Black Locust as a Bioenergy Feedstock: a Review
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- Straker, K.C., Quinn, L.D., Voigt, T.B. et al. Bioenerg. Res. (2015) 8: 1117. doi:10.1007/s12155-015-9597-y
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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.
KeywordsRobinia pseudoacacia Black locust Feedstocks Plantations Chemical composition Yield
A mandated increase in cellulosic biofuels , 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 , 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 . 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 . Furthermore, many SRWC species are stress-tolerant and well suited for marginal land not suited for food or feed production .
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 [7, 8, 9, 10]. Black locust can also conserve soil and water [11, 12, 13], sequester carbon [14, 15, 16], and improve biodiversity . 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.
Black locust fixes nitrogen through a root symbiont, forming perennial nodules  of an indeterminate type . It can be nodulated by a variety of strains of bacteria [28, 29, 30, 31] and more than one strain may occupy a nodule . Rhizobium spp. are most common [11, 28, 29], but several other taxa can nodulate black locust [28, 29, 31], depending on the soil microclimate . In general, nodule mass varies in response to soil pH, nitrogen, phosphorus, and other nutrient concentrations . For example, nitrogen fixation occurs at a greater rate in soils that are either neutral or weakly acidic with low calcium concentrations .
Distribution and Current Uses Worldwide
Black locust yield and survival by location as affected by spacing, cultivated variety, rotation length, rotation year, and fertilization
Spacing (m × m)
Species or cultivar
Rotation length (year)
Rotation year sampled
Yield (mg ha−1 year−1)
1.5 × 1.0
1.5 × 1.0
1.5 × 1.0
1.5 × 1.0
1.5 × 1.0
Common black locust
1 × 1 to 0.5 × 0.25
0.75 × 0.6
0.75 × 0.6
0.75 × 0.6
Seed from Brandenburg
Seed from Brandenburg
Seed from Romania
1 year before planting
Seed from Brandenburg
1 year before planting
0.3 × 0.3
1.8 × 1.8
Kentucky, Tennessee, and West Virginia, US
(16–32–8) at rate of 452 kg ha−1
Genetics and Breeding Opportunities
Genetic diversity , polymorphism, and heterozygosity  tend to be high within naturally occurring black locust populations, regardless of geographic variation or potential clines [50, 52, 53, 54]. 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 . However, one study suggests that tetraploid and mixoploid individuals may be inferior to diploid clones in biomass growth for timber or energy .
Characteristics of varieties and cultivated varieties of black locust
Variety or cultivar
Robinia pseudoacacia var. rectissima (Shipmast locust)
Straighter, taller, and slightly more durable wood
Improved for sawmilling, fuelwood, use on energy plantations, and high volume of wood
Improved for sawmilling, duel use as bee forage and forestry, and slight frost resistance
Improved for sawmilling, fuel wood, use on energy plantations, and high volume of wood, but suffered severe frost damage
Improved for sawmilling and frost resistance
Improved for sawmilling and fuel wood
Improved for sawmilling, fuel wood, and use on energy plantations
Improved for pole and postproduction and duel use as bee forage and forestry
Improved for pole and postproduction and dual use as bee forage and forestry
Improved for pole and postproduction and higher volume of wood
Improved for pole and postproduction
Improved for pole and postproduction
Improved for pole and postproduction and dual use as bee forage and forestry
Improved for bee forage and decorative purposes, fuel wood, and use on energy plantations
Improved for bee forage and decorative purposes and fuel wood
Improved for bee forage and decorative purposes
Improved for bee forage and decorative purposes
Improved for bee forage and decorative purposes
Improved for duel use as bee forage and forestry
Improved for frost resistance, fuel wood, use on energy plantations, and high volume of wood
Improved for higher volume of wood and use on energy plantations where it produced higher yields than common black locust
Seed propagation of black locust can be reliable, mechanized, and low cost . 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 , and scarification , 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 , as is planting between November and March and avoiding highly sloped land . 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 .
Black locust also can be propagated asexually from root cuttings or tissue culture . Transplants or cuttings can be planted in a furrow 4 cm deep, with the plant density dependent upon survival and production needs . Establishment of black locust from root cuttings often requires irrigation [18, 66].
