BioEnergy Research

, Volume 8, Issue 3, pp 1117–1135

Black Locust as a Bioenergy Feedstock: a Review

  • Kaitlin C. Straker
  • Lauren D. Quinn
  • Thomas B. Voigt
  • D. K. Lee
  • Gary J. Kling
Article

DOI: 10.1007/s12155-015-9597-y

Cite this article as:
Straker, K.C., Quinn, L.D., Voigt, T.B. et al. Bioenerg. Res. (2015) 8: 1117. doi:10.1007/s12155-015-9597-y

Abstract

Short rotation woody bioenergy crops (SRWC) could contribute a substantial portion of the biomass required to meet federal mandates and offset carbon emissions. One SRWC with strong bioenergy potential is black locust (Robinia pseudoacacia L.), planted extensively for wood and energy applications globally, but under-studied in its native US. This member of the Fabaceae family can fix nitrogen, tolerate stress, and sequester carbon while generating biomass yields up to 14 Mg ha-1 yr-1. This article offers a comprehensive state-of-the-art review of production practices, biomass and energy yield estimates, environmental risks and benefits, and economic considerations for this promising feedstock.

Keywords

Robinia pseudoacaciaBlack locustFeedstocksPlantationsChemical compositionYield

Introduction

A mandated increase in cellulosic biofuels [1], along with an increased awareness of the need to replace fossil fuels with cleaner energy sources [2, 3], is driving a marked increase in research and development of novel feedstocks in the US and around the world. Short-rotation woody energy crops (SRWCs), which are harvested every 1–15 years [4], may offer particularly favorable options for biomass production. These perennial crops can be harvested throughout the year after a relatively short juvenile period, which can reduce storage time/costs and any associated biomass degradation [5]. The option of year-round harvest allows farmers to avoid losses related to drought or other environmental fluctuations and offers a relatively steady biomass stream compared with annually harvested herbaceous crops [5]. Furthermore, many SRWC species are stress-tolerant and well suited for marginal land not suited for food or feed production [6].

Black locust (Robinia pseudoacacia L., Fabaceae) possesses many characteristics that make it an important candidate SRWC species: it can be coppiced, displays a high relative growth rate, produces large amounts of biomass, produces high-density wood that can be dried, harvested, and processed easily, and combusts well [710]. Black locust can also conserve soil and water [1113], sequester carbon [1416], and improve biodiversity [17]. Because it fixes nitrogen and can be grown on low fertility soils, black locust plantations will require fewer inputs during cultivation and may provide a profitable use for otherwise unusable marginal land. Despite these and other benefits, black locust is not widely grown in the US for biomass. Here, we review existing reports relating to production practices, yield estimates, environmental benefits and risks, and economic considerations to inform the growing bioenergy industry about this feedstock.

Basic Description

Biological Profile

Native to a small region in the southeastern US and the Appalachians (Fig. 1a) [1821], black locust is a deciduous, thorny (Fig. 2c), nitrogen-fixing pioneer tree species of medium size (usually 12–18 m, but occasionally up to 30 m; Fig. 2) [19, 22, 23]. Its bole often separates naturally into several branches at 3–5 m [22]. Pinnately compound leaves appear alternately along short branches in an open canopy structure (Fig. 2a) [24]. Large white flowers attract honeybees in late spring and produce leathery seed pods that persist on the tree through winter [24]. Floral structure and protogynous phenology encourages outcrossing, although selfing is possible [25]. Higher seed germination rates occur as a result of outcrossing, but the flexibility of black locust’s breeding system is useful for a pioneer species [25]. In addition to sexual reproduction by seeds, black locust reproduces asexually by producing numerous suckers (Fig. 2b) [22, 25, 26]. Mother and daughter clones are strongly integrated, with transfer of nitrogen and phosphorus to daughters through a common root system [27]. In addition, it has been shown that black locust mothers will selectively place daughter ramets in alkaline patches of soil [27].
Fig. 1

Maps showing Robinia pseudoacacia native distribution (a) from Huntley [22] and naturalized distribution (b) from EDDMapS [221]. Darker areas indicate presence of R. pseudoacacia

Fig. 2

Robinia pseudoacacia compound leaves (a), suckering after coppice (b), and thorns (c). Large image is a plantation of approximately 2.5-m height 1 year after coppice at the University of Illinois (Urbana, IL). Photos courtesy of Gary Kling

Black locust fixes nitrogen through a root symbiont, forming perennial nodules [28] of an indeterminate type [29]. It can be nodulated by a variety of strains of bacteria [2831] and more than one strain may occupy a nodule [28]. Rhizobium spp. are most common [11, 28, 29], but several other taxa can nodulate black locust [28, 29, 31], depending on the soil microclimate [28]. In general, nodule mass varies in response to soil pH, nitrogen, phosphorus, and other nutrient concentrations [32]. For example, nitrogen fixation occurs at a greater rate in soils that are either neutral or weakly acidic with low calcium concentrations [33].

Distribution and Current Uses Worldwide

Although black locust is native to a small region in the US, it has since become naturalized in other regions of the US (Fig. 1b) [21] and has been planted across the globe, becoming an important commercial species in both temperate and subtropical regions. Historically, black locust lumber has served many purposes. It was once used for wagon hubs, treenails, and insulator pins [11, 34] and has been used for flooring, firewood, fence posts, and as bee forage for honey production [7, 8, 11, 18]. Black locust is now the third most commonly planted hardwood tree, with an estimated 2.5 million ha in lumber and energy production globally [21]. In Hungary, black locust plantations account for more than 20 % of forested land [35, 36], more than any other single tree species, and has been studied as a SRWC there [7, 8, 18, 37]. Black locust is also an important species in other areas of Europe, especially the Mediterranean region [37, 38]. It is the most prevalent exotic species in Bulgaria where it is used for afforestation, industrial wood production, bee forage, and erosion control [8]. Italy and Greece have evaluated black locust as a SRWC (Table 1) [38, 39], and in Germany, it has been grown as a SRWC on former surface mines to improve soil quality [40]. Studies in Slovakia and Germany showed that black locust produced greater biomass when compared with other trees (Table 1) [4143]. Black locust has also become an important plantation species in Asia, including Japan [44], South Korea [18, 45], and the Loess region of northwest China, where it is used in reforestation and soil and water conservation [12, 13, 18, 46, 47]. In the US, black locust is not often grown in plantations at this time, but is commonly used in mine reclamation here [1113, 21, 40, 48]. When used outside of timber plantations, it was found that black locust improved pasture quality by adding nitrogen to soil, increasing protein content of understory forage, and reducing erosion [49].
Table 1

