Spatial Assessment of Degraded Lands for Biofuel Production in Indonesia

This study spatially estimated degraded lands in Indonesia that have limited functions for food production, carbon storage, and conservation of biodiversity and native vegetation and examined their suitability to grow biodiesel species (Calophyllum inophyllum, Pongamia pinnata, and Reutealis trisperma) and biomass species (Calliandra calothyrsus and Gliricidia sepium). Results showed ~3.5 million ha of degraded lands potentially suitable for these species in Indonesia. With the all-five-species scenario, these lands had the potential to produce 1105 PJ year−1 of biomass and 3 PJ year−1 of biodiesel. With the biodiesel-only-species scenario, these lands showed the potential to produce 10 PJ year−1 of biodiesel. Despite this energy potential, however, the land sizes were too small to support economies of scale for biofuel production. The study findings contribute to identifying lands with limited functions, modeling the growth of biofuel species on regional lands, and estimating carbon stocks of restored degraded lands in Indonesia.


Introduction
Bioenergy production from degraded lands may help society meet increasing energy demands and secure a new source of renewable energy for its sustainability.These potential benefits have attracted global attention on the feasibility of using degraded lands to produce bioenergy [1].In Indonesia, for example, energy demand is growing rapidly due to its urbanization, economic growth, and population increase [2].For these reasons, the government of Indonesia set ambitious targets in 2015 to increase its biodiesel and bioethanol consumption to 30% and 20%, respectively, of the total energy consumption by 2025 (Presidential Regulation No. 12/2015) [3].However, current biofuel production in Indonesia is far from meeting these targets.In 2016, biofuel production was only 0.05% (or 3.66 billion liters) of the total fuel consumption for the year (or 70 billion liters) [3].Moreover, according to the Indonesian National Energy Council [4], the average energy demand would increase by around 4.9% per year from 2015 to 2025.This surge in expected demand has increased interest in the feasibility of using degraded lands to provide a new source of renewable energy in Indonesia [5][6][7][8].
In order to realize these potential benefits, however, bioenergy production must be sustainable in various ways.The expansion of biofuel production can result in reduced food production, which is particularly the case for palm oil.Indonesia is the largest palm oil producer and exporter in the world, and palm oil is a major feedstock for the production of liquid biofuels in the country [9].In addition, the expansion of biofuel production through conversion of rainforests and peatlands would release large amounts of carbon from both aboveground and belowground reservoirs and create a biofuel carbon debt [10,11].Such expansion could also threaten-or destroy-rich biodiversity and native ecosystems in these lands [12].Thus, for renewable energy to be sustainable, biofuel production from degraded lands should avoid compromising food production, carbon stocks, biodiversity, and native vegetation.In many studies on degraded lands, however, data on the availability of such lands and their feasibility to deliver sustainable biofuel cannot be compared directly due to the diverging definitions of degraded lands used [1] and because of the many potential biofuel species available in Indonesia [5][6][7][8].
To reduce this knowledge gap, this study (1) assesses degraded lands that have limited functions to produce food, to sequestrate carbon stocks on land, and to maintain vegetation and biodiversity by adopting the definition of degraded lands from the Indonesia Climate Change Center (ICCC) [5]; and (2) examines the suitability of the degraded lands to grow key species for biodiesel production (Calophyllum inophyllum, Pongamia pinnata, and Reutealis trisperma) and biomass production (Calliandra calothyrsus and Gliricidia sepium).Indeed, biofuel production from degraded lands needs to overcome various obstacles as well, including improving the capacity of refineries, building business models for landowners and refineries, securing the property rights of the land, resolving potential conflicts among stakeholders, encouraging smallholder participation, competing with low-price fuels, and mitigating potential invasion by biofuel species [5,7,8,[13][14][15].However, the investigation of these challenges first requires an understanding of the degraded lands available for biofuel production and potential biofuel species.Thus, this study analyzes these lands and species and estimates their potential energy production.

