Pongamia: A Possible Option for Degraded Land Restoration and Bioenergy Production in Indonesia

Abstract

Indonesia has 14 million ha of degraded and marginal land, which provides very few benefits for human wellbeing or biodiversity. This degraded land may require restoration. The leguminous tree Pongamia pinnata syn. Milettia pinnata (pongamia) has potential for producing biofuel while simultaneously restoring degraded land. However, there is limited information on this potential for consideration. This paper aims to address the scientific knowledge gap on pongamia by exploring its potential as a biofuel and for restoring degraded land in Indonesia. We applied a literature review to collect relevant information of pongamia, which we analyzed through narrative qualitative and narrative comparative methods with careful compilation and scientific interpretation of retrieved information. The review revealed that pongamia occurs naturally across Indonesia, in Sumatra, Java, Bali, Nusa Tenggara and Maluku. It can grow to a height of 15–20 m and thrive in a range of harsh environmental conditions. Its seeds can generate up to 40% crude pongamia oil by weight. It is a nitrogen-fixing tree that can help restore degraded land and improve soil properties. Pongamia also provides wood, fodder, medicine, fertilizer and biogas. As a multipurpose species, pongamia holds great potential for combating Indonesia’s energy demand and restoring much of the degraded land. However, the potential competition for land and for raw material with other biomass uses must be carefully managed.

1. Introduction

An ever-growing demand for energy has increased the importance of new and renewable sources of energy [1,2]. Petroleum fuel is the primary source of energy used by communities in Indonesia to run their vehicles, generators and other apparatus with combustion engines [1]. In recent years, Indonesia has switched from being a petroleum-exporting to a petroleum-importing country, and its own natural reserves are expected to be depleted by 2030 [2]. This realization has highlighted the need to seek and develop alternative energy sources. Biofuel is considered an important alternative source of energy [2]. Therefore, the Government of Indonesia’s national energy policy supports new and renewable energy, which could provide up to 23% of national energy needs by 2025 and 31% by 2050 [3].

Globally, most biofuels are currently produced from oil palm, coconut, cassava, corn, sorghum and other edible food crops and are known as first-generation biofuels [4]. Second-generation biofuels use non-food crops as feedstocks and involve more advanced technologies in their production [5,6]. Some non-food crops, e.g., jatropha (Jatropha curcas), have biofuel potential but require fertile land to achieve high yields and the resulting competition with subsistence and cash crops limits their overall production prospects [7]. Therefore, there is an urgent need to identify suitable plant species that can be used as energy sources and can grow on abandoned lands, i.e., marginal or degraded lands. Pongamia (Pongamia pinnata syn. Milettia pinnata) is one such species. Its seeds are valued for their biofuel properties and it can grow on marginal and degraded land [8].

As biofuels are produced from renewable feedstocks via photosynthesis using atmospheric CO₂, their combustion is less harmful to the atmosphere [9,10]. Biofuels have received much attention because, with the exception of a few unhealthy compounds found in oil cakes, they are non-toxic, renewable and more biodegradable in nature than petroleum-derived fuels [11,12,13]. Toxic pollutants, such as carbon monoxide (CO), unburned hydrocarbons (UHC), and particulate matter (PM) are also significantly lower when biofuels rather than petroleum fuels are burned in compression ignition (CI) engines [14]. Further, an important consideration for biofuel-producing crops is their ability to grow on degraded land, as they can present a sustainable solution to the bioenergy land-use perplexity [15].

Indonesia has around 14 million ha of degraded and marginal land, which is of limited benefit for food production and environmental services [16]. The Government of Indonesia has committed to restoring 12 million ha of this degraded land in an effort to achieve climate resilience in the food, water and energy sectors [17,18]. In relation to energy production, a scientific study has revealed that 3 million ha of severely and critically degraded land is suitable for biofuel and biomass plantations [19]. Several Government agencies have land restoration targets. These include the Peatland Restoration Agency or Badan Restorasi Gambut, which aims to restore more than 2 million ha of degraded peatlands in Riau, Jambi, South Sumatra, West Kalimantan, Central Kalimantan, South Kalimantan and Papua provinces [20] (see Figure 1). As a potential bioenergy species, pongamia could provide an opportunity to restore degraded lands while enhancing ecosystem services [21] and supporting local economies [22,23,24]. (The restoration of degraded areas covered with Imperata cylindrica (alang-alang) in Indonesia may also requires crops like pongamia that can shade out such areas and enhance ecosystem services and financial benefits).