Black locust yield and survival by location as affected by spacing, cultivated variety, rotation length, rotation year, and fertilization

Location

Spacing (m × m)

Species or cultivar

Rotation length (year)

Rotation year sampled

Fertilization

Yield (mg ha−1 year−1)

Survival (%)

Source

Hungary

1.5 × 1.0

Üllői

2–3

3

 

3.0

 

[8, 9, 35, 37]

5

8.0

7

9.8

Hungary

1.5 × 1.0

Nyírségi

2–3

3

 

2.4

 

[8, 9, 35, 37]

5

5.7

7

6.7

Hungary

1.5 × 1.0

Kiscsalai

2–3

3

 

4.2

 

[8, 9, 35, 37]

5

6.2

7

7.1

Hungary

1.5 × 1.0

Jászkiséri

2–3

3

 

2.4

 

[8, 9, 35, 37]

5

7.4

7

7.6

Hungary

1.5 × 1.0

Common black locust

2–3

3

 

3.6

 

[8, 9, 35, 37]

5

6.7

7

8.4

Italy

1 × 1 to 0.5 × 0.25

 

2–3

  

10

 

[38]

Germany

0.75 × 0.6

 

3

2

None

5.8

 

[41]

Germany

0.75 × 0.6

 

1

2

Mineral

3.2

 

[41]

Germany

0.75 × 0.6

 

1

2

Compost

4.6

 

[41]

Germany

 

Seed from Brandenburg

3

3

Hedgerow fertilized

7.6

80

[42]

Germany

 

Seed from Brandenburg

3

2

Hedgerow fertilized

6.9

75

[42]

Germany

 

Seed from Romania

4

1

1 year before planting

3.2

84

[42]

Germany

 

Unknown

4

1

None

3.1

98

[42]

Germany

 

Seed from Brandenburg

9

1

Hedgerow fertilized

7.4

83

[42]

Germany

 

Unknown

14

1

1 year before planting

9.5

95

[42]

Kansas, US

0.3 × 0.3

 

1

1

 

6.5

17

[238]

6

7.0

Kansas, US

1.8 × 1.8

 

5

1

 

11

92

[238]

2

9.1

73

3

8.6

67

4

8.6

58

Kentucky, Tennessee, and West Virginia, US

  

1

2

(16–32–8) at rate of 452 kg ha−1

1.7

 

[182]

13

4.9

 

Genetics and Breeding Opportunities

Genetic diversity [50], polymorphism, and heterozygosity [51] tend to be high within naturally occurring black locust populations, regardless of geographic variation or potential clines [50, 5254]. Naturally occurring black locust trees are diploid, but fast-growing and stress-tolerant tetraploids have been produced using a colchicine treatment [55, 56]. As previously mentioned, tetraploids tend to have greater stress tolerance than diploid clones and are therefore desirable for production in marginal areas. In addition, tetraploids could potentially be used as parent material for producing sterile triploid clones, should the interest arise [56]. However, one study suggests that tetraploid and mixoploid individuals may be inferior to diploid clones in biomass growth for timber or energy [37].

The goals of previous breeding programs have included frost tolerance, straighter trees for the use as poles, longer flowering period and increased nectar for honey production [11], and greater stress tolerance (Table 2). In addition, herbicide (glufosinate ammonium)-resistant black locust plants have been created [57]. For future improvements, it is important to note that heritability and variation can vary depending on the trait in question and the age of the tree [58]. Dry weight [53], caloric content [59], and net photosynthetic and respiration rates [60] appear to be highly heritable, while heritability of lignin content, specific gravity, and total extractives appears low [59]. Variation in heritability was found for traits such as height, diameter, thorn length, dry weight, and photosynthetic rate [61, 62]. Resistance to insects has been reported to have no individual heritability [54], but some variation in heritability among related individuals has been found [61], so its ability to be improved to resist attacks is unclear.
Table 2

Characteristics of varieties and cultivated varieties of black locust

Variety or cultivar

Description

Sourcea

Robinia pseudoacacia var. rectissima (Shipmast locust)

Straighter, taller, and slightly more durable wood

[66, 239]

Nyirsegi

Improved for sawmilling, fuelwood, use on energy plantations, and high volume of wood

[18, 37, 63, 96, 99]

Kiskunsagi

Improved for sawmilling, duel use as bee forage and forestry, and slight frost resistance

[18, 37, 63]

Jaszkiseri

Improved for sawmilling, fuel wood, use on energy plantations, and high volume of wood, but suffered severe frost damage

[18, 37, 63, 96, 99]

Penzesdombi

Improved for sawmilling and frost resistance

[18, 37, 63]

Rojtokmuzsaji

Improved for sawmilling and fuel wood

[18, 37, 63]

Gori

Improved for sawmilling, fuel wood, and use on energy plantations

[18, 37, 63]

Zalai

Improved for pole and postproduction and duel use as bee forage and forestry

[18, 37, 63]

Csaszarto1tesi

Improved for pole and postproduction and dual use as bee forage and forestry

[18, 37, 63]

Szajki

Improved for pole and postproduction and higher volume of wood

[18, 37, 63, 99, 240]

HC-4146

Improved for pole and postproduction

[18, 37, 63]

Ricsikai

Improved for pole and postproduction

[18, 37, 63]

Vati-46

Improved for pole and postproduction and dual use as bee forage and forestry

[18, 37, 63]

Rozsaszin-AC

Improved for bee forage and decorative purposes, fuel wood, and use on energy plantations

