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Open AccessShort Note

Screening Potential Bioenergy Production of Tree Species in Degraded and Marginal Land in the Tropics

1
Center for International Forestry Research, Jalan CIFOR, Situ Gede, Sindang Barang, Bogor 16115, Indonesia
2
Natural Resources Institute Finland (Luke), Plant Production, 00790 Helsinki, Finland
3
Ruhr-University Bochum, Institute of Geography, Soil Science/Soil Ecology, Universitätsstrasse 150, 44801 Bochum, Germany
4
Institute of Plant Production and Agroecology in the Tropics and Subtropics, University of Hohenheim, 70593 Stuttgart, Germany
5
National Institute of Forest Science, 57 Heogi-ro, Dongdaemu-gu, Seoul 02455, Korea
*
Author to whom correspondence should be addressed.
Forests 2018, 9(10), 594; https://doi.org/10.3390/f9100594
Received: 2 August 2018 / Revised: 15 September 2018 / Accepted: 18 September 2018 / Published: 23 September 2018
(This article belongs to the Section Forest Ecology and Management)

Abstract

Bioenergy can produce at least 25% of the global energy demand to combat climate change through reducing emissions in the energy sector. However, information on the bioenergy production potential of woody species and their suitability for silviculture on various soils in the humid tropics is limited. This review aims to identify tree species suitable for bioenergy production under these conditions. Data were compiled from 241 publications and nine freely available databases to assess environmental and silvicultural information on tropical tree species. Energy outputs were derived from the estimated productivity of the reviewed species and ranged from 0.2 to 24.0 Mg biomass ha−1 yr−1, 0.1 to 9.0 Mg bio-oil ha−1 yr−1, and 0.2 to 20.0 Mg sugar ha−1 yr−1, equivalent to an energy yield between 2 and 444 GJ ha−1 yr−1. As such, these bioenergy yields are within the range reported for the lignocellulosic biomass of energy crops cultivated in Europe, the USA, and Brazil. Our review identified some high-yielding species (e.g., Dyera polyphylla (Miq.) Steenis, Metroxylon sagu (Rottb.), Pongamia pinnata (L.)) and leguminous species that could be beneficial in mixed stands (e.g., Elaeis oleifera (Kunth) and Pongamia pinnata) or are suitable species to grow on wet or re-wetted peatland (Dyera polyphylla). However, there are limitations to cultivate woody bioenergy species on wet peatland. Sustainable methods for managing and harvesting forests, particularly on wet or re-wetted peatland, need to be developed.
Keywords: tropics; paludiculture; biomass; biofuel; biodiesel; bioethanol tropics; paludiculture; biomass; biofuel; biodiesel; bioethanol

1. Introduction

It is predicted that global energy demand will increase by 45% by 2040 compared to consumption in 2013, resulting in a total CO2 emission increase of 40% [1]. At the same time, achieving the 2 °C limit on global warming requires new policies to reduce the energy sector’s CO2 emissions by replacing traditional and fossil fuels with renewable energies [1,2,3]. Bioenergy, energy produced from biological sources, is one such renewable energy [2,3]. Globally, bioenergy has the potential to produce 100–400 exajoules (EJ) yr−1 [2,4], which is equivalent to 25%–100% of the total energy consumed in 2014 [5]. Despite such enormous potential, in 2014, only about 4% of the electricity and heat consumed were generated from biofuels, while traditional use of biomass (e.g., cooking) represented 9% of the energy consumed [5]. Traditional use of biomass, although common in developing countries, remains inefficient and hazardous to health; bioenergy could provide clean and affordable energy to meet increasing demands in these countries [1,2,6]. In the tropics, oil palm (Elaeis guineenis (Jacq.)) dominates biofuel production from tree species [7,8]. However, in comparison to forests, oil palm monoculture results in a loss of ecosystem functions [8]. This is less severe in mixed systems [9]; however, the advantages of mixed systems are countered by a reduction in palm oil yields due to the reduced number of stems per hectare. Diversifying or replacing palm oil plantations by including other oil crops (e.g., Pangamia pinnata) may offset oil yield loss. This has yet to be assessed.
The aim of this study was to identify tropical tree species that could produce biological resources for bioenergy production and are able to grow on various type of soils. The potential biofuel and energy yields were estimated by assessing yields based on silvicultural information (e.g., stem density) and productivity (e.g., biomass per ha and year), which were converted into energetic values (e.g., GJ ha−1 yr−1). Due to the huge body of literature on species used and recommended for bioenergy production in the tropics, this study specifically focused on tree species for bioenergy production [10,11,12,13]. We therefore excluded bamboo and other non-woody species to avoid repetition of recent results produced by Abel et al. [11], Darabant et al. [14], Pfister [15], and Wi et al. [16].