Pongamia is a leguminous species native to Bangladesh, India, China, Pakistan, Sri Lanka, Vietnam, Malaysia, Indonesia, Japan, Fiji and Australia and has been introduced to the United States, Puerto Rico and many African countries, including Egypt [26]. The species occurs naturally in humid and sub-tropical regions and grows well in a wide range of agro-climatic conditions [2] (common names for the species include Indian beech, karum tree (English); pongam (Gujarati); dalkaramch (Tamil); karanj, karanja, kanji (Hindi); kanuga oil tree (Telugu); honge (Kannada); shuihuang pi (Chinese); day mau (Vietnamese); kranji, malapari (Indonesian); mempari (Malay) [27,28]). Pongamia has been utilized traditionally as a pharmaceutical plant [26]. It is a preferred species for controlling soil erosion and binding sand dunes because of its dense network of lateral roots. It can also be productive on degraded land [29].

This paper compiles information on pongamia, including its natural distribution, growth, yields, biofuel potential and land restoration capacity, to corroborate scientific understanding. It may provide a valuable resource for practitioners in planning bioenergy and restoration projects.

Section 1 of this paper comprises the motivation of this research; Section 2 contains data collection and analysis methods used for this work; Section 3, which is divided into ten sub-sections, is results and consequent discussion of the analysis following the focus area of pongamia, as stated in the paragraph above; and the last, Section 4, summarizes the results and provides suggestions for ways forward.

2. Materials and Methods

This study is based on a literature review of both peer-reviewed and grey literature. The review mainly focused on four scientific areas of interest, i.e., distribution and growth, potential yield, potential biofuel, and landscape restoration capacity of pongamia. A preliminary scoping study was conducted based on a Google Scholar search targeted at finalizing key words and search phrases, and contributing to the framing of the manuscript. After finalizing key words and phrases, as well as inclusion criteria (

Table 1. Search sites, key words and inclusion criteria to generate targeted information from the literature review used in this study.

Search Sites	Key Words and Search Phrases	Inclusion Criteria
Google Scholar Mendeley Scopus Web of Science	‘pongamia’ OR ‘bioenergy’ OR ‘biofuel’ OR ‘jet fuel’, ‘pongamia’ AND ‘bioenergy’, ‘pongamia’ AND ‘biofuel’, ‘pongamia’ AND ‘jet fuel’, ‘pongamia’ AND ‘oil’, ‘pongamia’ AND ‘yield’, ‘ pongamia’ AND ‘growth’, ‘bioenergy’ AND ‘Indonesia’, ‘biofuel’ AND ‘Indonesia’, ‘pongamia’ AND ‘Indonesia’, ‘biofuel’ AND ‘Indonesia’, ‘pongamia’ AND ‘land restoration’, ‘pongamia’ OR ‘land restoration’ AND ‘Indonesia’, ‘pongamia’ AND ‘nitrogen’, ‘pongamia’ OR ‘land restoration’ AND ‘nitrogen’, ‘pongamia’ AND ‘benefit’, ‘pongamia‘ AND ‘potential’, ‘pongamia’ AND ‘wood’, ‘pongamia’ AND ‘medicine’, ‘pongamia’ AND ‘landscape’, ‘land tenure’ AND ‘Indonesia’, ‘land tenure’ AND ‘tree planting’, ‘land tenure’ AND ‘pongamia plantation’.	Evidence-based information on pongamia, i.e., distribution, growth, yields, biofuel potential, land restoration capacity

Note: as ‘Millettia’ is a synonym of ‘Pongamia’, and mistakenly overlooked as a search word, may cause excluding some relevant articles.

