Assessment of Suitability of Tree Species for Bioenergy Production on Burned and Degraded Peatlands in Central Kalimantan , Indonesia

Large areas of deforested and degraded land, particularly degraded peatlands, need a viable long-term solution for restoration, ideally one that ensures energy security without compromising food security or biodiversity conversation. To address a knowledge gap on the most adaptive bioenergy crop(s) for degraded lands, this research project assessed the survival and growth performance of potential bioenergy crops to restore burned and degraded peatlands. Our methodology compared the bioenergy species with the potential to survive in extreme environments, i.e., gamal [Gliricidia sepium (Jacq.) Walp.], kaliandra (Calliandra calothyrsus Meissner), kemiri sunan [Reutealis trisperma (Blanco) Airy Shaw], and nyamplung (Calophyllum inophyllum L.). Observed parameters are plant survival rates, tree height, and circular stem growth. The experiment was conducted between March 2016 to February 2017 in a two-hectare demonstration plot on burned and degraded peatland in Buntoi village, Pulang Pisau, Central Kalimantan province. Using a split plot design, two treatments were given to each species, i.e., monoculture plantation and agroforestry (intercropped with Ananas comosus (L.) Merr.); with each treatment, the species were replicated on two separate plots. Results indicate that nyamplung is the most adoptable species followed by kemiri sunan, however both species performed very well under agroforestry treatment when compared with monoculture. Further study is needed to assess the productivity and associate biofuel yield.


Introduction
Indonesian energy demand has significantly increased, primarily due to population growth, urbanization, and economic development [1].At the same time, sources of fossil fuel have depleted and they are unable to fulfill the increasing energy demands of the future [2].Whilst responding to interests in renewable energy and degraded land restoration, bioenergy can provide a potential alternative to meet growing energy demands.The Indonesian government has mandated for increases in renewable energy production, including bioenergy from plant sources (e.g., Calophyllum inophyllum L., Elaeis guineensis Jack.), with the aim of it meeting 23% of total energy use by 2025 [3].However, such expansion of plantations for energy production could trigger competition with other land uses, such as food production and biodiversity conservation 1 .To avoid such competition, and to diversify bioenergy production, degraded and underutilized land has been identified as a potential target area for bioenergy production [4][5][6][7].
Central Kalimantan province has one of the largest amounts of degraded land in Indonesia, estimated at approximately 7.2 million hectares (ha) [8].Forest conversion to other types of land use, e.g., agriculture and open mining, is one of the key driving factors land degradation [9][10][11].The frequent occurrence of forest fires, particularly in recent years, has driven an escalation in degraded land, including peatland [11,12].The occurrence of fire has also affected agricultural land that is managed by local farmers and declined its productivity.Most of the burned land, including peatland, has been abandoned due to its declining fertility [13].The Central Kalimantan province is also facing energy deficits, with large number of households (42%) in the province having no access to electricity [14].Consumption of traditional biomass for cooking purposes is also relatively high [15].To increase community access to energy, the central government, through the Ministry of Energy and Mineral Resources (ESDM) in collaboration with district and provincial governments, initiated a bioenergy program, called Bioenergi Lestari.The program aims to establish bioenergy plantations on approximately 62,500 ha of degraded and abandoned lands in two districts, i.e., Pulang Pisau and Katingan, with the expectation of increasing bioenergy production [16].However, the progress so far is slow due to very few studies providing useful information on bioenergy crops that are suitable for growing on degraded lands, particularly in Central Kalimantan.To fill this scientific knowledge gap, this research project aimed to identify the most promising bioenergy crop(s) for degraded lands.