2. Materials and Methods

The aim of this narrative review [17] was to identify tree species suitable for bioenergy systems in the tropics from literature [11,12,13,18,19,20] by combining their silvicultural information (Table 1) and potential energy yields per hectare per year (Table 2). A literature search using Google Scholar was conducted for silvicultural information using species names as keywords.
The search produced 241 documents and 9 freely available databases (Table S1). These were used to assess the following aspects of woody bioenergy crops: (i) botanical information (e.g., species and origin, synonyms, common name, typical use), (ii) ecological settings (e.g., temperature, mean annual precipitation, soil properties), and (iii) cropping and yields (e.g., stem density, biomass yield, bio-oil yield) (Table S2). Data extracted from original resources were taken directly from the publication; thus, our dataset represents original information without any conversion into a single system (e.g., FAO soil classification). Extracted soil pH values were mostly (i.e., 93%, Table S2) published without further clarification on solutions used (e.g., H2O, KCl, CaCl2) or salt concentration, which affects the comparability of pH values [21,22]. Thus, due to the lower accuracy of pH values and ranges presented, this review can provide only approximate information on the soil pH values tolerated. Yield data in mass or volume per unit area were used as presented in surveys or calculations, based on single tree productivity (e.g., dry biomass, fruit yield, oil content) and stand density per unit area (Table S2). Conversion factors used to derive energy yields (GJ ha−1 yr−1) were: (i) carbon density of 0.5 in dry mass of wood [23], (ii) energy of 37 MJ stored per kg carbon or 19 MJ per kg dry biomass [3], (iii) a bio-oil:biodiesel conversion rate of 90% (adapted from values published by Meher et al. [24], (iv) biodiesel density of 0.9 g cm³ [24,25], (v) energy of 33 MJ stored per liter of biodiesel [24,25], (vi) a sugar:bioethanol conversion rate of 51% [26], (vii) bioethanol density of 0.8 g cm−3 [25], and (vii) energy of 21 MJ per liter of bioethanol [25].