), relevant literature was gathered using scientific research search sites, i.e., Google Scholar, Mendeley, Scopus and Web of Science. In our literature search, we only considered published scientific papers available online. At the outset of the study, we conducted a quick review of the abstracts and contents of the retrieved literature to evaluate their relevance for inclusion in further extensive reviews. After removing any duplicates, and considering the timeframe for this study, we selected 84 of the 770 pieces of literature for thorough review by considering their relevance. A basic checklist of quality criteria (i.e., clear aim and replicable methodology, accurately and reliably measured outcomes, and consistently reported findings with methodologies and empirical data provided) was used to select these 84 pieces of literature. A total of nine months from January 2018 to September 2019 (and again from June to September 2021 for revision) was required for four reviewers (one full-time and three part-time) to extract relevant data. Further supporting data was gathered from the Indonesian Ministry of Environment and Forestry (MoEF) and is presented in Annex 1 of this paper.

Relevant information was carefully compiled point-by-point, and scientific interpretations were made by using narrative qualitative and narrative comparative analysis methods, including tables and figures [30,31,32]. (Narrative analysis methods are characterized by perspective and context, which deal with points of view regarding what has happened, and describing what may be significant in the near future [33]. They simply provide meaning and coherence to, and perspective on, experience and knowledge [34].) The analysis process was designed to scrutinize relevant concepts in a transparent and subjective way, following the objective of this paper and the inclusion criteria (Table 1), i.e., the growth, distribution, yield and biofuel production potential, and landscape restoration capacity of pongamia. Careful attention was paid to a more discursive interpretation and to represent a view of reality through a process of decontextualization and recontextualization with appropriate scientific order as presented in Section 3 below. It is also important to mention that some terms in this manuscript are stated in general without having precise quantification (e.g., pongamia growth rates), which reflects the original literature source.

3. Results and Discussion

3.1. Potential of Pongamia as a Biofuel Species

3.1.1. Distribution

Pongamia grows naturally across the Indonesian archipelago, mostly in Berbak National Park in Jambi province, Sembilang National Park in South Sumatra province; Berikat Gulf in Bangka Belitung province; Ujung Kulon in Banten province; Batu Karas, Pangandaran in West Java province; Alas Purwo National Park and Baluran National Park in East Java province; Senipah, Samboja, Sekerat and Tanjung Batu in East Kalimantan province; Lovina in Bali province; and Sembelia in East Lombok, West Nusa Tenggara province. It can also be found in the western part of Seram island, Maluku province (Figure 2) [28,35,36,37]. Pongamia has many local names in Indonesia, including malapari (Simeulue), mabai (Bangka), kipahanglaut (East Java), bangkongan and kepik (Java), kranji (Madura), marauwen (Minahasa, Sulawesi), hate hira (Ternate) and butis and sikam (Timor) [35].

3.1.2. Growth

Pongamia grows well naturally in lowland forests on calcareous soils, in rocky coastal areas, along the edges of mangrove forests, and along streams and estuaries. It is a hardy woody plant and can survive temperatures ranging 5 to 50 °C and elevations up to 1200 m [36,38]. It grows well in both full sunlight and partial shade and can grow in most soil types from stony through sandy to clay. Although it is salinity tolerant, it does not survive well in dry sands [39]. Studies have found pongamia to have potential for growing as a restoration species in highly degraded forest areas [38] and on land which has been degraded physically, chemically and biologically due to mining operations [40]. Pongamia plantation trials in a hot, dry area with limestone soil at Bukit Jimbaran, Bali showed 100% survivability of plants two months after planting without irrigation. Trial plants also showed vigorous growth in height (7.5–13.60 m), stem diameter (20.70–63.69 cm at breast height) and numbers of compound leaves (crown width 6.00–20.0 m) [41], indicating pongamia’s adaptability to the hot, dry conditions associated with marginal land. Similarly, another study [42] observed survival rates ranging 88 to 100% with average height of 83.75 cm and diameter of 0.85 cm six months after planting in Java. In a trial plot on degraded peatland in Buntoi village, Pulang Pisau district, Central Kalimantan province, pongamia trees started flowering 1.5 years after planting (Figure 3) [43]. Another study showed four-month-old pongamia seedlings demonstrating tolerance to 200 and 150 mM NaCl saline drain and saline waterlogged conditions respectively [44].