Materials and Methods
This study was conducted in Buntoi village (located between 102 • 48 59.4 S and 114 • 10 47.3 E) in the district of Pulang Pisau, Central Kalimantan, Indonesia (Figure 1).Buntoi, with a total land area of 16,261.595ha, is dominated by forest and agricultural land (Figure 2).The soil domination is mainly peat and alluvial.Buntoi has a tropical and humid climate with a temperature ranging from 26.5 to 27.5 • C. It has two distinct seasons, dry (April-October) and rainy (November-March).The village was selected as one of the locations for bioenergy crop plantation initiated by the Ministry of Energy and Mineral Resources (ESDM) and the local government, under the Bioenergi Lestari project.
The total population of Buntoi is 2729; this population is mainly dependent on rubber plantation and other subsistence agriculture [17].In late 2015, Buntoi village was affected by forest and peatland fires that destroyed large areas of farmers' productive land, including approximately 461 ha of rubber plantation.The burned land has since been abandoned, and the farmers are now looking for alternative land uses to meet their livelihood needs.
As an example, oil palm has been the main source of bioenergy in Indonesia, however, expansion of oil palm production has raised concerns about compromising food production and destroying forests and consequent biodiversity [18][19][20].The experiment was carried out between March 2016 and February 2017 on two hectares of degraded peatland.Having a total of 16 sub plots, a split plot design was applied to test the performance of four biofuel crop species with two different treatments, i.e., under monoculture and agroforestry conditions; with agroforestry conditions involving intercropping with pineapple.
3 Gamal and kaliandra are native species to Central America, while kemiri sunan is known native to the Philippines, and nyamplung to Indonesia [32].These species require mean annual temperatures 18 to 33 • C with rainfall ranging 60 to 5000 mm, to grow (see Table A1 in Appendix A).The tested species were naturally distributed, and cultivated in Indonesia [32,33].However, for these tested species, we did not find any literature that can explain commercial scale cultivation in the peatlands.Gamal and kaliandra are utilized to produce energy from its woody biomass [33,34].Meanwhile, nyamplung and kemiri sunan are utilized for its seeds to be converted to biofuel [35,36].Observed parameters in our study included plant height (in cm) and circular stem growth (in mm), measured from 10 cm above the ground.Survival rate was also observed by counting the total number of survived saplings in each plot.Data was recorded every month using above parameters.
As the research site is a fire prone area, for the safety of the experimental plot, we used a six-meter firebreak from natural vegetation, and four-meter firebreaks from rubber trees and the road.We also used a six-meter break between different treatments (Figure 3).In terms of plant spacing, species were spaced as follows: kaliandra and gamal (2 m × 1 m), kemiri sunan and nyamplung (8 m × 8 m), and pineapple (Ananas comosus) (1 m × 1 m).Fertilizer, i.e., NPK using slow release method, was used in all of the plots.
The peatland depth profile and pH value were also measured from four sample locations of our study plots by measuring their distance from the river, i.e., two samples 50 m from river and two samples 200 m from the river.
Besides descriptive statistics, a nonparametric test, i.e., kruskal-wallis and post-hoc test results of a wilcoxon rank sum test in R software (version 3.4.4),were used to analyze the data.