Potential Biofuel Species in Indonesia
While many energy crops exist in Indonesia, in this study, we assessed five tree species with the potential for biodiesel production (C.inophyllum, P. pinnata, and R. trisperma) or biomass production (C.calothyrsus and G. sepium) on degraded lands [8,[16][17][18][19][20][21].These species are native to Indonesia and tolerant to lands with harsh conditions (e.g., drought and acidic soil) that are normally unsuitable for agriculture; thus, these species do not have the capacity to compete with food production (Table 1).The study intentionally excluded bamboo and other nonwoody species as it mainly focuses on tree species for bioenergy production.Oil palm was excluded due to its large potential to compromise food production.
C. inophyllum is a medium-to-large tree that is 8-20 m in height [17].Called "nyamplung" in Indonesia, the tree is used for its wood (e.g., building canoes) and seed oil (medicines and cosmetics) [17,22].The oil is slightly toxic for human consumption [17].As it tolerates windy and sandy conditions, its major habitats include coastal areas, but it also grows inland at high elevations [17,22].Several studies have analyzed biofuel production from C. inophyllum oil because this species can yield up to 20 tons of inedible oil per hectare [17,19,20,[22][23][24].R. trisperma is a tree that is 10-15 m in height [41].In Indonesia, it is called "kemiri sunan" and is used as a natural pesticide and fertilizer [28].It is also used for land rehabilitation owing to its capacity to mitigate land erosion.Although one tree can yield about 25-30 kg of seeds per year, they are toxic and inedible [41,42].There is interest in biofuel production from R. trisperma oil because of its high oil yield [28,43,44].
P. pinnata is a fast-growing leguminous tree that is 12-15 m in height [45].In Indonesia, it is called "malapari" and is used for wood, firewood, and medicine [29,30,33].However, all parts of the plant are toxic for human consumption [46].It tolerates salinity and drought and grows in a wide range of habitats from humid tropical and subtropical regions to cooler and semiarid zones [47].Many studies have analyzed biofuel production from P. pinnata as it is nitrogen-fixing, tolerates various habitats, and has a high oil yield [16,33,47,48].
C. calothyrsus is a fast-growing shrub that is 5-6 m in height [27].In Indonesia, it is called "kaliandra" and is used for firewood and land restoration due to its fast growth and good adaptability to a wide range of habitats [18,36].The shrub is also used for animal feed [35,49].It grows in various soil types, including sandy clays and acid soil [34,50].There is emerging interest in biofuel production from C. calothyrsus because it is highly cellulosic (46-48%), fast-growing, suitable for a short rotation, and adaptable to diverse habitats [18,34,36,51,52].
G. sepium is a medium-sized species that is 2-15 m in height [27].In Indonesia, it is called "gamal" and is used for firewood, cattle feedstock, and medicine [36,39,40].Although it has been dominantly used as firewood, its seeds are now also considered to have the potential to become a feedstock for biodiesel [40].Its leaves, fruits, seeds, roots, and bark can be toxic for human consumption [40,53].It tolerates various soil types, including slightly saline and clay soils [40].There is interest in biofuel production from G. sepium as it not only grows fast and tolerates harsh soil conditions, but it also has low moisture content, high energy potency, and high carbon and volatile content [36,40].

Lands Available for Producing Biomass in Indonesia
Several studies have analyzed the available lands for biofuel production in Indonesia (Table 2).These studies employed different definitions of lands (e.g., degraded or suitable lands) and various methodologies (e.g., spatial or policy analysis), resulting in different land estimates.Several studies have also investigated degraded land for palm oil plantations.Colchester et al. [54] indicated there are about 27 million hectares (Mha) of unproductive degraded forestlands in Indonesia.Gingold et al. [55] showed about 7 Mha of degraded lands in the provinces of West and Central Kalimantan.Harahap et al. [9] demonstrated about 20.9 Mha of degraded land in Indonesia.Other studies have investigated non-palm-oil species for potential biofuel production in Indonesia.The ICCC [5] identified about 23.78 Mha of degraded lands suitable for bioethanol species, such as sugarcane, cassava, sweet sorghum, corn, and sago.Milbrandt and Overend [56] identified about 3.7 Mha of marginal lands for biomass production.Nijsen et al. [57] suggested approximately 30 Mha of lands with degraded soils for production of grasses and woody crops.Wulandari et al. [6] showed about 0.9 Mha of potentially suitable lands on which R. trisperma can be grown in the province of West Java.None of these studies, however, examined the availability of lands that are degraded but potentially suitable for growing C. calothyrsus, C. inophyllum, G. sepium, P. pinnata, or R. trisperma in Indonesia.Although Wulandari et al. [6] analyzed R. trisperma, the study only focused on the province of West Java, and its objective was to estimate all suitable lands, including nondegraded lands.Thus, our study contributes to analyzing the potential growth of multiple biofuel species on degraded lands at a national level in Indonesia.