[18, 37, 63]

Debreceni-2

Improved for bee forage and decorative purposes and fuel wood

[18, 37, 63]

Halvamyearozsaszin

Improved for bee forage and decorative purposes

[18, 37, 63]

Debreceni 3-4

Improved for bee forage and decorative purposes

[18, 37, 63]

Matyusi 1-3

Improved for bee forage and decorative purposes

[18, 37, 63]

Egyleyelfi

Improved for duel use as bee forage and forestry

[18, 37, 63]

Kiscsalai

Improved for frost resistance, fuel wood, use on energy plantations, and high volume of wood

[18, 37, 63, 99, 240]

Ulloi

Improved for higher volume of wood and use on energy plantations where it produced higher yields than common black locust

[18, 99, 240]

aBlack locust was first introduced to Europe in 1601 [18] and Hungary between 1710 and 1720 [18], where it was widely planted across Hungary for afforestation. Superior trees were initially propagated by seed and later clonally by cuttings and micropropagation, allowing for the naming of cultivated varieties by plant breeders at the Forest Research Institute (FRI) in Budapest. Following the discovery and description of the variety rectissima (Shipmast locust) by Raber in 1936 [239], germplasm from the new variety with a straight central leader was widely incorporated into many of the crosses. The variety rectissima is found in the New England area of the US, including parts of New York, Massachusetts, and New Jersey. The cultivars in Table 2 were developed at the FRI, without provenance information being published

Production Considerations

Propagation

Seed propagation of black locust can be reliable, mechanized, and low cost [37]. Although seeds can remain dormant for several years, viability and germination rate can vary widely [37, 63]. Dormancy is due to the hard seed coat [64], and scarification [65], boiling water, or sulfuric acid treatments are recommended before sowing [18, 37, 64, 66, 67]. When direct seeding in the field, a furrow 5–8 cm wide and 3 cm deep is recommended [37], as is planting between November and March and avoiding highly sloped land [68]. However, some older sources suggested transplanting 1-year-old seedlings was more successful than direct seeding [34, 66]. If grown from seeds, exogenous irrigation may not be necessary if seasonal precipitation is adequate [66].

Black locust also can be propagated asexually from root cuttings or tissue culture [37]. Transplants or cuttings can be planted in a furrow 4 cm deep, with the plant density dependent upon survival and production needs [18]. Establishment of black locust from root cuttings often requires irrigation [18, 66].

Micropropagation techniques, particularly, axillary bud multiplication, can quickly increase planting stock [55, 6974]; as many as 10,000 plantlets (with 80 % survival rate) could be produced annually from 100 cultured shoots [70]. Shoot and root growth from axillary buds can be induced using a hormone-supplemented growth medium (Table 3) [55, 69, 70, 7274]. Through cambial explants, it is also possible to use mature trees for regeneration [75]. Black locust can also be propagated from stem cuttings, with rooting ability being higher in juvenile cuttings harvested in spring, and in auxin-treated cuttings (NAA 500 mg l−1) [76]. Stems can be stored for 4–8 months before use [75].
Table 3

Micropropagation techniques, media, and supplements used to propagate black locust

Micropropagation technique

Medium

Supplements

Source

Shoot multiplication

Murashige and Skoog (MS)

0.4–0.6 mg l−1 6-benzylaminopurine (BAP) and 0.05 mg l−1 indole-3-butyric acid (IBA)

[72]

Shoot multiplication

MS

0.25–1.0 mg l−1 BAP for establishment and naphthalene acetic acid (NAA) for increased length

[74]

Rooting

½ Strength MS

0.5–1 mg l−1 IBA

[74]

Shoot multiplication

MS

Low levels of cytokine and auxin

[73]

Rooting

½ Strength MS

Low salt and low auxin

[73]

Shoot multiplication

MS

BAP

[69]

Rooting

½ Strength MS

IBA

[69]

Rooting

½ Strength MS

5 ppm of indole acetic acid (IAA)

[18, 70]

Rooting

½ Strength MS

500 mg l−1 of NAA and auxin

[76]

Shoot multiplication

MS

1 M thidiazuron (TDZ)

[241]

Shoot multiplication

MS

0.5 mg l−1 BAP, 0.5 mg l−1 kinetin, and 0.1 mg l−1 NN

[55]

Rooting

½ strength MS

to 0.25 mg l−1 IBA

[55]

Edaphic Requirements

Although black locust requires fewer agricultural inputs than many other hardwood species [77], careful field preparation can increase establishment success and biomass yields. Noncompacted soil is preferred [48], and inoculation with soil symbionts Glomus mosseae and Rhizobium spp. can improve growth [78]. For optimal growth, soil pH should be 5.5–7.5, with 3–4 % organic matter, less than 5 % calcium carbonate, 150–200 mg kg−1 phosphorus, and 100 mg kg−1 potassium [18]. While it can tolerate some acidity, pH levels lower than 4 contributed to establishment failure [68]. Even if ideal conditions are not met, black locust can likely survive as long as the soil is well drained [8, 18, 37, 63] and the trees receive adequate solar radiation.

Production Management for Timber

In the past, due to its use as a valuable timber crop in the US, best plantation procedures were developed and disseminated to help farmers grow black locust [34, 66, 79, 80]. Management recommendations included breaking up the soil [66] and maintaining cultivation and fertilization [80]. In plantations for timber, pruning, and thinning to remove dead limbs was touted as an important part of cultivation [79]. More recently, it has been shown that thinning decreases wind damage [8, 18, 63], achieves regular spacing [81], and increases stem diameter; two moderate thinnings are commonly used to avoid shock (height or diameter reduction following thinning) [82]. After a harvest, clear cutting was suggested so that sprouts and suckers could regenerate in the sunlight [79]. Likewise, due to its shade intolerance, open-spaced trees were said to grow faster than closely spaced trees except on poor soils where closely planted trees grew better [66]. Goggans and May [83] noted that without proper preparation and care or on land with an eroded topsoil and very compact subsoil, black locust will not grow well.