Results and Discussion
Peatland depth and pH in the study plot ranged from 56 cm to 87 cm and 2.88 to 3.19, respectively (Table 2); which, showing that with the medium acidity level peatland depth is relatively higher when in proximity to the river.The survival rate of the bioenergy crops is shown in Figure 4. Results indicate that nyamplung and kemiri sunan are adaptable to degraded peatland, with respective survival rates of 88% and 48%.However, kaliandra and gamal did not survive in our experimental plot.Therefore, nyamplung and kemiri sunan are useful for planting in burned and degraded peatlands.Figures 5 and 6 show the growth rate on degraded peatland of nyamplung and kemiri sunan, the two adaptable trialed species.For nyamplung, the growth rate is steady in all conditions, except agroforestry (plot B) conditions where the growth rate from month 5 to 6 is comparatively high, and after that becomes steady again.For kemiri sunan, under all conditions, the growth rate remains steady, except monoculture (plot B) where growth rate during the first and last month is comparatively high.Higher growth rates in a specific month for both species, as mentioned above, might be due to external inputs, i.e., fertilizer application and weather conditions, e.g., rainfall or sunlight.The figures also indicate that with intercropping, both species see better growth than under monoculture.However, further investigation is needed to examine the external factors that affected growth.Our data also illustrated that the circular stem growth of nyamplung and kemiri sunan steadily increased both under intercropping and monoculture systems (Figures 7 and 8).Figures 5 and 6 show the growth rate on degraded peatland of nyamplung and kemiri sunan, the two adaptable trialed species.For nyamplung, the growth rate is steady in all conditions, except agroforestry (plot B) conditions where the growth rate from month 5 to 6 is comparatively high, and after that becomes steady again.For kemiri sunan, under all conditions, the growth rate remains steady, except monoculture (plot B) where growth rate during the first and last month is comparatively high.Higher growth rates in a specific month for both species, as mentioned above, might be due to external inputs, i.e., fertilizer application and weather conditions, e.g., rainfall or sunlight.The figures also indicate that with intercropping, both species see better growth than under monoculture.However, further investigation is needed to examine the external factors that affected growth.Our data also illustrated that the circular stem growth of nyamplung and kemiri sunan steadily increased both under intercropping and monoculture systems (Figures 7 and 8).Figures 5 and 6 show the growth rate on degraded peatland of nyamplung and kemiri sunan, the two adaptable trialed species.For nyamplung, the growth rate is steady in all conditions, except agroforestry (plot B) conditions where the growth rate from month 5 to 6 is comparatively high, and after that becomes steady again.For kemiri sunan, under all conditions, the growth rate remains steady, except monoculture (plot B) where growth rate during the first and last month is comparatively high.Higher growth rates in a specific month for both species, as mentioned above, might be due to external inputs, i.e., fertilizer application and weather conditions, e.g., rainfall or sunlight.The figures also indicate that with intercropping, both species see better growth than under monoculture.However, further investigation is needed to examine the external factors that affected growth.Our data also illustrated that the circular stem growth of nyamplung and kemiri sunan steadily increased both under intercropping and monoculture systems (Figures 7 and 8).Our wilcoxon rank sum test further shows that nyamplung performs better for both tree height and circular stem growth as compared to kemiri sunan (Figures 9 and 10).Looking at the two different treatments (i.e., agroforestry and monoculture), both species performed better for tree height growth under agroforestry (Figure 11), however, only nyamplung performed well for circular stem growth under agroforestry (Figure 12).Our wilcoxon rank sum test further shows that nyamplung performs better for both tree height and circular stem growth as compared to kemiri sunan (Figures 9 and 10).Looking at the two different treatments (i.e., agroforestry and monoculture), both species performed better for tree height growth under agroforestry (Figure 11), however, only nyamplung performed well for circular stem growth under agroforestry (Figure 12).Our wilcoxon rank sum test further shows that nyamplung performs better for both tree height and circular stem growth as compared to kemiri sunan (Figures 9 and 10).Looking at the two different treatments (i.e., agroforestry and monoculture), both species performed better for tree height growth under agroforestry (Figure 11), however, only nyamplung performed well for circular stem growth under agroforestry (Figure 12).Our research shows that out of four tested species, nyamplung is the most adoptable species, followed by kemiri sunan, when grown on burned and degraded peatland in Central Kalimantan.However, both species performed very well under agroforestry treatment when compared to monoculture.This is a win-win solution, as growing biofuel using an agroforestry system can be a better land use strategy, considering its potential to enhance farm production and income, protect biodiversity, and support sustainable development [37,[41][42][43][44][45].If the target is also to motivate local farmers to use their degraded land for biofuel production, it is important to consider that tree growing by farmers is often associated with multiple objectives influenced by livelihood necessities and local cultures [46][47][48][49].Current literature emphasizes that farmers' capacity to adopt tree planting is also dependent on production technology, adequate physical infrastructure, and developed markets for tree products [47,49,50].Improved understanding of these circumstances is crucial for policy improvements to succeed in making tree planting feasible, acceptable, and ultimately profitable for local people and related stockholders [51].
Planting millions of square miles of biofuel could store between 1.2 and 6.3 billion tons of carbon/year; enough to make a very large dent in global greenhouse gas emissions [52], while also providing sufficient energy stock [53].However, there is a risk that doing so could lead to forest clearance, compete with agricultural production and put additional pressure on biodiversity [52].As a solution, producing biofuel on degraded land can avoid compromising agricultural production and the related negative environmental consequences.