Pongamia is a semi-deciduous tree the seeds of which contain non-edible oil, which can be processed into biodiesel. However, recent technological advances have allowed the seeds to be processed as food. It is a forest tree, demands only low levels of moisture and is therefore drought resistant, and needs only minimum input and management to grow well [27,45]. It can reach heights of 15–20 m and has a large and wide crown [27]. It grows very rapidly and reaches its full height and maturity within 4–5 years [46]. Pongamia can be propagated by generative or vegetative means. It can be propagated vegetatively from cuttings and root suckers (with new plants growing from lateral roots of the parent tree) [26]. Pongamia is also propagated from seeds in nursery beds or polybags and via in-situ sowing of seeds in plantations [8,47]. It has also been reported that seeds stored for three months or more result in lower germination and plant vigor [47]. Seeds take approximately one week to germinate and around 85% of seeds do so with appropriate nursery management. Study findings have also indicated a direct relationship between seed size and germination efficiency but only for fresh seeds [8]. The long-term viability of pongamia trees also depends on appropriate pruning practices. Information on pongamia growth rates from four trial sites is presented in Annex 1.

3.1.3. Yields

Pongamia produces large quantities of seeds. However, yields vary according to soil and climatic conditions, as well as management practices [27,39,48,49,50,51]. There is limited information on pongamia seed yields in Indonesia, with most literature coming from India and Australia [48,50,51,52]. This is because pongamia grows naturally or in plantations in a wide range of regions in India, while pongamia is cultivated extensively in Queensland, Australia [8,50].

Pongamia can produce 9 to 90 kg of seeds annually per adult tree in India, equivalent to a potential yield of between 900 kg and 9000 kg per hectare [27,48]. This differs slightly from yields of between 20 and 80 kg per tree reported in Australia [52]. Another report noted average annual seed production of 20 kg per tree in Australia [50]. Another study [53] reported seed yield of pongamia trees improving significantly in the northern part of Western Australia after the introduction of Apis mellifera beehives (up to 4.9 kg per 1-year-old tree), as bees are effective pollinators for pongamia. Incorporating Apis mellifera bees into pongamia plantations could be a win–win solution for successful pongamia pollination and honey production.

In Bangladesh, young pongamia (15 years old or younger) produced more than 25 kg of seeds per tree annually. However, yields increased as trees grew older, i.e., annual yields of more than 100 kg for 20-year-old trees [54]. A trial plot in Parung Panjang, West Java, Indonesia showed that pongamia trees as young as 8 years old cultivated in an agroforestry system can provide 3.80 kg of seeds per year (Figure 4, Appendix A). Pongamia trees can live for up to 100 years, can produce seeds every year, and require minimal maintenance once they reach 30 years old [54].

3.1.4. Biofuel Production Potential

The most useful product from pongamia is biodiesel. Biodiesel is produced by the transesterification of vegetable oils or animal fats using alcohol (methanol or ethanol) and a catalyst (e.g., potassium hydroxide (KOH) or sodium hydroxide (NaOH) [54]. The biodiesel produced is a clean burning fuel that has no sulfur emissions and is non-corrosive [55]. At low pressures and temperatures, transesterification produces 80% methyl ester, and 20% glycerin as by-products [27]. The major fatty acids in crude pongamia oil are oleic (51%), linoleic (19%), palmitic (11%), stearic (6%), linolenic (4.5%) and behenic (4.5%) [44]. Pongamia oil extracts exhibit good chemical properties and could be used as good biodiesel feedstock [27]. Fatty acid methyl ester from pongamia and other potential biodiesel plants such as Azadirachta indica, Calophyllum inophyllum and Jatropha curcas meet the major specifications of biodiesel standards required by American and European standards organizations [56].