Materials and Methods
The study methods consisted of two steps.The first step identified degraded lands in Indonesia.The second step analyzed the suitability of growing five biofuel species on the degraded lands and estimated their potential energy production (Figure 1).

Identification of Degraded Lands in Indonesia
The first step of the study involved identifying degraded lands in Indonesia.The analysis employed four types of geographic information system (GIS) data to identify potentially degraded land in Indonesia using an overlaying analysis.These data included severely degraded land data, conservation area data, land cover data, and land system data (Figure 1).Degraded lands were identified by overlaying these spatial data based on inclusion and exclusion criteria as described below.
First, severely degraded land data [58] were used to define the initial scope of degraded lands in Indonesia.The data were developed by the Directorate General of Watershed Management and Social Forestry, under the Ministry of Environment and Forestry of Indonesia, based on technical guidelines for the development of spatial data on severely degraded land (Petunjuk Teknis Penyusunan Data Spasial Lahan Kritis) set out in Regulation No. P.4/V-SET/2013.These severely degraded lands indicate the degree of land degradation in Indonesia in terms of land cover, slope, potential erosion, land productivity, and land management.The regulation categorizes land degradation as follows: (1) not severe, (2) potentially severe, (3) somewhat severe, (4) severe, and (5) very severe.Of these categories, this study selected the categories of "severe" and "very severe" to identify the initial scope of degraded lands.
Second, conservation area data [59] were used to exclude protected and conserved forests that prohibit production activities on degraded lands.The data were used to identify protected forest (Hutan Lindung) and conservation forest (Hutan Konservasi) defined by the Basic Forestry Law, UU No. 41, 1999.The law defines protected forest as an area that protects life support systems by regulating water cycles, maintaining soil fertility, and preventing floods, erosion, and salt water intrusion.Conservation forest is defined as an area that protects life support systems by preserving biodiversity and utilizing bionatural resources and ecosystems sustainably.
Third, land cover data [60] were utilized to exclude lands that are used for other purposes and not feasible for biomass production, such as crop estates, forests, swamps, paddy fields, mining areas, fish ponds, water bodies, and built-up areas.The data were collected from the Indonesian Ministry of Environment and Forestry.Land cover is classified into 23 classes based on the physiognomy or

Identification of Degraded Lands in Indonesia
The first step of the study involved identifying degraded lands in Indonesia.The analysis employed four types of geographic information system (GIS) data to identify potentially degraded land in Indonesia using an overlaying analysis.These data included severely degraded land data, conservation area data, land cover data, and land system data (Figure 1).Degraded lands were identified by overlaying these spatial data based on inclusion and exclusion criteria as described below.
First, severely degraded land data [58] were used to define the initial scope of degraded lands in Indonesia.The data were developed by the Directorate General of Watershed Management and Social Forestry, under the Ministry of Environment and Forestry of Indonesia, based on technical guidelines for the development of spatial data on severely degraded land (Petunjuk Teknis Penyusunan Data Spasial Lahan Kritis) set out in Regulation No. P.4/V-SET/2013.These severely degraded lands indicate the degree of land degradation in Indonesia in terms of land cover, slope, potential erosion, land productivity, and land management.The regulation categorizes land degradation as follows: (1) not severe, (2) potentially severe, (3) somewhat severe, (4) severe, and (5) very severe.Of these categories, this study selected the categories of "severe" and "very severe" to identify the initial scope of degraded lands.
Second, conservation area data [59] were used to exclude protected and conserved forests that prohibit production activities on degraded lands.The data were used to identify protected forest (Hutan Lindung) and conservation forest (Hutan Konservasi) defined by the Basic Forestry Law, UU No. 41, 1999.The law defines protected forest as an area that protects life support systems by regulating water cycles, maintaining soil fertility, and preventing floods, erosion, and salt water intrusion.Conservation forest is defined as an area that protects life support systems by preserving biodiversity and utilizing bionatural resources and ecosystems sustainably.
Third, land cover data [60] were utilized to exclude lands that are used for other purposes and not feasible for biomass production, such as crop estates, forests, swamps, paddy fields, mining areas, fish ponds, water bodies, and built-up areas.The data were collected from the Indonesian Ministry of Environment and Forestry.Land cover is classified into 23 classes based on the physiognomy or appearance of biophysical cover, which is visually distinguished using the available cloud-free Landsat imagery.Visual classification is carried out by a digitizing on-screen technique using the key elements of image interpretation [61].Fourth, land system data [62] were used to obtain information on slope, pH, rainfall, soil depth, temperature, and altitude of the degraded lands.The data were built by the Regional Physical Planning Programme for Transmigration (RePPProT).Land systems are natural ecosystems in which rocks, climate, hydrology, topography, soils, and organisms are correlated in a specific way [62].In addition, missing data of the systems at a regional level were collected from the Land Resources Department (1989).