Weed Control

Competition or chemical interactions with co-existing herbaceous species can substantially affect growth of black locust. For example, herbaceous cover was reported to reduce mean stem volume by 88 % compared to black locust trees grown without herbaceous cover [84]. Increases in black locust dry weight were also reduced by allelochemicals from goldenrod (Solidago altissima), broomsedge (Andropogon virginicus), crownvetch (Securigera varia), wild carrot (Daucus carota), tall fescue (Festuca arundinacea, now Schedonorus arundinaceus), and timothy (Phleum pratense) in the litter [85]. Specifically, goldenrod reduced growth by 90 % and wild carrot litter reduced growth by 77 % [85]. Thus, these and other resident species should be cleared—not simply tilled under—when preparing and maintaining sites for black locust plantations. Preemergence herbicides should be applied in early spring; various formulations and mixtures have been recommended, including a combination of glyphosate and atrazine or 2,4-D [18]. While soil sterilization is not necessary unless nematodes are present, aeration and insecticide application are recommended [18]. However, other reports suggest that black locust can be planted without intensive site preparation unless competition is “extreme” and indicate that it requires less initial weed control than poplar and ash plantations [86].

Fertilizer

Although black locust fixes nitrogen, a number of studies have examined the effects of exogenous nitrogen fertilization on its growth [8791]. In the majority of cases, black locust grew faster and accumulated greater biomass when treated with nitrogen fertilizer at the seedling stage [87] and within the first few seasons [90, 91]; however, positive effects of nitrogen fertilization were less detectible or absent in older trees [87]. Because fertilization decreases nodulation and nitrogen fixation [88], some researchers suggest that inoculation with nodulating bacteria may result in greater biomass production than with fertilization [91]. Indeed, inoculation with Rhizobium spp., Hebeloma mesophasem, and Glomus caledonium was shown to significantly increase nitrogen fixation and growth in black locust [92]. Because high levels of nitrogen inhibit nodule formation, it has been suggested that unless the soil is devoid of nitrogen, only phosphorus and potassium fertilization are likely needed [89]. Phosphorus application can increase nodulation, shoot height and mass of black locust [89]. Recommended rates of phosphorus and other soil amendments are given in Keresztesi [18]. Fertilizers, manure, or compost should be added in fall and incorporated to a depth of 35–40 cm before winter [18].

Stress Tolerance

Although it is described as shade- and flooding-intolerant [22, 9395], black locust can tolerate a number of stressors typical of marginal lands including drought, thermal extremes, salinity, acidity and alkalinity [8, 9, 18, 23, 40, 66, 77, 80, 9699]. While wild-type diploid black locusts are somewhat drought-tolerant [100, 101], tetraploid black locust clones display greater biomass, water use efficiency, and photosynthesis rates under drought than diploids [102]. Similarly, tetraploid clones are better able to tolerate sodium chloride and sodium sulfate salts than diploid clones [8]. Black locust also shows adaptation to cold climates by increasing fatty acid concentration [103] and protein synthesis in bark cells [104, 105], including a glycoprotein to help prevent ice formation in cells [106]. However, stem dieback has been reported in response to cold [23], and frost can decrease black locust growth rate and height and can exacerbate insect damage [107, 108]. Therefore, it is important to choose cold-tolerant cultivars for locations with freezing winters. For example, Hungarian cultivars Penzesdombi and Kiscsalai are comparatively frost-tolerant [7]. Black locust is considered relatively heat-tolerant [109], but a substantial degree of stem dieback was associated with hot, dry conditions in Oklahoma plantings [110]. To our knowledge, however, no empirical studies have been published on the mechanisms or degree of heat tolerance in this species.

Pests and Disease

Black locust can be damaged or killed by insect pests, diseases, fungi, herbivory, and extreme abiotic stress (Table 4). The principal insect pest of black locust is the locust borer, Megacyllene robiniae Forster, a beetle in the Cerambycidae family whose distribution is sympatric with that of black locust [66, 79, 111113]. Larvae burrow into and weaken the tree’s branches and trunk [114]. Although black locust can survive borer attacks, growth is often stunted and trees can become more susceptible to damage from other stressors including wind damage [111, 112, 114]. Adult locust borers feed on goldenrod (Solidago spp.) pollen [114] and the presence of goldenrod near black locust stands (up to 800 m away) can increase the number of attacks on black locust [111]. Black locust genotypes vary in their susceptibility to locust borers [115]. Other plantation studies found that vigorous trees, trees under 0.5 in. (1.27 cm) or over 6 in. (15.24 cm) in diameter, trees in mixed stands, and trees further from heavily infested brood trees were all less likely to be attacked by borers [116], as were trees in high-quality sites with less herbaceous biomass and lower elevation [117].
Table 4

Description and damage caused by insect and disease pests of black locust

Scientific name

Common name

Size

Appearance

Lifecycle description

Damage

Source

Megacyllene robiniae (Forster)

Locust borer

Larvae 2.5 cm

Larvae white, legless

Larvae bore into wood during the spring and summer and emerge as adults around September

Larvae bore into the wood as thin as 3.5 cm in diameter and left tunnels and a honeycomb effect. Damage was noticed by sap-like wet spot on the outside of bark, boring dust by the base on the trunk; caused wind breakage, stunted growth, a place for fungi to enter the tree along with making the wood useless for commercial purposes like posts

[66, 79, 111113]

Adults 1.9 cm

Adults black with yellow stripes, one of which makes a noticeable “W” shape on back

Adults often live on goldenrod, lay eggs in cracks, crevices, and under the bark in late summer to early fall

Fomes rimosus

Heart rot fungus

 

Conk dark brown, hard, and woody, attaches directly to the bark

Enters the tree through a wound, spores are windblown to infect other wounds

Fungus makes the heartwood lightweight, crumbly, and unusable for commercial products

[79, 112]

Ecdytolopha insiticiana (Zeller)

Locust twig borer

Caterpillar 1.2–1.9 cm

Caterpillar red to dark yellow

Cocoons contain full-grown larvae over winter in soil, and pupate in spring

Larvae cause irregular galls and growth loss in young trees or mortality in seedlings

[66, 111113]

Moth wingspan of 1.9–2.5 cm

Moth dark brown grey wings with pink patches

Adults emerge mid-spring to early summer and in late summer or early fall

Odontota dorsalis (Thunberg)

Locust leaf miner

Larvae slightly larger than adults

Larvae yellow

Larvae live and feed in leaf blisters

Larvae create large blister-like mines on leaves, which later turn brown; weakens the tree; can be fatal in combination with other stressors. Adults eat holes in and skeletonize leaves.