During the past few decades, pongamia oil has attracted considerable attention as a potential renewable, biodegradable, eco-friendly, non-toxic fuel [27] and as being economically viable [52]. Studies have determined oil yield and properties with 1000 kg of pongamia seeds yielding 270–300 kg of crude pongamia oil [49]. In other studies, oil yield was reported to reach up to 35% by weight [27,57], with some reports showing yields of up to 40% [50,58]. Crude pongamia oil needs further processing (transesterification) to give methyl esters. Around 85–90 L of biodiesel and 15–16 L of glycerin (considered a by-product) can be obtained from 100 L of crude pongamia oil by transesterification [49]. Meanwhile, other studies have found approximately 4 kg of pongamia seeds being required to produce one liter of crude pongamia oil, which in turn could yield 0.896 L of biodiesel [59]. The major cost of biodiesel production from pongamia is the feed stock, which can account for 60% of total production costs, followed by chemical costs for transesterification at 17%, and operating costs at 10% [60]. Therefore, high seed yield is the key to successful pongamia biodiesel production. A study on the economic viability of pongamia biodiesel production in Fiji [61] showed the levelized cost of biodiesel to be USD 1.44 per liter and the benefit–cost ratio to be 1:06.

Table 2. Physio-chemical properties of crude pongamia oil.

Property	Unit	Value
Color	-	Yellowish red
Density	g/cc	0.924
Viscosity	mm2/s	40.2
Acid value	mgKOH/g	5.40
Iodine value	-	87
Saponification value	-	184
Calorific value	Kcal/kg	8742
Specific gravity	-	0.925
Unsaponifiable matter	-	2.9
Flash point	°C	225
Fire point	°C	230
Cloud point	°C	3.5
Pour point	°C	−3
Boiling point	°C	316
Cetane number	-	42
Copper strip corrosion	-	No corrosion observed
Ash Content	%	0.07

Adapted from Ref. [27].

,

Table 3. Fatty acid composition of crude pongamia oil.

Fatty Acid (%)	Molecular Formula	Percentage	Structure
Palmitic acid	C16H32O2	11.65	CH3(CH2)14COOH
Stearic acid	C18H36O2	7.50	CH3(CH2)16COOH
Oleic acid	C18H34O2	51.59	CH3(CH2)14(CH=CH)COOH
Linoleic acid	C18H32O2	16.64	CH3(CH2)12(CH=CH)2COOH
Eicosanoic acid	C20H40O2	1.53	CH3(CH2)18COOH
Dosocasnoic acid	C22H44O2	4.45	CH3(CH2)20COOH
Tetracosanoic acid	C24H48O2	1.09	CH3(CH2)22COOH

Adapted from Ref. [27].

and

Table 4. Properties of pongamia methyl ester.

Property	Unit	ASTM Test Method	Pongamia Biodiesel	Diesel
Density	g cc−1	D1498	0.860	0.824
Calorific value	Kcal kg−1	D240/D4868	3700	4285
Cetane number	Number	D613	41.7	49
Acid value	mg KOHg−1	D664	0.46	0.36
Iodine value	Number	D1510	91	-
Water and sediments	% vol, max	D2709	0.005	-

Adapted from Ref. [27].

below detail properties of crude pongamia oil. It is worth noting that pongamia can yield a considerable volume of biodiesel in comparison with other biofuel-producing species (see Figure 5). However, the oil content of pongamia may vary depending on seed source, processing methods (i.e., hydraulic press, mechanical press or solvent extraction) and equipment used (

Table 5. Pongamia oil content from different seed sources and extraction methods.