Conclusions
This study demonstrated that among four trial species, nyamplung is the most adaptive (88%) bioenergy species for growth on degraded peatland in Central Kalimantan, followed by kemiri sunan (48%).Growth performance indicators showed that nyamplung grew better in agroforestry sub-plots when compared to monoculture sub-plots both in terms of height and circular stem growth; likewise, kemiri sunan performed better in terms of height growth in agroforestry sub-plots.Our research shows that out of four tested species, nyamplung is the most adoptable species, followed by kemiri sunan, when grown on burned and degraded peatland in Central Kalimantan.However, both species performed very well under agroforestry treatment when compared to monoculture.This is a win-win solution, as growing biofuel using an agroforestry system can be a better land use strategy, considering its potential to enhance farm production and income, protect biodiversity, and support sustainable development [37,[41][42][43][44][45].If the target is also to motivate local farmers to use their degraded land for biofuel production, it is important to consider that tree growing by farmers is often associated with multiple objectives influenced by livelihood necessities and local cultures [46][47][48][49].Current literature emphasizes that farmers' capacity to adopt tree planting is also dependent on production technology, adequate physical infrastructure, and developed markets for tree products [47,49,50].Improved understanding of these circumstances is crucial for policy improvements to succeed in making tree planting feasible, acceptable, and ultimately profitable for local people and related stockholders [51].
Planting millions of square miles of biofuel could store between 1.2 and 6.3 billion tons of carbon/year; enough to make a very large dent in global greenhouse gas emissions [52], while also providing sufficient energy stock [53].However, there is a risk that doing so could lead to forest clearance, compete with agricultural production and put additional pressure on biodiversity [52].As a solution, producing biofuel on degraded land can avoid compromising agricultural production and the related negative environmental consequences.

Conclusions
This study demonstrated that among four trial species, nyamplung is the most adaptive (88%) bioenergy species for growth on degraded peatland in Central Kalimantan, followed by kemiri sunan (48%).Growth performance indicators showed that nyamplung grew better in agroforestry sub-plots when compared to monoculture sub-plots both in terms of height and circular stem growth; likewise, kemiri sunan performed better in terms of height growth in agroforestry sub-plots.This awareness of nyamplung and kemiri sunan's survivability in degraded peatland, as well as their improved performance using agroforestry, can promote the benefits of agroforestry and enhance farmers' livelihoods, as well as supporting sustainable development.However, further study on the productivity and associate biofuel yields of both species is needed.Further studies are also needed for these four selected, and different trial species on different peat and degraded land areas, including more accurate extended measurement variables, e.g., soil nutrients, peat water table, and peat depth, with more controlled environment.That may help to get additional data, such as in our study it was not clear why there are significant differences between the same species and the same treatment (e.g., Figure 9).Selecting tree species with multiple benefits in terms of livelihoods, local culture familiarity, and strong market value, might be beneficial to improve farmers' motivation to utilize degraded land for biofuel production.

Figure 2 .Figure 1 .
Figure 2. Land uses in Buntoi village [16].The experiment was carried out between March 2016 and February 2017 on two hectares of degraded peatland.Having a total of 16 sub plots, a split plot design was applied to test the performance of four biofuel crop species with two different treatments, i.e., under monoculture and

Figure 3 .
Figure 3. Split plot design for the treatment of four biofuel species in Buntoi research site in Pulang Pisau, Central Kalimantan.

Figure 4 .
Figure 4. Survival rate of the four selected bioenergy species in Buntoi research site in Pulang Pisau, Central Kalimantan.

Figure 5 .
Figure 5. Height growth of nyamplung in monoculture vs. agroforestry plots in Buntoi research site in Central Kalimantan.

Figure 4 .
Figure 4. Survival rate of the four selected bioenergy species in Buntoi research site in Pulang Pisau, Central Kalimantan.

Figure 5 .
Figure 5. Height growth of nyamplung in monoculture vs. agroforestry plots in Buntoi research site in Central Kalimantan.

Figure 5 .
Figure 5. Height growth of nyamplung in monoculture vs. agroforestry plots in Buntoi research site in Central Kalimantan.

Figure 6 .
Figure 6.Height growth of kemiri sunan in monoculture vs. agroforestry plots in Buntoi reseaech site in Central Kalimantan.