Suitability of Degraded Lands to Grow Biofuel Species
The second step of the study involved analyzing the suitability of the degraded lands to grow potential biofuel species and estimating their energy production.Five biofuel species were analyzed: C. calothyrsus, C. inophyllum, G. sepium, P. pinnata, and R. trisperma (Table 3).The study categorized suitable lands as highly and moderately suitable lands by modeling:  H score and M score were the total scores of criteria for highly and moderately suitable lands, respectively.HSs were dummy variables of highly suitable (HS) land criteria for altitude, annual rainfall, temperature, slope, soil pH, and soil depth of the biofuel species, whose values were 1 for land meeting the species criteria and 0 for land not meeting the criteria.Similarly, MSs were dummy variables of moderately suitable (MS) land criteria for altitude, annual rainfall, temperature, slope, soil pH, and soil depth of the biofuel species.Highly and moderately suitable lands were determined only when H score and M score were 6, which indicate that a land meets all six criteria for the growth of biofuel species.Values of all dummy variables (HCs and MCs) were estimated by applying Monte Carlo analysis (e.g., Reference [13]) to the spatial analysis results (Step 1 results in Figure 1) based on Gaussian distribution.The averages of 1000 simulation results were used to estimate the probabilities of each land to meet all growth criteria in Table 3 for all five biofuel species.The total areas of these suitable lands for these species were calculated by multiplying these probabilities and sizes of the lands.
To analyze land sizes and parcel numbers, the degraded lands were categorized into small, medium, and large sizes.Size categories were developed based on the literature on palm oil production [55,68].In palm oil production, smallholder lands are up to 50 ha [68]; this criterion was used to categorize small-sized lands for biofuel species production.For industrial palm oil production, 5000 ha is considered to be the minimum land size [55]; this criterion was used to define large-sized lands for biofuel species production.In this study, therefore, "small-sized lands" were lands smaller than 50 ha, "medium-sized lands" were lands bigger than 50 ha but smaller than 5000 ha, and "large-sized lands" were lands bigger than 5000 ha.After categorizing the lands with their sizes, the numbers of land parcels were estimated for each land size.
To analyze energy productivity from degraded lands suitable for the selected biofuel species, we developed and investigated two scenarios: (1) the all-five-species scenario and (2) the biodiesel-only-species scenario.The all-five-species scenario analyzed all five of the biofuel species, including those for biodiesel production (C.inophyllum, P. pinnata and R. trisperma) and those for biomass production (C.calothyrsus and G. sepium).The scenario estimated potential energy productivity from each species assuming that their biomass or seed yields would be lower on moderately suitable land compared with highly suitable land (Table 4).Later, we chose only one species with the highest energy productivity when multiple species were suitable on the same degraded lands so that energy productivity could be maximized from these lands.The biodiesel-only-species scenario was treated using identical analytical procedures, but it only examined those species intended for biodiesel production. 1 It was assumed that seed yield per tree would be 150 kg on highly suitable land and 40 kg on moderately suitable land and that 133 trees could be planted per hectare (e.g., maximum 20 tons of seed yield per ha = about 133 trees × 150 kg of seeds) following Reference [24].It was also assumed that 65% of seed is oil, and 80% of the oil could be converted to biodiesel [69]; 2 [70]; 3 [44]; 4 [41]; 5 It was assumed that oil yield per hectare would be 2250 kg for highly suitable land and 225 kg for moderately suitable land [71,72] and that 80% of oil could be converted to biodiesel [69]; 6 [45]; 7 [27,52]; 8 Based on 4205 kcal/kg for C. calothyrsus and 4027 kcal/kg for G. sepium [36]; 9 [67].