No.	Oil Content (%)	Method	Equipment	Seed/Seed Source	Reference
1.	15.44–15.82	Mechanical press	Simple screw expeller press	Bulk seed/Banten, Indonesia	[37]
2.	15.92–19.60	Mechanical press	Fabricant screw expeller press	Bulk seed/Banten, Indonesia	[62]
3.	14.25–14.67	Mechanical press	Simple screw expeller press	Bulk seed/West Java, Indonesia	[37]
4.	13.05–13.23	Mechanical press	Simple screw expeller press	Bulk seed/East Java, Indonesia	[37]
5.	24.00–26.00	Mechanical press	Simple screw expeller press	Bulk seed/India	[63]
6.	27.34–39.26	Solvent extraction	Soxhlet extractor	Bulk seed/Banten, Indonesia	[62]
7.	26.61–44.68	Solvent extraction	Soxhlet extractor	Individual seed/Banten, Indonesia	[62]
8.	26.30–32.00	Solvent extraction	Soxhlet extractor	Bulk seed/Bali, Indonesia	[41]
9.	28.00–31.00	Solvent extraction	Soxhlet extractor	Bulk seed/Bali, Indonesia	[64]
10.	27.00–39.00	Solvent extraction	Soxhlet extractor	Bulk seed/India	[63]
11.	33.31–39.01	Solvent extraction	Soxhlet extractor	Bulk seed/Madhya Pradesh, India	[65]
12.	28.18–41.32	Solvent extraction	Soxhlet extractor	Bulk seed/Carmen, Philippines	[66]
13.	33.00–40.30	Solvent extraction	Soxhlet extractor	Bulk seed/Queensland and Northern Territory, Australia	[44]

).

3.1.5. Pongamia for Bio-Jet Fuel

One potential product from pongamia is bio-jet fuel. Most aircraft are fueled by conventional jet fuel, which is non-renewable, costly and emits large amounts (80%) of carbon, e.g., one tonne of conventional jet fuel emits 0.8 tonnes of carbon when burned [67]. Therefore, the aviation industry is looking for renewable jet fuels [67]. However, compared to other industries, aviation has a limited range of alternative renewable fuel options that can replace fossil fuels. Bio-derived jet fuel could be a viable alternative for aviation industries [68]. Camelina sativa, Jatropha spp., Elaeis guineensis and algae have already been used to produce fuel for several test flights. Pongamia oil has yet to be tested, but has significant potential [50], as it can abate 43% of greenhouse gases on a lifecycle basis [69].

3.2. The Potential of Pongamia for Land Restoration

3.2.1. Nutrition Enhancement for Degraded Land

Degraded land is land that has lost its productivity [20]. Such land often has low soil nutrient content, low productivity, suffers from erosion, and is unsuitable for growing crops. There are two main ways of restoring degraded land: (i) physical, technical or engineering restoration; or (ii) biological restoration [70].

Pongamia trees have several benefits for restoring degraded land. Studies have shown five-year-old pongamia plantations having carbon sequestration potential of around 13.43 tons per ha [71,72]. Pongamia is capable of withstanding drought stress, can grow on saline soils, and needs little topsoil as it has a dense network of lateral roots and long thick taproots. Pongamia plantations can help alleviate compaction and crusting [73]. It is a sturdy plant with no special nutritional requirements and can grow in extreme environmental conditions. It is tolerant to soil sodicity, pH imbalances, high temperatures, heavy metal contamination, drought and poorly drained soils. Consequently, pongamia can achieve phytostabilization, i.e., the long-term stabilization and containment of pollutants [74,75]. Iron, chromium, copper, manganese and magnesium in fly ash dykes have been phytostabilized by establishing pongamia plantations on the dykes [75]. Therefore, establishing pongamia biofuel plantations on degraded land can be a win–win solution for energy production and land restoration, especially compared to Elaeis guineensis (oil palm), where links to deforestation are a worldwide concern [70,73,74,75,76].

3.2.2. Nitrogen Fixation

Chemical nitrogen fertilizer is widely used for growing crops. However, it is costly and its production causes high levels of greenhouse gas emissions [77]. The restoration of degraded land also requires the stabilization of its nitrogen content. Pongamia is a leguminous tree that fixes nitrogen, while also producing raw material for biofuel [78,79]. In contrast, other common biofuel crops, such as canola, sugarcane, sweet sorghum, maize and woody trees (e.g., eucalyptus and willow), deprive soils of nitrogen rather than increasing nitrogen content [79]. Pongamia fixes nitrogen throughout its life [79]. A study of the phenotypic characteristics of rhizobia isolates from pongamia showed isolates growing well at temperatures ranging 29 to 39 °C, within pH levels 7 to 9, and tolerating less than 1% salinity. It also showed isolates from Rhizobium and Bradyrhyzobium genera being effective microsymbionts under controlled conditions [44]. Relative effectiveness of the symbiosis between pongamia as a host and rhizobia is determined by dividing shoot dry weight of plants inoculated with rhizobia isolate by shoot dry weight of plants treated with nitrogen fertilizer expressed as a percentage of weight [80]. By using this method, the highest relative effectiveness of rhizobia isolates was 85.9% [44].

