (hydrogels) can absorb and retain water thereby reducing the effects of drought stress and improving early seedling growth (Tongo et al. 2014).

At the species level, direct seeding of C. inophyllum sown when the rain began to stabilize in December (ST-2) using seed briquettes and adding aquasorb, gave the highest seedling survival (68.7%). Although C. inophyllum naturally grows well in deep soil near the coast and will thrive on pure sand (Orwa et al. 2009), this species can grow in a wide ecological range. This species can grow in a variety of habitats (Calibo et al. 2021), can grow in poor soil, is more resistant to stress, both biotic (weed competition) and abiotic (drought and light) (Gilman and Watson, 1993; Sudrajat et al. 2018). In addition, because of its long, broad, and evergreen leaves, dense crown, and horizontal branches (Allen 2002; Kainuma et al. 2016), C. inophyllum has a high land cover capability and has strong potential to adapt in various regions and environmental conditions, assuring for its popular use in reforestation and rehabilitation of degraded or marginal lands (Calibo et al. 2021).

Conclusion

Sowing time, seed treatment, and sowing engineering are methods that can be used to increase the success of direct seeding. Implementation of direct seeding which was carried out when the rain began to stabilize in December (ST2) with the use of seed briquettes and the addition of aquasorb 5 g was able to increase the success of direct seeding of C. pentandra, E. cyclocarpum, and C. inophyllum with seedling survival respectively 23.9%, 55.0%, and 68.7%. The use of seed briquettes and aquasorb also provided the best growth as shown in the height and root collar diameter of the seedlings at each stage of sowing time. C. inophyllum gave higher seedling survival compared to the other two species which indicates that this species is very prospective for direct seeding applications due to its high adaptability. In general, the application of direct seeding using seed briquettes and the addition of aquasorb is a promising alternative technology for forest and landscape restoration, especially for tropical remote areas.

Acknowledgment

This work was funded by the Asia-Pacific Network for Sustainable Forest Management and Rehabilitation under Grant No. APFNet-Agreement-2020-045. The authors were grateful to the authorities of the Institute for Standard and Instrument Implementation of Environment and Forestry, Bogor, for facilitating the field research activities at Parungpanjang Forest Research Station, Bogor, Indonesia.

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.