Degraded Lands in Indonesia
The study results showed about 5.8 Mha of degraded lands in Indonesia that have limited ability to produce food, to sequestrate carbon on land, and to maintain vegetation and biodiversity (Figures 2  and 3).Of them, 72% were categorized as severely degraded lands and 28% as very severely degraded lands.The largest area of degraded lands was located in Sumatra, totaling about 1.8 Mha.The second largest area of lands was in Kalimantan, totaling about 1.5 Mha.The smallest areas of land were in the Java and Bali regions, totaling about 0.1 Mha of degraded lands.

Degraded Lands in Indonesia
The study results showed about 5.8 Mha of degraded lands in Indonesia that have limited ability to produce food, to sequestrate carbon on land, and to maintain vegetation and biodiversity (Figures 2 and 3).Of them, 72% were categorized as severely degraded lands and 28% as very severely degraded lands.The largest area of degraded lands was located in Sumatra, totaling about 1.8 Mha.The second largest area of lands was in Kalimantan, totaling about 1.5 Mha.The smallest areas of land were in the Java and Bali regions, totaling about 0.1 Mha of degraded lands.moderately suitable land [71,72] and that 80% of oil could be converted to biodiesel [69]; 6 [45]; 7 [27,52]; 8 Based on 4205 kcal/kg for C. calothyrsus and 4027 kcal/kg for G. sepium [36]; 9 [67].

Degraded Lands in Indonesia
The study results showed about 5.8 Mha of degraded lands in Indonesia that have limited ability to produce food, to sequestrate carbon on land, and to maintain vegetation and biodiversity (Figures 2 and 3).Of them, 72% were categorized as severely degraded lands and 28% as very severely degraded lands.The largest area of degraded lands was located in Sumatra, totaling about 1.8 Mha.The second largest area of lands was in Kalimantan, totaling about 1.5 Mha.The smallest areas of land were in the Java and Bali regions, totaling about 0.1 Mha of degraded lands.

Suitability of Degraded Lands to Grow Biofuel Species
Of the degraded lands identified, about 3.5 Mha (or 57%) had the potential to grow at least one of the five biofuel species (Figure 3).The distribution of suitable lands was slightly different from the distribution of degraded lands in general.For instance, Maluku and Nusa Tenggara had a larger suitable land area than Kalimantan.Of these degraded lands, 2.85 Mha were suitable for C. calothyrsus, 1.64 Mha for G. sepium, 0.21 Mha for R. trisperma, 0.14 Mha for P. pinnata, and 0.05 Mha for C. inophyllum (Figures 4 and 5).For many of these species, the area of highly suitable lands was significantly smaller than that of moderately suitable lands.Moreover, the lands suitable for biomass species (C.calothyrsus and G. sepium) were approximately 11 times larger (or 4.49 Mha) than the lands suitable for biodiesel species (C.inophyllum, P. pinnata, and R. trisperma) (or 0.4 Mha).
Figure 3. Distribution of degraded lands and lands suitable for growing biofuel species (* Nusa Tenggara).Note: (a) degraded lands in Indonesia identified as having limited functions for food production, carbon storage, and conservation of biodiversity and native vegetation; and (b) degraded lands identified as suitable for cultivating at least one of the following: C. calothyrsus, G. sepium, C. inophyllum, P. pinnata, and R. trisperma.