3. Results

Although numerous woody species are suitable for forest-based bioenergy systems in humid tropical regions, the estimation of potential bioenergy yields per unit of area (i.e., GJ ha1 yr1) was limited to 33 species due to the scarcity of silvicultural and biorefinery data (Table 1 and Table 2). This study provides species-specific information on environments preferred by each species, silvicultural information (e.g., stem density per hectare), and yield data (Mg dry biomass ha−1 yr−1). About 50% of the species (n = 16) are adapted to mineral soils and able to tolerate acidic and nutrient-poor soils (e.g., eroded Acrisols) and droughts (Table 1). Trees that can tolerate drought include Aleuritis moluccana (L.), Calophyllum inophyllum (L.), and Pongamia pinnata. Although their cropping on terrestrial soils potentially produces high yields, such yields will be reduced by flooding and wet soil conditions. In addition, soil wetness, soil acidity, and low nutrient status may also limit plant productivity [27]. In particular, biological nitrogen fixation by leguminous species that have been widely used to rehabilitate degraded land (e.g., Calophyllum inophyllum, Gliricidia sepium (Jacq.)) is drastically reduced in acidic soils. Based on tree productivity data, and information on their silvicultural recommendations, species suitable for growth on mineral soils and (re)-wetted peatland (Table 1 and Table 2) can potentially produce between 0.2 and 24.0 Mg biomass ha−1 yr−1, 0.1 and 9.0 Mg bio-oil ha−1 yr−1, and between 0.2 and approximately 20.0 Mg sugar ha−1 yr−1, which is equal to an energy yield between 2 and 444 GJ ha−1 yr−1 (Table 2 and Table S2).
Seventeen species are potentially suitable for bioenergy activities on wet land and land which is regularly flooded (Table 1). Three tree species tolerate brackish environments, namely Cerbera manghas (L.), Nypa fructicans (Wurmb.), and Melaleuca cajuputi (Powell). The energy yield potential of these species ranges between 71 and 295 GJ ha−1 yr−1 (no data for Melaleuca cajuputi, Table 2). Calamus caesius (Blume) and Symphonia globulifera (L.f.) are adapted to wet soils rich in organic matter, while Combretocarpus rotundatus (Miq.) Danser, Dyera polyphylla, and Palaquium ridleyi (King & Gamble) can grow on permanently wet organic soils (i.e., peatland). Although peatland species produce raw material for bioenergy activities, data on productivity and energy yields are rarely reported, with productivity data found only for Dyera polyphylla (Table 2). The remaining nine tree species presented in Table 2 tolerate flooding and produce biomass, bio-oil, and sugar. Again, although information found on yields and productivity are minimal, the estimated energy output of some species may be too low for bioenergy activities (e.g., Euterpe oleracea (Mart.), Fleroya ledermannii (K.Krause), Spondias mombin (L.)), while the estimated productivity of Pentadesma butyracea Sabine and Sesbania bispinosa (Jacq.) seems to be promising for bioenergy activities (Table 2). Other species in this group are promising bioenergy crop candidates, but information on their productivity is not readily available (Table 2 and Table S2).

4. Discussion

The species presented that tolerate acidic soils and droughts are known and often used to produce raw material for bioenergy in tropical countries [12,13,28]. However, initiatives that aim to produce bioenergy require silvicultural information and yield data. The information presented here can be used to assess the economic feasibility of bioenergy projects and cropping system types [29,30,31]. Silvicultural and yield data are scarce for tropical tree species adapted to permanently wet and regularly flooded environments, but such data are required to develop feasible bioenergy strategies for wetlands (e.g., peatland). Two reasons could explain this knowledge gap: (i) limited interest in most of these tree species, except for sugar- and starch-producing palm trees (Metroxylon sagu, Nypa fructicans) and (ii) a lack of machinery for harvesting [32]. To avoid competition between food production and the production of raw materials for bioenergy, non-food crops should be cultivated on less productive land (e.g., eroded soil) [3,6,28]. The simplest approach to rehabilitating eroded land is the establishment of plantations [33]. Optimizing initial plant growth on eroded land for biomass production may require the application of fertilizer, which can cause the emission of N2O [3,34]. A less-assessed, but promising, way to reduce the amount of N-fertilizer is to mix non-leguminous and leguminous crops (e.g., Elaeis oleifera (Kunth) Cortés and Pongamia pinnata). Rehabilitation may require initial site preparation by planting species that can shade out weeds, fix nitrogen, and improve soil organic matter [33]. Trees suitable for site preparation are fast-growing, nitrogen-fixing species, e.g., Calliandra calothyrsus (Meisn.), Gliricidia sepium, and Zapoteca tetragona (Willd.). The cultivation of non-native tree species risks invasive competition [35,36,37]. Thus, introducing for example species native to Africa (i.e., Croton megalocarpus (Hutch)) and America (i.e., Spondias mombin) to Southeast Asia and could have negative impacts on biodiversity and environmental services.
The rehabilitation of wet land requires selection of species that can tolerate wet soils and are adapted to natural conditions of peat swamp forests (e.g., Dyera polyphylla) [32], yet there is limited information available on suitable trees for peat-swamp rehabilitation activities. In this study, bioenergy yields are compared to those of palm oil trees (Elaeis guineenis), which produce 3–6 Mg bio-oil ha−1 yr−1 [38,39,40], equivalent to an energy output of 90–194 GJ ha−1. Most of the assessed species have the potential to produce raw material (Palaquium ridleyi, Sandoricum koetjape (Burm.f.)) generating the same level of energy. For some species, very high yields have been reported (e.g., Dyera polyphylla, Metroxylon sagu, Pongamia pinnata) [19,41,42], potentially far above yields that are possible on degraded land. Other species with an estimated energy output of <90 GJ ha−1 yr−1 (i.e., the lowest energy output estimated for Elaeis oleifera) might not be feasible for bioenergy activities in tropical countries.