Micropropagation techniques, media, and supplements used to propagate black locust
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)
0.25–1.0 mg l−1 BAP for establishment and naphthalene acetic acid (NAA) for increased length
½ Strength MS
0.5–1 mg l−1 IBA
Low levels of cytokine and auxin
½ Strength MS
Low salt and low auxin
½ Strength MS
½ Strength MS
5 ppm of indole acetic acid (IAA)
½ Strength MS
500 mg l−1 of NAA and auxin
1 M thidiazuron (TDZ)
0.5 mg l−1 BAP, 0.5 mg l−1 kinetin, and 0.1 mg l−1 NN
½ strength MS
to 0.25 mg l−1 IBA
Although black locust requires fewer agricultural inputs than many other hardwood species , careful field preparation can increase establishment success and biomass yields. Noncompacted soil is preferred , and inoculation with soil symbionts Glomus mosseae and Rhizobium spp. can improve growth . 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 . While it can tolerate some acidity, pH levels lower than 4 contributed to establishment failure . 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  and maintaining cultivation and fertilization . In plantations for timber, pruning, and thinning to remove dead limbs was touted as an important part of cultivation . More recently, it has been shown that thinning decreases wind damage [8, 18, 63], achieves regular spacing , and increases stem diameter; two moderate thinnings are commonly used to avoid shock (height or diameter reduction following thinning) . After a harvest, clear cutting was suggested so that sprouts and suckers could regenerate in the sunlight . 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 . Goggans and May  noted that without proper preparation and care or on land with an eroded topsoil and very compact subsoil, black locust will not grow well.
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 . 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 . Specifically, goldenrod reduced growth by 90 % and wild carrot litter reduced growth by 77 % . 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 . While soil sterilization is not necessary unless nematodes are present, aeration and insecticide application are recommended . 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 .
Although black locust fixes nitrogen, a number of studies have examined the effects of exogenous nitrogen fertilization on its growth [87, 88, 89, 90, 91]. In the majority of cases, black locust grew faster and accumulated greater biomass when treated with nitrogen fertilizer at the seedling stage  and within the first few seasons [90, 91]; however, positive effects of nitrogen fertilization were less detectible or absent in older trees . Because fertilization decreases nodulation and nitrogen fixation , some researchers suggest that inoculation with nodulating bacteria may result in greater biomass production than with fertilization . Indeed, inoculation with Rhizobium spp., Hebeloma mesophasem, and Glomus caledonium was shown to significantly increase nitrogen fixation and growth in black locust . 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 . Phosphorus application can increase nodulation, shoot height and mass of black locust . Recommended rates of phosphorus and other soil amendments are given in Keresztesi . Fertilizers, manure, or compost should be added in fall and incorporated to a depth of 35–40 cm before winter .
Although it is described as shade- and flooding-intolerant [22, 93, 94, 95], 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, 96, 97, 98, 99]. 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 . Similarly, tetraploid clones are better able to tolerate sodium chloride and sodium sulfate salts than diploid clones . Black locust also shows adaptation to cold climates by increasing fatty acid concentration  and protein synthesis in bark cells [104, 105], including a glycoprotein to help prevent ice formation in cells . However, stem dieback has been reported in response to cold , 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 . Black locust is considered relatively heat-tolerant , but a substantial degree of stem dieback was associated with hot, dry conditions in Oklahoma plantings . To our knowledge, however, no empirical studies have been published on the mechanisms or degree of heat tolerance in this species.
Pests and Disease
Description and damage caused by insect and disease pests of black locust
Megacyllene robiniae (Forster)
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
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
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
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
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 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.
Adults 0.6 cm
Adults black head and orange red body and wings
Adults emerge in early spring and in late summer
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
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 . It is now spreading rapidly and impacting black locust plantations throughout the region ; for example, just 1 year after its discovery, it was found in 148 out of 161 Slovakian locations surveyed . Other recorded pests include the locust leaf miner (Odontota dorsalis Thunb., family Chrysomelidae) , sawfly larvae (suborder Symphyta) , bagworms (family Psychidae) [111, 112, 113], leaf and tree hoppers (superfamily Membracoidea) , 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 . 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 % .
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, 111, 112, 113]. One study found that fertilized trees acquired herbivory tolerance through increased photosynthetic area and resistance, despite initial losses . 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 . Black locust can also be damaged by branch cankers, broom viruses, leaf diseases and rusts, drought, frost injury, herbicide injury, mechanical damage, and chlorosis . 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 . Similarly, black locust can also be infected with peanut stunt virus (PSV) and could infect other legumes planted nearby . 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 . 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 . 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).
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 % . The same study showed a yield increase up to 14 % for P. alba, as well . 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 .
Black locust could be a profitable species for alley cropping systems (ACS) with other agricultural crops  due to black locust’s ability to improve soil without nitrogen fertilization . 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 . Black locust creates a less variable microclimate for forages  and has been determined to be a valuable candidate species for silvopastoral systems in the southeastern US .
Black locust responds well to coppicing, with up to 100 % greater biomass than uncut trees , 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 . 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 , but more modest yields were later reported within this region . In the Piedmont region, predicted yields from a 3-year rotation were lower (3 to 8 Mg ha−1 year−1) . 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) . 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 , although some cultivars yielded up to 9.7 mg ha−1 year−1 after 7 years . 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) . 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 .
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 . 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 . Black locust wood can be pyrolyzed to yield bio-oil and other combustible liquid products , with an 80–90 % energy recovery during conversion to bio-oil, bio-coal, and bio-gas . With the addition of 10 % perlite, an 11-min pyrolysis cycle yielded 62 % liquid product . 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 . 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 . 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 . Black locust can also be converted into bioethanol, yielding 40–45 g kg−1 . 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 .