Another study showed that nodules of pongamia formed on seed-derived seedlings within four weeks with visible nodulation and established symbioses by eight weeks at 28 °C. The nodules produced by these strains were uniformly filled with bacteroid zones [81]. Pongamia nodules can actively fix nitrogen as demonstrated by quantification by gas chromatography of ethylene in acetylene reduction assays, where C₂H₂ (acetylene) serves as a substrate for bacterially encoded nitrogenase [76]. Therefore, cultivating pongamia together with agricultural crops has the potential to produce favorable agricultural yields.

3.2.3. Other Services Provided by Pongamia

Restoring degraded land using pongamia could also provide a range of services to benefit local communities and nature by enhancing ecosystem functions (see

Table 6. Pongamia tree products and their various uses.

Attributes	Important Uses	References
Wood	Pongamia logs serve as the raw material for wood flour as lignocellulosic filler that can be further processed to produce wood–plastic composites.	[82]
	Pongamia wood is useful for making tool handles, combs, cabinets, cartwheels, posts, agricultural implements and paper pulp.	[26,48]
	Pongamia wood is used as fuelwood.	[48]
Medicine	Almost all parts of pongamia trees are used in folk medicine: Juice from the roots blended with coconut milk is used to treat gonorrhea. Stem bark extract has sedative and antipyretic qualities and reduces enlarged spleens. Juice from the leaves is used to treat diarrhea, colds and coughs, and to relieve rheumatism. The fruits are used to treat abdominal tumors. The seed is used to treat keloid tumors, skin ailments and hypertension, and as an expectorant for bronchitis and whooping cough. The flowers are used to treat certain diabetic conditions. The oil is used to treat leprosy, chronic fever, skin diseases and rheumatism.	[26,29]
	A crude decoction of pongamia leaves is used as an antidiarrheal with efficacy against cholera.	[83]
Fodder	The leaves are commonly used for cattle feed and, less so, for goat feed, and are a valuable source of fodder in arid regions. Seed residue, press cake and seedcake contain much protein and are used for poultry feed; but should not exceed 75% of feed as they contain several toxic compounds.	[26,46,48]
Fertilizer and biogas	Seedcake and leaves are used as fertilizer. Seedcake can generate biogas in household biogas generators.	[49,51]
Biodiversity restoration	Pongamia trees can restore biodiversity by improving soil quality, controlling erosion, and enhancing vegetation cover at the landscape level (including in sandy, heavy clay, rocky and waterlogged areas).	[8,29,84,85,86,87,88]
Other services	Pongamia trees serve as windbreaks, are fire tolerant, and are ornamental trees. The oil is used as a lubricant, as a leather dressing, and for manufacturing soap, varnish and paint. The flowers are a good source of pollen and nectar, yielding a dark honey. The bark is used to make rope. Pounded and roasted seeds used to be utilized as a fish poison. Dried leaves are useful to store with grain to repel insects.	[8,26,29,48,56,57,84,85,89,90,91,92,93,94,95]

). Such services could be enhanced with appropriate pongamia plantation design by counting costs and benefits as well as trade-offs and synergies associated with different options in specific locations, e.g., various understory crop combinations considering local economic, cultural and environmental values (e.g., Figure 3).