5. Conclusions

Tree species adapted to tropical wetlands and peatlands are potentially useful for bioenergy production, but published data are available only for a small number species. The estimated bioenergy yields of the reviewed woody species are in the range reported for lignocellulosic biomass of energy crops cultivated in Europe, the USA, and Brazil (110–370 GJ ha−1 yr−1) [2,3]. However, the values and coefficients used to estimate energy yields per unit area may fail to reflect the real variability of caloric values of biomass from various species [43,44]. Thus, this study provides initial estimations, which should be verified by experiments to test the impact of silviculture and biorefinery methods on energy yields.

Supplementary Materials

The following are available online at https://www.mdpi.com/1999-4907/9/10/594/s1, Table S1: References, Table S2: Data survey.

Author Contributions

Study concept and design: N.B. and H.B. Data collection: M.B., A.M.H., M.U., and N.B. Analysis and interpretation of data: N.B. and M.B. Drafting of the manuscript: N.B., M.B., A.M.H., and H.B. Critical revision of the manuscript: N.B., M.B., A.M.H., M.U., S.M.L., and H.B. Administrative, technical, and material support: H.B. and S.M.L. All the authors contributed to the discussion of the results and the final edition of the manuscript.

Acknowledgments

This research was carried out by the Center for International Forestry Research (CIFOR) as part of the CGIAR Research Program on Forests, Trees and Agroforestry (CRP-FTA), and funded by the National Institute of Forest Sciences, Korea. Nils Borchard was placed as an integrated expert at CIFOR by the Centre for International Migration and Development (CIM). CIM is a joint operation of the Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH and the International Placement Services of the German Federal Employment Agency. Many thanks go to GIZ’s Advisory Service on Agricultural Research for Development (BEAF) for supporting co-author Ms. Medha Bulusu during her thesis research at CIFOR. We also thank Jutta Zeitz for valuable comments and suggestions and Achmat Solichin for assisting during the literature survey. We thank three anonymous reviewers for their highly constructive comments.

Conflicts of Interest

The authors declare no conflict of interest.