Suitability of Degraded Lands to Grow Biofuel Species
Of the degraded lands identified, about 3.5 Mha (or 57%) had the potential to grow at least one of the five biofuel species (Figure 3).The distribution of suitable lands was slightly different from the distribution of degraded lands in general.For instance, Maluku and Nusa Tenggara had a larger suitable land area than Kalimantan.Of these degraded lands, 2.85 Mha were suitable for C. calothyrsus, 1.64 Mha for G. sepium, 0.21 Mha for R. trisperma, 0.14 Mha for P. pinnata, and 0.05 Mha for C. inophyllum (Figures 4 and 5).For many of these species, the area of highly suitable lands was significantly smaller than that of moderately suitable lands.Moreover, the lands suitable for biomass species (C.calothyrsus and G. sepium) were approximately 11 times larger (or 4.49 Mha) than the lands suitable for biodiesel species (C.inophyllum, P. pinnata, and R. trisperma) (or 0.4 Mha).The degraded lands were analyzed in terms of their sizes and numbers of parcels (Figure 6).Small-sized lands (less than 50 ha) consisted of 81.2% of the total number of land parcels, but their areas were only 8.4% of the lands.Medium-sized lands (between 50 and 5000 ha) represented 18.7% of the total number of parcels, but their total area comprised 69.8% of the lands.Large-sized lands (larger than 5000 ha) consisted of only 0.1% of the total number of land parcels, but their total area The degraded lands were analyzed in terms of their sizes and numbers of parcels (Figure 6).Small-sized lands (less than 50 ha) consisted of 81.2% of the total number of land parcels, but their areas were only 8.4% of the lands.Medium-sized lands (between 50 and 5000 ha) represented 18.7% of the total number of parcels, but their total area comprised 69.8% of the lands.Large-sized lands (larger than 5000 ha) consisted of only 0.1% of the total number of land parcels, but their total area represented 21.8% of the degraded lands.

Hypothetical Maximum Energy Productivity
The all-five-species scenario, which assessed all the biofuel species, resulted in the identification of suitable lands for C. calothyrsus, P. pinnata, R. trisperma, G. sepium, and C. inophyllum (Figure 7).Of the species assessed, C. calothyrsus was the one that could grow well on the largest area of suitable lands (2.8 Mha) because it was not only the most suitable for the degraded lands (Figure 4) but also had the highest potential energy productivity compared with the other species (Table 4).Suitable lands for this species were largely located in Sumatra (0.93 Mha), and the smallest areas were identified in Java and Bali (0.07 Mha).Suitable lands identified for the other species in this scenario were smaller in area: G. sepium had 430,002 ha; P. pinnata had 30,559 ha; R. trisperma had 21,013 ha; and C. inophyllum only had 132 ha.This scenario resulted in about 1.105 EJ year −1 of hypothetical maximum energy productivity (Table 5).The energy productivity from biomass was about 1.102 EJ year −1 (99%), while that from biodiesel was only about 0.003 EJ year −1 .

Hypothetical Maximum Energy Productivity
The all-five-species scenario, which assessed all the biofuel species, resulted in the identification of suitable lands for C. calothyrsus, P. pinnata, R. trisperma, G. sepium, and C. inophyllum (Figure 7).Of the species assessed, C. calothyrsus was the one that could grow well on the largest area of suitable lands (2.8 Mha) because it was not only the most suitable for the degraded lands (Figure 4) but also had the highest potential energy productivity compared with the other species (Table 4).Suitable lands for this species were largely located in Sumatra (0.93 Mha), and the smallest areas were identified in Java and Bali (0.07 Mha).Suitable lands identified for the other species in this scenario were smaller in area: G. sepium had 430,002 ha; P. pinnata had 30,559 ha; R. trisperma had 21,013 ha; and C. inophyllum only had 132 ha.This scenario resulted in about 1.105 EJ year −1 of hypothetical maximum energy productivity (Table 5).The energy productivity from biomass was about 1.102 EJ year −1 (99%), while that from biodiesel was only about 0.003 EJ year −1 .
The biodiesel-only-species scenario, which assessed biodiesel species only, resulted in the identification of suitable lands for R. trisperma, C. inophyllum, and P. pinnata (Figure 7).Of them, R. trisperma was the species with the biggest potential, with about 0.18 Mha of suitable lands.These lands were distributed across several regions in Indonesia, but no suitable land was found in Kalimantan.In this scenario, C. inophyllum and P. pinnata had only 40,625 ha and 30,739 ha of suitable lands, respectively.The scenario resulted in about 0.01 EJ year −1 of hypothetical maximum energy productivity (Table 5).The biodiesel-only-species scenario, which assessed biodiesel species only, resulted in the identification of suitable lands for R. trisperma, C. inophyllum, and P. pinnata (Figure 7).Of them, R. trisperma was the species with the biggest potential, with about 0.18 Mha of suitable lands.These lands were distributed across several regions in Indonesia, but no suitable land was found in