3.3. Community Involvement

As stakeholders, local communities can be directly affected by fuel shortages, and their potential contributions [96] should be taken into account during pongamia cultivation processes. Such contributions could be overseen through local technical and administrative capacity building [97,98] to strengthen pongamia cultivation at the landscape level. Community involvement could also enhance local incomes, innovative spirit, technical proficiency and enthusiasm through the distribution of degraded land in areas surrounding settlements to communities for pongamia cultivation, or by villagers using their own degraded land for the same purpose [96,99]. It may also increase transparency and accountability for all parties (local communities, government and investors), foster a sense of responsibility and encourage support and mutual interest for land restoration efforts [100].

3.4. Other Considerations

To restore forestland, it has to be a biodiversity-rich, self-regenerating system, consisting of a microclimate and a wide variety of plants and animals in mutual coexistence [101]. Monoculture plantations may not provide as high levels of biodiversity as forest, and require ongoing human intervention including the use of herbicides and pesticides during land preparation [47,102,103,104]. Profitable pongamia plantations might also become a new driver of land clearing and an indirect cause of deforestation, especially where land tenure is not clear, as witnessed in various countries [101,105]. Secure land tenure is crucial for the successful implementation of tree-planting activities [106,107]. If local communities have insecure rights to use land and to harvest produce from trees, they are less likely to tend trees. When farmers lack secure land titles, they are deprived of access to the credit essential as initial capital for investing in tree planting [108]. Therefore, policy support to provide secure land titles to local people will be essential to enable pongamia adoption.

Development of the Government of Indonesia’s policy on biofuel-based energy is carried out using a SWOT analysis approach to analyze existing conditions, formulate problem-solving strategies, and develop policies for sustainable biofuel [109]. The General National Energy Plan (RUEN) has a number of long- and short-term programs to support the National Energy Policy (Government Regulation No. 79 of 2014). RUEN targets to produce 15.6 and 54.2 million kiloliters of biofuel by 2025 and 2050, respectively [110]. To support government policy, sustainable energy plantations of adaptive and productive species could play a crucial role, especially when implemented on ex-mining and degraded land. Besides growing biofuel, such species can also fertilize soils and provide other benefits, e.g., income generation, improved biodiversity, and provision of multiple ecosystem services [43,111].

Considering the diversity of biofuel production (and system components) across locations, there may be a shortage of specific evidence about variation in contributions to the delivery of various ecosystem services. Improving this knowledge base (e.g., through modeling) can be facilitated by research that produces a more sophisticated approach to incorporate the economic, social and environmental characteristics of biofuel.

Further, through strengthening and socializing biofuels as a strategic industry, increasing productivity and diversification of biofuel-producing plants, providing incentives for investment in biofuel facilities, encouraging market links, and increasing research budgets for the development of biofuel commodities and products can strengthen sustainable biofuel production.

4. Conclusions

Pongamia trees are well suited to growing in adverse environmental conditions. The species can grow in most soil types, in partial shade or full sunlight, and at various temperatures. Pongamia is a multipurpose tree that fixes atmospheric nitrogen, improves soil health and can produce large amounts of oil for biodiesel. It can produce bioenergy on degraded land unsuitable for food production. As Indonesia’s large areas of degraded land deliver limited benefits to people and nature, restoring such land through pongamia cultivation could provide an opportunity to enhance ecosystem services and reverse biodiversity loss. Although several other species produce biofuel (e.g., oil palm, coconut or jatropha), with its multiple benefits (see Table 6), pongamia is a prime candidate for planting as a bioenergy feedstock on degraded land.

The Government of Indonesia’s initiation of a national policy on new and renewable energy use, which includes biofuel making up 5% of the energy mix by 2025 [112], has significantly increased the importance of domestic biofuel production [111]. As palm oil production is widely questioned, pongamia could be a potential new alternative for cultivation on degraded land. However, it will necessitate long-term monitoring to prevent forest being cleared for biodiesel crop production [111].

It was apparent from our literature review that scientific knowledge gaps remain, i.e., up-to-date pongamia production technology, long-term plantation management, community involvement, various added-value options (e.g., understory crop association), identifying potential biofuel producers and consumers, developing effective business models for various biofuel stakeholders, and the feasibility of building stable biofuel markets. Therefore, new studies on pongamia focusing on these issues could help to fill knowledge gaps and benefit scientific communities, managers and other stakeholders.