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Table 1. Potential bioenergy species that tolerate unfavorable soil conditions. Soil conditions that are most relevant for species selection are presented. Potential ecological adaptations are also shown to inform about tolerances toward to for instance droughts and flooding. Primary data and their corresponding references are shown in Table S2.
Table 1. Potential bioenergy species that tolerate unfavorable soil conditions. Soil conditions that are most relevant for species selection are presented. Potential ecological adaptations are also shown to inform about tolerances toward to for instance droughts and flooding. Primary data and their corresponding references are shown in Table S2.
SpeciesSoil pHSoil TextureSoil MoistureSoil FertilityAdditional Adaptations
Species that tolerate poor soils, moist and dry environments
Agathis borneensis (Warb.)<7-/--/--/-Deep, well-drained, acidic soil
Aleurites moluccana (L.)5.0–8.0-/-Moist to dryPoorTolerates droughts
Arenga pinnata (Wurmb.)-/-SandMoist to dry-/-Tolerates dry environments
Azadirachta indica (A. Juss.)6.0–7.0-/--/-Poor-/-
Borassus flabellifer (L.)5.0–6.0-/-Moist to dry-/-Tolerates droughts and short-term flooding
Calliandra calothyrsus (Meisn.)5.0–6.5-/-Moist to dryPoorPioneer species, tolerates droughts
Calophyllum inophyllum (L.)4.0–7.5-/-Moist to dry-/-Xerophytic species, tolerates droughts
Ceiba pentandra (L.)-/-SandyMoist-/-Deep, well-drained, light soil, Andosol
Croton megalocarpus (Hutch.)-/-SandyMoist-/-Pioneer species; deep, well-drained, light soil
Croton tiglium (L.)4.5–7.5-/--/--/--/-
Gliricidia sepium (Jacq.)4.5–8.5VariousMoist-/-Pioneer species, deep, well-drained soil
Neolamarckia cadamba (Roxb.)-/--/-Moist-/-Deep, alluvial soils
Pongamia pinnata (L.)-/-SandyMoist to dry-/-Deep soils, tolerates droughts and acidity
Reutealis trisperma (Blanco)5.4–7.1-/--/-Poor-/-
Vernicia fordii (Hemsl.)6.0–6.5SandyMoist to dry-/-Deep, well-drained, light soils
Zapoteca tetragona (Willd.)-/--/--/--/--/-
Species that tolerate permanently wet and waterlogged or temporarily flooded soils
Calamus caesius (Blume)-/-Peat, clayish, siltyMoist to wet-/-Margins of peat and swamp land, tolerates flooding
Cerbera manghas (L.)-/--/-Moist to wet-/-Riparian, swamp and mangrove environment
Combretocarpus rotundatus (Miq.)3.0–4.5PeatWet-/-Peat-swamp forest (Shorea spp.), tolerates waterlogged soils
Dyera polyphylla (Miq.)3.0–4.5PeatWet-/-Peat-swamp forest, wet soils, peat
Erythrina excelsa (Baker)-/-VariousMoist to wet-/-Riparian and swamp land, high water table
Euterpe oleracea (Mart.)-/-SandyMoist-/-Light soils, tolerates flooding
Melaleuca cajuputi (Powell)-/-Sandy-/-PoorPoor, well-drained soils, brackish and acidic sulfate soils
Metroxylon sagu (Rottb.)>4.5VariousMoist to wet-/-Tolerates flooding
Fleroya ledermannii (K.Krause)-/--/--/--/-Anemochory, tolerates flooding
Nypa fruticans (Wurmb.)5.0ClayishMoist to wet-/-Mangrove species
Palaquium ridleyi (King & Gamble)3.0–4.5PeatWet-/-Peat-swamp forest
Pentadesma butyracea (Sabine-/--/--/--/-Riparian forests, deep soils
Phoenix reclinata (Jacq.)-/-Various-/--/-Medium-to-fine textured soil, tolerates flooding
Sandoricum koetjape (Burm.f.)≥7Various-/-PoorPioneer species, riparian areas
Sesbania bispinosa (Jacq.)<10VariousDry to wet-/-Alkaline soils, riparian areas, tolerates droughts