Black locust wood properties
Wood (% of dry mass)
Heartwood (% of dry mass)
Bark to wood ratio 1 year old shoots
Bark to wood ratio 3–5 years old shoots
Fiber length (mm)
Heating value (cal g−1)
Hydrogen content (% of dry mass)
Carbon content (% of dry mass)
Ash content (% of dry mass)
Ash softening temperature (°C)
Cellulose (% of dry mass)
Holocellulose (% of dry mass)
Alpha cellulose (% of dry mass)
Hemicelluloses (% of dry mass)
Lignin (% of dry mass)
Xylan (% of dry mass)
Arabinan (% of dry mass)
Mannan (% of dry mass)
Acetyl group (% of dry mass)
Acetate (% of dry mass)
Pentosan (% of dry mass)
Extractives (% of dry mass)
EtOH extractives (% of dry mass)
Benzene-EtOH extractives (% of dry mass)
H2O (hot) extractives (% of dry mass)
Dichloromethane extractives (% of dry mass)
Black locust wood also contains extractives that correlate with higher energy content  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 [159, 160, 161, 162]. Variations in extractive concentrations were found between sapwood and heartwood  and were found to vary according to origin, clone , and season .
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].
Black locust stands are associated with elevated levels of carbon and nitrogen in the soil [12, 14, 16, 18, 167, 168, 169, 170, 171, 172, 173, 174] up to 4 m from the tree , 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 . Soil under black locust also had higher rates of nitrification and N mineralization compared to pine–oak stands , and nitrate levels were two orders of magnitude greater under black locust than pine . In addition, nitrogen cycling and litter decomposition rates were faster for black locust stands than oak (Quercus liaotungensis) stands . Litter decomposition is associated with soil enzymatic activity  and lower organic matter . 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 .
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)  to 4.0 Mg C ha−1 year−1 (Germany)  to 7.0 Mg C ha−1 year−1 (Germany) . 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, 181, 182, 183] and landfill sites .
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 . Black locust can also improve the hydrologic processes of the area where it is planted , 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 . 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 , although it may out-compete native species along coastal windbreaks .
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 . Increased carbon uptake under elevated CO2  in black locust leads to an increased net photosynthetic rate . Black locust could be cultivated in new areas in a warmer climate , although this potential range expansion could also mean that escaped black locust could establish and invade in new habitats . 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  and accumulates lead and cadmium in leaves, indicating a tolerance to soil and air pollution . Specifically, leaf stomatal density and the upper cuticle layer increase in response to air pollutants, while spongy mesophyll is reduced .
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 [192, 193, 194]. 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 , particularly, in R. pseudoacacia var. monophylla . Twigs and seeds also provide food for wildlife including deer, rabbits, and quails . 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 . 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 . Another study indicates that certain bird species may even be more abundant in stands invaded with black locust . Black locust canopies have also been shown to be important day roosts for male northern bats (Myotis septentrionalis) .
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 , and is widely naturalized across the country (Fig. 1b). Introduced to Europe in the early 1600s , black locust is now one of the top three invaders there [205, 206] and has also invaded in Japan , South Korea , and Kenya . In fact, black locust has been called one of the top 100 woody invaders worldwide . It is commonly found in disturbed areas [45, 210, 211, 212] like roadsides, old fields, prairies, woods, savannas  and in urban areas . 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 , are dispersed primarily by gravity (barochorously), resulting in a relatively small seed shadow around the parent plant . However, longer-distance dispersal mechanisms have been observed, with fruits rolling down steep slopes into moving streams  or traveling up to 100 m from the parent through wind dispersal (anemochory) . 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 . 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 . Horizontal soil mixing is required after removal of black locust to encourage recolonization of native taxa .
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  and leaching allelochemicals into soil  and can facilitate the colonization of nonnative plants through its ability to add nitrogen to soil . For example, both richness and abundance of nonnative plants were greater under naturalized black locust than native pine and pine–oak forest . Specifically, ruderal nitrophilous plant species  (of which many are themselves nonnative invaders) proliferate, and oligotrophs and acidophilus species decrease under black locust . With proper soil preparation, native grasses can rebound after removal of black locust . Lichen communities suffer losses and even extinction of native species in favor of eutrophication-tolerant species , and lichen diversity does not rebound over time in older black locust stands . 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 . 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 .
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 . 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 . 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 . However, it should be pointed out that some Rubus species are also invasive . 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 , although other authors suggest this strategy may not be feasible for some industrial uses .
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 . For any energy crop to be profitable, biomass yield must be maximized while production costs are minimized . Unfortunately, Gasol et al.  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 . 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 . 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 . 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 , 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 . 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 . 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 .
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  or through regular monitoring of adjacent landscapes and careful handling and transport of vegetative fragments , 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.
The authors wish to acknowledge funding from the Energy Biosciences Institute.