[66, 79, 111113]

Adults 0.6 cm

Adults black head and orange red body and wings

Adults emerge in early spring and in late summer

Obolodiplosis robiniae

Black locust gall midge

 

Larvae pale yellow

 

Forms irregular galls on leaf margins; can cause premature leaf drop. However, it tends to be a problem primarily for ornamental black locusts

[118120]

Adults 2.6–3.2 mm

Adults yellowish brown

Adults emerge soon after pupation; females oviposit on young leaves. First generation: mid-May until early July (in European locations); second generation: July and August; third generation: September to mid-November (pupation occurs in rolled leaf margins); last generation: larvae over winter and pupate in soil.

Other insect pests include the black locust gall midge, Obolodiplosis robiniae Haldeman, discovered in Europe in 2003, which causes gall formation and premature leaf drop [118]. It is now spreading rapidly and impacting black locust plantations throughout the region [119]; for example, just 1 year after its discovery, it was found in 148 out of 161 Slovakian locations surveyed [120]. Other recorded pests include the locust leaf miner (Odontota dorsalis Thunb., family Chrysomelidae) [121], sawfly larvae (suborder Symphyta) [122], bagworms (family Psychidae) [111113], leaf and tree hoppers (superfamily Membracoidea) [111], aphids (superfamily Aphidoidea) [112, 113], and carpenter moths (family Cossidae) [111, 113]. Black locust genotypes may vary in their resistance to insects; it was found that locust leaf miner adults preferred to oviposit in black locusts from particular seed sources in Maryland [123]. Occasionally, minor insect pests can cause significant damage. In one study, it was found that the Hymenopteran Bruchophagus mutabilis Nikol’skaya reduced black locust seed survival by nearly 74 % [124].

While insecticide application directly on the tree may be useful for individual trees or certain infestations, prevention is the best form of control [66, 79, 111113]. One study found that fertilized trees acquired herbivory tolerance through increased photosynthetic area and resistance, despite initial losses [125]. Furthermore, interplanting with other species may also prevent attacks [66, 79, 112, 113].

In addition to insect pests, a number of diseases affect black locust. Several fungi have been found to infect black locust. Phomopsis oncostoma (Thum.) var. Hohn, Aglaospora profusa (Fr.) de Not., and Cucurbitaria elongata (Ft) Grev can cause branch and tree death [126]. Black locust can also be damaged by branch cankers, broom viruses, leaf diseases and rusts, drought, frost injury, herbicide injury, mechanical damage, and chlorosis [112]. Researchers have found that black locust can serve as a host for witches’ broom disease (phytoplasma) [127, 128], leading the authors to speculate that it could potentially spread it to other agricultural crops [127]. Similarly, black locust can also be infected with peanut stunt virus (PSV) and could infect other legumes planted nearby [129]. Therefore, for use as an alley-cropping tree, prevention of witches’ broom and PSV will be important. During micropropagation, several bacterial genera, including Acidovorax, Dyella, Microbacterium, and Sphingomonas, can infect clones and decrease propagation potential [130]. However, it was found that thyme or lemongrass oil in 0.03 % concentration or a combination of thyme and lemongrass oil at 0.015 % concentration reduced bacterial infection [130]. Short-rotation seedling black locusts grown at the University of Illinois Energy Biosciences research farm have been relatively free from insect and disease problems over a 5-year period with the exception of potato leaf hoppers during an extreme drought in 2012 (Gary Kling, unpublished data).

Intercropping

When grown in mixed stands with other species, black locust can be less susceptible to disease and pest attacks [66, 79, 131], and total biomass yield can increase. In Hungary, for example, yield of black locust grown in approximately equal ratios with the energy species Populus alba L. can increase up to 18 % [132]. The same study showed a yield increase up to 14 % for P. alba, as well [132]. The trials in Hungary were not replicated and could not be statistically analyzed [132, 133], but observations from mixed poplar black locust stands in China corroborate these results [18, 63]. Conversely, a mixed stand of sycamore and black locust grown on eroded soil was found to decrease yields of both species, though the decrease was substantially greater for sycamore [134].

Black locust could be a profitable species for alley cropping systems (ACS) with other agricultural crops [135] due to black locust’s ability to improve soil without nitrogen fertilization [136]. Barley biomass decreased when black locust was pruned and mulched in an ACS field; however, the combined biomass yield increased, and the need for fertilizer was significantly decreased [137]. Black locust creates a less variable microclimate for forages [138] and has been determined to be a valuable candidate species for silvopastoral systems in the southeastern US [139].