Discussion
The study results showed that degraded lands in Indonesia might support bioenergy production by growing biodiesel species (C.inophyllum, P. pinnata and R. trisperma) and biomass species (C.calothyrsus and G. sepium).We found that Indonesia potentially had about 3.5 Mha of severely degraded lands that not only have limited functions for food production, carbon storage, and conservation of biodiversity and native vegetation but might also support the growth of these biofuel species.These degraded lands were smaller in area than other types of degraded lands identified in Indonesia, such as lands with degraded soils (about 30 Mha) [57], unproductive degraded forest lands (about 27 Mha) [54], degraded lands including forest covers with low carbon stocks (about 24 Mha) [5], degraded lands administratively available for biofuel production (about 21 Mha) [9], degraded lands for palm oil production in West and Central Kalimantan (about 7 Mha) [55], and degraded lands that have poor climate, poor physical characteristics, or degraded lands on which cultivation is difficult (about 3.7 Mha) [56].Moreover, the estimated biomass energy productivity from C. calothyrsus and G. sepium (about 1.1 EJ year −1 ) in our study was smaller than the expected biomass energy from woody crops (about 5 EJ year −1 ) and grasses (about 7 EJ year −1 ) from degraded soil lands in Indonesia [57].These differences stem from different definitions, different methods, different spatial data, and different species analyses among the studies.
Although the two-scenarios analyses showed their potential support for increasing the supply of biodiesel in Indonesia, there are still challenges to apply these scenarios in achieving the biodiesel consumption target of 30% of total energy consumption by 2025 (Presidential Regulation No. 12/2015) in Indonesia.First, these lands might be limited in their ability to support economies of scale for biofuel production and might only reflect a hypothetical maximum land area.The sizes of many degraded lands were smaller than 5000 ha, which is considered the minimum land size on which economies of scale from palm oil production can be achieved [55].Although palm oil is not solely used for biofuel production, lessons from palm oil production would support growth of other biofuel species, since palm oil has been used as a dominant biofuel species in Indonesia [9].Thus, the sizes of these degraded lands must be considered in analyzing their potential business models for bioenergy production in Indonesia.
In addition, the study results indicate the maximum energy productivity as the study assumed that all degraded lands would be utilized for biofuel production by growing the five biofuel species.In reality, however, this bioenergy production would be discounted by many socioeconomic factors, such as the cost-benefit of the production to farmers and refineries [5,13,15], higher opportunity costs for bioenergy production compared with palm oil production [55], competition with low-price energy such as gasoline [5], and conflicted stakeholder interests [54].Furthermore, this energy would be reduced further when it is converted into other types of energy, such as electricity, for final consumption.These factors are likely to reduce the biofuel production estimates from the study; therefore, these factors must be analyzed further to understand how many of these degraded lands might in reality support bioenergy production in Indonesia.
Despite these challenges, the study findings might still contribute to the analysis of potential biofuel species in Indonesia and the investigation of carbon sequestration from bioenergy production from degraded lands.First, the study results support the identification and comparison of potential species for biodiesel production (e.g., C. inophyllum, P. pinnata, and R. trisperma) and biomass production (e.g., C. calothyrsus and G. sepium).For instance, C. calothyrsus was more suitable on degraded lands compared with the other biofuel species in this study.Among the biodiesel species, R. trisperma had better suitability than P. pinnata and C. inophyllum.The identified lands suitable for these species would serve as input data for analyzing their potential growth on degraded lands by applying more sophisticated plant growth modeling (e.g., Reference [13]).Moreover, suitable lands for these five species can be reference data for studies on other biofuel species in Indonesia, such as sugarcane, cassava, sweet sorghum, corn, and sago [5].
Second, the study findings might support the investigation of potential carbon sequestration from bioenergy production from degraded lands.The locations and sizes of the suitable lands support the modeling of carbon sequestration by the five species from aboveground and belowground on degraded lands.Such carbon modeling would support the analysis of net-positive carbon sequestration of the bioenergy production from degraded lands.Other required studies that the study findings might support are analyses of the following: conventionally consumed biomass species in Indonesia that might be replaced with biomass production from C. calothyrsus and G. sepium; increased carbon stocks by avoiding the harvests of replaced biomass species; impacts of climate changes on the growth of the five biofuel species on degraded lands and their capacities to store carbon stocks; saved carbon stocks from other uses of biomass from degraded lands; carbon emissions from diesel blended with biodiesel from C. inophyllum, P. pinnata and R. trisperma; and impacts of forest restoration of degraded lands on national carbon emissions in Indonesia as well as a scheme of reducing emissions from deforestation and forest degradation (REDD+) in Indonesia (e.g., Reference [5]).