Spondias mombin (L.)4.3–8.0Various-/--/-Various mineral soils, tolerates flooding
Symphonia globulifera (L.f.)-/--/-Moist to wet-/-Lowland rainforest to swamp forest
-/- no data available.
Table 2. Potential annual biomass, bio-oil, sugar (Su), and starch (St) productivity in Mg ha−1 yr−1 of species used/potentially suitable for forest-based bioenergy production in tropical regions. Biomass data were also converted into volumetric values (mL ha−1 yr−1) and energy values (GJ ha−1 yr−1). A ‘Yes’ indicates a promising species, but due to a lack of information in the literature, yield could not be estimated. Primary data and their corresponding references are shown in Table S2.
Table 2. Potential annual biomass, bio-oil, sugar (Su), and starch (St) productivity in Mg ha−1 yr−1 of species used/potentially suitable for forest-based bioenergy production in tropical regions. Biomass data were also converted into volumetric values (mL ha−1 yr−1) and energy values (GJ ha−1 yr−1). A ‘Yes’ indicates a promising species, but due to a lack of information in the literature, yield could not be estimated. Primary data and their corresponding references are shown in Table S2.
SpeciesBiomassBio-Oil and BiodieselSugar or Starch and Bioethanol
Mg ha−1 yr−1GJ ha−1 yr−1Mg ha−1 yr−1kL ha−1 yr−1GJ ha−1 yr−1Mg ha−1 yr−1kL ha−1 yr−1GJ ha−1 yr−1
Species that tolerate poor soils, moist and dry environments
Agathis borneensis (Warb.)1.0–1.719–31-/--/--/--/--/--/-
Aleurites moluccana (L.)3.6–5.767–1050.5–6.00.5–6.016–194-/--/--/-
Arenga pinnata (Wurmb)-/--/--/--/--/-20 (Su)2.0–12.843–268
Azadirachta indica (A.Juss.)-/--/-0.1–2.70.1–2.74–87-/--/--/-
Borassus flabellifer (L.)-/--/--/--/--/-20 (Su)1.2–12.825–268
Calliandra calothyrsus (Meisn.)6.0–24.0111–444-/--/--/--/--/--/-
Calophyllum inophyllum (L.)-/--/-2.0–6.02.0–5.965–194-/--/--/-
Ceiba pentandra (L.)-/--/-1.3–4.81.3–4.842–155-/--/--/-
Croton megalocarpus (Hutch)-/--/-1.6–4.51.6–4.552–145-/--/--/-
Croton tiglium (L.)-/--/-0.2–0.90.2–0.96–29-/--/--/-
Gliricidia sepium (Jacq.)2.0–12.037–222-/--/--/--/--/--/-
Neolamarckia cadamba (Roxb.)1.8–12.933–239-/--/--/--/--/--/-
Pongamia pinnata (L.)-/--/-0.9–9.00.9–8.929–290-/--/--/-
Reutealis trisperma (Blanco)-/--/-Yes-/--/--/--/--/-
Vernicia fordii (Hemsl.)-/--/-0.3–1.00.2–1.08–32-/--/--/-
Zapoteca tetragona (Willd.)Yes-/--/--/--/--/--/--/-
Species that tolerate continuously wet and waterlogged or temporarily flooded soils
Calamus caesius (Blume)1.5–3.028–56-/--/--/--/--/--/-
Cerbera manghas (L.)-/--/-2.22.271-/--/--/-
Combretocarpus rotundatus (Miq.)-/--/--/--/--/--/--/--/-
Dyera polyphylla (Miq.)5.4–14.0100–259-/--/--/--/--/--/-
Erythrina excelsa (Baker)Yes-/--/--/--/--/--/--/-
Euterpe oleracea (Mart.)-/--/--/--/--/-0.2–3.8 (Su)0.1 –2.42–50
Melaleuca cajuputi (Powell)Yes-/--/--/--/--/--/--/-
Metroxylon sagu (Rottb.)-/--/--/--/--/-15–24 (St)9.6–15.3201–321
Fleroya ledermannii (K.Krause)2.7–3.249–59-/--/--/--/--/--/-
Nypa fruticans (Wurmb.)-/--/--/--/--/-3–22 (Su)1.9–14.040–295
Palaquium ridleyi (King & Gamble)-/--/--/--/--/--/--/--/-
Pentadesma butyracea (Sabine)-/--/-0.6–8.00.6–7.920–258-/--/--/-
Phoenix reclinata (Jacq.)Yes-/--/--/--/--/--/--/-
Sandoricum koetjape (Burm.f.)-/--/--/--/--/-Yes-/--/-
Sesbania bispinosa (Jacq.)8.0–17.0148–315-/--/--/--/--/--/-
Spondias mombin (L.)0.2–0.64–10-/--/--/--/--/--/-
Symphonia globulifera (L.f.)Yes-/--/--/--/--/--/--/-
-/- no data available.
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