Yield Estimates

Biomass Yield

Black locust responds well to coppicing, with up to 100 % greater biomass than uncut trees [140], but reported biomass yields vary according to location, harvest timing, and agronomic practices (Table 1). In the mountainous terrain of South Korea, where black locust is used for fuel—wood and fodder, dry wood yields reach 13 Mg ha−1 year−1 [18]. In the Great Plains region in the US, yields of 6.5–11year and 14 Mg ha−1 year−1 were obtained at 0.3 and 1.6 m2 spacing, respectively [141], but more modest yields were later reported within this region [142]. In the Piedmont region, predicted yields from a 3-year rotation were lower (3 to 8 Mg ha−1 year−1) [143]. Short-rotation black locusts planted on a 1.5 × 1.8-m spacing at the University of Illinois Energy Biosciences research farm produced 11.3 Mg ha−1 year−1 at the first coppice (1-year-old seedlings planted and grown for two seasons prior to coppice) and 12.2 Mg ha−1 year−1 in the first post-coppice harvest (year 2 post-coppice; Gary Kling, unpublished data). On former mining lands in Germany, black locust yields ranged from 3 to 10 Mg ha−1 year−1 (mean was 4.0 and 6.0 Mg ha−1 year−1 at 3- and 6-year rotations, respectively) [42]. In Hungary, the average yield across black locust varieties was 6.5 Mg ha−1 year−1 at dense spacing (22,000 plants ha−1) and harvested on a 5-year rotation [144], although some cultivars yielded up to 9.7 mg ha−1 year−1 after 7 years [10]. This was similar to yields from harvest of black locust hedgerows planted adjacent to arable land (6.9 to 7.6 Mg ha−1 year−1) [42]. In Hungary, trees planted on a 1.5 × 0.3-m spacing and harvested at 3- to 5-year intervals produced the greatest biomass and avoided pest and mortality problems associated with longer or shorter harvest intervals (Table 1) [8, 9, 18, 35, 37]. Black locust performed poorly compared with poplar and willow on poor soils in Poland; however, second-year black locust yields averaged 1.14 Mg ha−1 year−1 across soil enrichment treatments [145].

Composition and Energy Generation

The thermal behavior of black locust in inert and oxidative atmospheres was found to be similar to other SRWC crops such as willow and poplar as well as to Miscanthus sinesis [146]. However, black locust has a higher energy content than many hardwoods, with 3- to 9-year-old trees yielding 33.8 to 76.8 × l09 J ha−1 year−1 [147]. Black locust wood can be pyrolyzed to yield bio-oil and other combustible liquid products [148], with an 80–90 % energy recovery during conversion to bio-oil, bio-coal, and bio-gas [38]. With the addition of 10 % perlite, an 11-min pyrolysis cycle yielded 62 % liquid product [149]. In a test of energy production from black locust chips, a down draft gasifier with a 2.4 gas-to-feed mass ratio produced 5.2–5.6 MJ m−3 [150]. Another study found black locust burned at a rate of 1.9 kg h−1 and had a lower heat value (LHV) of 16,676 kJ kg−1 [36]. A proximate analysis of the fuel from black locust in terms of the percentage of the material burned in gas showed 80.9 % volatiles, 18.3 % fixed carbon, and 0.8 % ash [151]. Black locust can also be converted into bioethanol, yielding 40–45 g kg−1 [152]. Ethanol yield can be increased with increased saccharification by ammonia fiber expansion pretreatment at 180 °C, along with milling to achieve smaller particle size and loading higher enzyme concentrations [152].

Several authors have quantified the chemical composition of black locust wood (Table 5). Biomass harvested from 10-year-old trees had a relatively low moisture content of 39 % [151], and has been shown to combust readily even when wet [8, 18, 151]. 1-year-old seedlings grown in the field for two additional years had a moisture content of 34.3 % when harvested during the dormant season in University of Illinois field tests (Gary Kling, unpublished data). 3-year-old plantation-grown black locusts in the Midwestern US varied in carbon (48.7–49.6 %), ash (0.6–2.7 %), and hydrogen (6.4–7.2 %) concentrations, but carbon and ash were higher on upland sites than bottomland sites (Table 5) [153]. The same study also found that a plant spacing of 31 × 46 cm resulted in higher hydrogen concentrations than a narrower 23 × 23 cm spacing. Another study reported black locust ash content at 0.77 %, greater than willow or poplar [154]. The same study reported that black locust also had greater wood density (602 kg m3) and calorific value (21.2 MJ kg−1) than other species. Similarly, black locust wood has a high specific gravity (Table 5), allowing it to burn hotter and longer than other taxa [150, 151]. Cellulose (45–50 %) [103, 155], lignin (19–29.4 %) [103, 155157], and other wood properties vary according to growing conditions, production spacing, and wood type (heartwood vs. sapwood; Table 5).
Table 5

Black locust wood properties

Property

Value

Source

Wood (% of dry mass)

85

[59]

Heartwood (% of dry mass)

54

[59]

Bark to wood ratio 1 year old shoots

17:83

[242]

Bark to wood ratio 3–5 years old shoots

16:84

[242]

Fiber length (mm)

0.71–1.13

[59, 103, 150]

Specific gravity

0.55–0.69

[59, 150]

Heating value (cal g−1)

4,405–4,745

[151]

Hydrogen content (% of dry mass)

6.43–7.19

[153]

Carbon content (% of dry mass)

48.67–49.45

[153]

Ash content (% of dry mass)

0.17–2.2

[103, 153, 155, 157]

Ash softening temperature (°C)

1,520

[41]

Cellulose (% of dry mass)

45–50

[103, 155]

Holocellulose (% of dry mass)

75–78

[156]

Alpha cellulose (% of dry mass)

41–42

[156]

Hemicelluloses (% of dry mass)

13.3–31.6

[103, 155]

Lignin (% of dry mass)

19–29.4

[103, 155157]

Xylan (% of dry mass)

16.2

[157]

Arabinan (% of dry mass)

0.4

[157]

Mannan (% of dry mass)

1.0

[157]

Acetyl group (% of dry mass)

3.8

[157]

Acetate (% of dry mass)

1.3

[155]

Pentosan (% of dry mass)

17

[156]

Extractives (% of dry mass)

6.8–8.5

[103]

EtOH extractives (% of dry mass)

1.1

[59]

Benzene-EtOH extractives (% of dry mass)

3.5

[59]

H2O (hot) extractives (% of dry mass)

2.8–13.49

[59, 164]

Dichloromethane extractives (% of dry mass)

0.48–4.03

[164]

Black locust wood also contains extractives that correlate with higher energy content [151] and resistance to fungal decay [59, 103]. A number of phenolic compounds and flavonoids (see Table 5) [103, 158, 159], some of which are associated with decay resistance, are found within wood, leaves, roots, and bark [159162]. Variations in extractive concentrations were found between sapwood and heartwood [163] and were found to vary according to origin, clone [164], and season [165].