Conclusions
The study identified 3.5 Mha of degraded lands in Indonesia that might not only avoid compromising food production, carbon storage, biodiversity, and native vegetation but might also support bioenergy production by growing biodiesel species (C.inophyllum, P. pinnata, and R. trisperma) and biomass species (C.calothyrsus and G. sepium).The study results revealed both opportunities and challenges for bioenergy production from these degraded lands in Indonesia.The two-scenario analyses showed that maximum biomass energy production from C. calothyrsus and G. sepium might support the increase of biofuel supply in Indonesia.However, the sizes of degraded lands were too small to support economies of scale for biofuel production, and the study results would be discounted by many socioeconomic factors in reality.The study findings support future studies on modeling the growth of biofuel species on degraded lands, comparing diverse potential biofuel species, and modeling carbon sequestration from restoration of degraded lands.Moreover, to better understand the potential of these degraded lands to produce bioenergy, further studies would be required on biofuel species in terms of their feedstock productions in various environmental conditions as well as their potential to produce different types of biofuel (e.g., G. sepium for biodiesel production).

Figure 1 .
Figure 1.Research methods used to estimate degraded lands in Indonesia and their suitability to grow biofuel species.

Figure 1 .
Figure 1.Research methods used to estimate degraded lands in Indonesia and their suitability to grow biofuel species.

Figure 2 .
Figure 2. Spatial distribution of degraded lands in Indonesia that have limited functions for food production, carbon storage, and conservation of biodiversity and native vegetation.

Figure 2 .
Figure 2. Spatial distribution of degraded lands in Indonesia that have limited functions for food production, carbon storage, and conservation of biodiversity and native vegetation.

Figure 2 .
Figure 2. Spatial distribution of degraded lands in Indonesia that have limited functions for food production, carbon storage, and conservation of biodiversity and native vegetation.

Figure 3 .
Figure 3. Distribution of degraded lands and lands suitable for growing biofuel species (* Nusa Tenggara).Note: (a) degraded lands in Indonesia identified as having limited functions for food production, carbon storage, and conservation of biodiversity and native vegetation; and (b) degraded lands identified as suitable for cultivating at least one of the following: C. calothyrsus, G. sepium, C. inophyllum, P. pinnata, and R. trisperma.

Figure 4 .
Figure 4. Total area of degraded lands in Indonesia identified as suitable for growing individual biofuel species.

Figure 4 . 18 Figure 5 .
Figure 4. Total area of degraded lands in Indonesia identified as suitable for growing individual biofuel species.Sustainability 2018, 10, x FOR PEER REVIEW 10 of 18

Sustainability 2018 , 18 Figure 6 .
Figure 6.Total areas and number of land parcels for small, medium, and large-sized degraded lands in Indonesia suitable for at least one of the biofuel or biomass species (C.calothyrsus, G. sepium, C. inophyllum, P. pinnata, and R. trisperma).

Figure 6 .
Figure 6.Total areas and number of land parcels for small, medium, and large-sized degraded lands in Indonesia suitable for at least one of the biofuel or biomass species (C.calothyrsus, G. sepium, C. inophyllum, P. pinnata, and R. trisperma).

Figure 7 .
Figure 7.Total area of degraded lands in Indonesia identified as suitable for the all-five-species and biodiesel-only-species scenarios (* Nusa Tenggara).

Figure 7 .
Figure 7.Total area of degraded lands in Indonesia identified as suitable for the all-five-species and biodiesel-only-species scenarios (* Nusa Tenggara).

Table 1 .
Potential biofuel species in Indonesia.

Table 2 .
Studies on potential lands for biofuel production in Indonesia.
HS altitude + HS rainfall + HS tempt + HS soil slope + HS soil pH + HS soil depth M score = MS altitude + MS rainfall + MS tempt + MS soil slope + MS soil pH + MS soil depth (1)

Table 3 .
Criteria for highly and moderately suitable lands.

Table 4 .
Energy productivity of five potential biofuel species in Indonesia.

Table 5 .
Potential energy production (TJ year −1 ) of selected biofuel species from degraded lands in Indonesia.

Table 5 .
Potential energy production (TJ year −1 ) of selected biofuel species from degraded lands in Indonesia.