Figure 6 .
Figure 6.Height growth of kemiri sunan in monoculture vs. agroforestry plots in Buntoi reseaech site in Central Kalimantan.

Figure 8 .
Figure 8. Circular stem growth of kemiri sunan in monoculture vs. agroforestry plots in Buntoi reseaech site in Central Kalimantan.

Figure 10 .
Figure 10.Results of Wilcoxon rank sum test on tree diameter for nyamplung and kemiri sunan (BAK-agroforestry kemiri sunan plot B; BAN-agroforestry nyamplung plot B; BMK-monoculture kemiri sunan plot B; BMN-monoculture nyamplung plot B; FAK-agroforestry kemiri sunan plot A; FAN-agroforestry nyamplung plot A; FMK-monoculture kemiri sunan plot A; and, FMN-monoculture nyamplung plot A).The letters a, b, and c on the figure show different performance levels of diameter.

Figure 11 .
Figure 11.Results of Wilcoxon rank sum test on tree height under two different treatments, i.e., agroforestry and monoculture, for nyamplung and kemiri sunan (AN-agroforestry nyamplung; KA-agroforestry kemiri sunan; KM-monoculture kemiri sunan; and, MN-monoculture nyamplung).The letters a and b on the figure show the different performance levels of height.

Figure 10 .
Figure 10.Results of Wilcoxon rank sum test on tree diameter for nyamplung and kemiri sunan (BAK-agroforestry kemiri sunan plot B; BAN-agroforestry nyamplung plot B; BMK-monoculture kemiri sunan plot B; BMN-monoculture nyamplung plot B; FAK-agroforestry kemiri sunan plot A; FAN-agroforestry nyamplung plot A; FMK-monoculture kemiri sunan plot A; and, FMN-monoculture nyamplung plot A).The letters a, b, and c on the figure show different performance levels of diameter.

Figure 10 .
Figure 10.Results of Wilcoxon rank sum test on tree diameter for nyamplung and kemiri sunan (BAK-agroforestry kemiri sunan plot B; BAN-agroforestry nyamplung plot B; BMK-monoculture kemiri sunan plot B; BMN-monoculture nyamplung plot B; FAK-agroforestry kemiri sunan plot A; FAN-agroforestry nyamplung plot A; FMK-monoculture kemiri sunan plot A; and, FMN-monoculture nyamplung plot A).The letters a, b, and c on the figure show different performance levels of diameter.

Figure 11 .
Figure 11.Results of Wilcoxon rank sum test on tree height under two different treatments, i.e., agroforestry and monoculture, for nyamplung and kemiri sunan (AN-agroforestry nyamplung; KA-agroforestry kemiri sunan; KM-monoculture kemiri sunan; and, MN-monoculture nyamplung).The letters a and b on the figure show the different performance levels of height.

Figure 11 .
Figure 11.Results of Wilcoxon rank sum test on tree height under two different treatments, i.e., agroforestry and monoculture, for nyamplung and kemiri sunan (AN-agroforestry nyamplung; KA-agroforestry kemiri sunan; KM-monoculture kemiri sunan; and, MN-monoculture nyamplung).The letters a and b on the figure show the different performance levels of height.

Figure 12 .
Figure 12. Results of Wilcoxon rank sum test on tree diameter under two different treatments, i.e., agroforestry and monoculture, for nyamplung and kemiri sunan (AN-agroforestry nyamplung; KA-agroforestry kemiri sunan; KM-monoculture kemiri sunan; and, MN-monoculture nyamplung).The letters a and b on the figure show different performance levels of diameter.

Figure 12 .
Figure 12. Results of Wilcoxon rank sum test on tree diameter under two different treatments, i.e., agroforestry and monoculture, for nyamplung and kemiri sunan (AN-agroforestry nyamplung; KA-agroforestry kemiri sunan; KM-monoculture kemiri sunan; and, MN-monoculture nyamplung).The letters a and b on the figure show different performance levels of diameter.

Table 1 .
Review of adaptability of selected bioenergy crops to different type of soils.

Table 2 .
Peatland depth profile and pH value of the study plots in Buntoi research site in Pulang Pisau, Central Kalimantan.