Lifecycle Analysis

When considering ethanol from black locust as a complete process, a well-to-wheel life-cycle analysis (LCA) can be a useful tool [155, 166]. Compared to conventional gasoline, ethanol from black locust reduced fossil fuel use (76 % reduction) and contributions to climate warming (97 %), acidification (42 %), and eutrophication (41 %) [155, 166]. These reductions are greater than for other bioenergy feedstocks, including poplar trees, due in part to the low input requirements of black locust during production [155, 166]. The use of sustainable agronomic practices along with improved ethanol conversion efficiency make black locust a favorable bioenergy choice [155, 166].

Environmental Benefits

Soil Improvements

Black locust stands are associated with elevated levels of carbon and nitrogen in the soil [12, 14, 16, 18, 167174] up to 4 m from the tree [175], and levels increase with stand age [12, 14, 16]. For example, soil nitrate, total nitrogen, organic carbon, pH, and microbial diversity were elevated under black locust, while ammonium concentration and catabolic diversity of microbes were decreased; these conditions are associated with enhanced nitrification [175]. Soil under black locust also had higher rates of nitrification and N mineralization compared to pine–oak stands [169], and nitrate levels were two orders of magnitude greater under black locust than pine [176]. In addition, nitrogen cycling and litter decomposition rates were faster for black locust stands than oak (Quercus liaotungensis) stands [177]. Litter decomposition is associated with soil enzymatic activity [178] and lower organic matter [47]. For example, 30-year-old restored black locust forest in the Loess Plateau in China had significantly greater nutrient content and soil enzymatic activity and lower organic matter than unrestored plant communities or climax forest dominated by other species [47].

Several studies have found substantial carbon sequestration rates under black locust, with values ranging from 2.4 Mg C ha−1 year−1 (Ohio, USA) [14] to 4.0 Mg C ha−1 year−1 (Germany) [16] to 7.0 Mg C ha−1 year−1 (Germany) [15]. Black locust is also associated with increased pH, Ca, Mg, K, and PO4–P concentrations relative to other species [171, 176, 179]. Because of these soil improvements, black locust is a good candidate for production on marginal land [7, 15, 47, 168, 169, 171, 172, 178, 180], and it has been used to reclaim former surface mines [7, 11, 14, 16, 17, 40, 68, 174, 181183] and landfill sites [184].

Due to its extensive root system and prolific suckering, black locust can prevent soil erosion [7, 11, 18, 34, 66, 79, 80]. It can also improve previously eroded soil by increasing porosity, soil organic matter, nitrogen, and phosphorus [185]. Black locust can also improve the hydrologic processes of the area where it is planted [46], which, along with its ability to be grown without excessive irrigation [7, 11, 18], makes it a good candidate for arid or drought-stricken regions [13]. Although considered somewhat drought-tolerant, our research showed some shoot dieback during the extreme drought of 2012 in Urbana, IL, although almost all plants survived (Gary Kling, unpublished data). It may also be used as a windbreak [11], although it may out-compete native species along coastal windbreaks [44].

Resilience to Climate Change and Air Pollution

Under elevated CO2, black locust shows increased biomass and root mass and an increase in root nodulation and nitrogen fixation rates [186]. Increased carbon uptake under elevated CO2 [187] in black locust leads to an increased net photosynthetic rate [60]. Black locust could be cultivated in new areas in a warmer climate [188], although this potential range expansion could also mean that escaped black locust could establish and invade in new habitats [189]. This issue is discussed in greater detail in Section “Environmental Risks (Invasiveness)” below. In addition, black locust is resistant to nitrogen oxide and ozone damage [63] and accumulates lead and cadmium in leaves, indicating a tolerance to soil and air pollution [190]. Specifically, leaf stomatal density and the upper cuticle layer increase in response to air pollutants, while spongy mesophyll is reduced [191].

Additional Uses of Black Locust

In addition to its use as an energy and lumber crop, black locust can provide fodder for livestock in summer when drought affects herbaceous forages [192194]. For example, it could be a useful feed for goats and other livestock [139, 194]; however, it is not as easily digested as alfalfa or other types of fodder [195, 196]. Pollarding and annual cutting have been shown to increase foliar (fodder) biomass in Greece [39], particularly, in R. pseudoacacia var. monophylla [197]. Twigs and seeds also provide food for wildlife including deer, rabbits, and quails [11]. Black locust has also been shown to be useful as bee forage [8, 11, 198], leading to the production of high-quality honey [8, 11, 198]. Further, Robinia pseudoacacia var. monophylla has been shown to produce greater and better quality forage biomass than common black locust [39]. However, grazing pressure from wildlife can affect growth and yield [7, 86, 199] and should be avoided if black locust is being raised as an energy crop.

Black locust stands can also provide habitat for a number of birds. Despite its nonnative provenance, black locust harbors the same bird diversity as native forest during the breeding season in Italy, although diversity decreased in black locust stands during winter [200]. Another study indicates that certain bird species may even be more abundant in stands invaded with black locust [201]. Black locust canopies have also been shown to be important day roosts for male northern bats (Myotis septentrionalis) [202].

Environmental Risks (Invasiveness)

Although it is native to a small region of North America, black locust is regulated as a noxious weed in Massachusetts, USA, listed as invasive by 12 states and three regional invasive species councils [203], and is widely naturalized across the country (Fig. 1b). Introduced to Europe in the early 1600s [204], black locust is now one of the top three invaders there [205, 206] and has also invaded in Japan [207], South Korea [45], and Kenya [208]. In fact, black locust has been called one of the top 100 woody invaders worldwide [209]. It is commonly found in disturbed areas [45, 210212] like roadsides, old fields, prairies, woods, savannas [211] and in urban areas [45]. Therefore, it will be important to minimize disturbance and control propagule escape in sensitive landscapes adjacent to production fields [45, 213].

Black locust seeds, which can be shed year-round in some locations [207], are dispersed primarily by gravity (barochorously), resulting in a relatively small seed shadow around the parent plant [212]. However, longer-distance dispersal mechanisms have been observed, with fruits rolling down steep slopes into moving streams [207] or traveling up to 100 m from the parent through wind dispersal (anemochory) [212]. While seed dispersal and seedling establishment are responsible for spread in some natural populations, seeds are hard and display dormancy up to 10 years; therefore, asexual reproduction via suckering appears more common [44, 167, 214, 215]. However, it will be important for farmers to monitor field margins and adjacent landscapes for escaped seedlings and saplings.

Black locust’s substantial impacts on soil properties and nutrient cycling can have negative impacts on native species adapted to low nitrogen [216, 217], in general, the ability of black locusts to fix nitrogen results in increased nitrate and ammonium [168, 218] and other nutrients in soil. Compared to a native pine–oak stand, black locust subsoils had up to 3.2 times more nitrogen, up to 120 times greater net nitrification rates, and higher phosphorus and calcium concentrations [169]. Increased soil nitrogen can affect not only local soils, but can result in nitrogenous runoff into low-lying streams [219, 220]. Although Mg and Ca concentrations return to baseline levels following the removal of black locust, P and N mineralization rates remained elevated for up to 2 years after black locust was removed [172]. Horizontal soil mixing is required after removal of black locust to encourage recolonization of native taxa [172].

Black locust also exerts direct and indirect effects on resident plants and animals. It can negatively impact native vegetation by forming dense shady stands of mature trees and suckers [221] and leaching allelochemicals into soil [222] and can facilitate the colonization of nonnative plants through its ability to add nitrogen to soil [171]. For example, both richness and abundance of nonnative plants were greater under naturalized black locust than native pine and pine–oak forest [171]. Specifically, ruderal nitrophilous plant species [168] (of which many are themselves nonnative invaders) proliferate, and oligotrophs and acidophilus species decrease under black locust [223]. With proper soil preparation, native grasses can rebound after removal of black locust [172]. Lichen communities suffer losses and even extinction of native species in favor of eutrophication-tolerant species [224], and lichen diversity does not rebound over time in older black locust stands [225]. Native birds may prefer black locust stands, reaching greater densities than in native forest [171, 226], but those birds achieved lower reproductive success than in their native habitat [226]. In addition to significant ecological effects, the economic impacts of black locust invasion can be substantial: costs to control and restore invaded areas have been estimated at more than $80,000 ha−1 [45].

As previously mentioned, farmers should monitor black locust seed establishment outside of cultivation. However, because black locusts reproduce more commonly by root suckering and stump sprouting and because clones are interconnected through a common root system [27, 211], control efforts must focus on killing the roots [211]. Black locust can be controlled by repeated girdling, cut-stump herbicide application, hand-pulling, and mowing, although complete eradication may not be achieved through these methods [213]. Again, minimizing soil disturbance in areas adjacent to black locust plantations will be important. In addition, farmers or land managers could plant cover species including blackberry (Rubus spp.) and herbaceous species that have been shown to suppress black locust [227]. However, it should be pointed out that some Rubus species are also invasive [228]. Similarly, it has been suggested that if forests are appropriately managed, later successional species may shade out black locusts [26, 171, 212]. It has also been proposed that existing naturalized black locust stands could be harvested as biomass for energy [229], although other authors suggest this strategy may not be feasible for some industrial uses [7].

Economic Considerations

Although black locust can produce favorable biomass yields and has a number of traits that can benefit the environment, it will be important to consider its production from an economic standpoint [230]. For any energy crop to be profitable, biomass yield must be maximized while production costs are minimized [231]. Unfortunately, Gasol et al. [232] found that black locust biomass yields were not sufficient to offset current production costs in Italy, and governmental support was recommended. Also, in Colorado, black locust production was not predicted to be economically viable even with federal subsidies, due to low market prices and insufficient yields [230]. Further, researchers in the Great Plains of the US concluded that black locust production was not economical due to locust borer infestations and high establishment costs [141]. It has been estimated that biomass yield of woody feedstocks could be improved by as much as 40 % simply through genetic selection, and harvesting costs could be lowered up to 30 % with technological advances [233]. It seems these advancements may yet be needed for black locust in some locations.

Despite these negative reports, several studies suggest black locust cultivation could be economically viable. For example, it was calculated that short-rotation woody crops can produce 3,676 l ha−1, similar to corn ethanol yields [234], and with improved technology and growth this could be increased. Further, black locust cultivation on a 3-year rotation is already thought to be economically competitive in Germany [42]. Also, in the North Central region of the US, some SRWC crops attain comparable yields to grain-based biofuels when grown on otherwise unprofitable marginal land [235]. There remain questions as to the profitability of short-rotation plantations of black locust, but more field studies into black locust yields, particularly, in its native range and on marginal land, could provide better insight into its potential. In addition, the use of programs such as cost share assistance or decreasing rental payments as land is kept in use may encourage the establishment of these plantations [235].

Conclusions

Despite the current economic challenges surrounding black locust cultivation, this species shows strong promise for use as an energy crop in the US and elsewhere. As previously discussed, black locust is easy to propagate even on marginal land, requires few agricultural inputs, sequesters carbon, produces high biomass yields, and is efficiently converted into a number of forms of energy. In addition, its cultivation may enrich poor soils and provide additional social and environmental benefits. If its potential to escape is intentionally and carefully avoided by restricting cultivation to its narrow native range [236] or through regular monitoring of adjacent landscapes and careful handling and transport of vegetative fragments [237], black locust invasion risk could be minimized. With continued research into improved lines (including less invasive and sterile forms), management techniques, as well as harvest and conversion technology, black locust biomass could provide a valuable and renewable energy source for the future.

Acknowledgments

The authors wish to acknowledge funding from the Energy Biosciences Institute.

Copyright information

© Springer Science+Business Media New York 2015

Authors and Affiliations

  • Kaitlin C. Straker
    • 1
  • Lauren D. Quinn
    • 1
  • Thomas B. Voigt
    • 1
  • D. K. Lee
    • 1
  • Gary J. Kling
    • 1
  1. 1.Energy Biosciences InstituteUniversity of Illinois at Urbana ChampaignUrbanaUSA