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Hanover, J.W. 1993. Black locust: An excellent fiber crop. p. 432-435. In: J. Janick and J.E. Simon (eds.), New crops. Wiley, New York.

Black Locust: An Excellent Fiber Crop*

James W. Hanover

  1. ATTRIBUTES OF BLACK LOCUST
  2. BLACK LOCUST FOR FIBER AND BIOMASS
  3. CULTURAL SYSTEMS
  4. SUMMARY
  5. REFERENCES
  6. Table 1
  7. Table 2
Black locust, (Robinia pseudoacacia L., Fagaceae), is a remarkable yet relatively neglected tree species with untapped potential which much of the rest of the world already appreciates. The native range is east- and west-central United States (Hanover 1992), but the species now grows wild in all of the contiguous states. It is ironic that one of our common native species may now be considered as a new crop for fiber. An intensive research and development effort for black locust has the goal of exploiting one of our most valuable plant resources with a multitude of uses including fiber production.

ATTRIBUTES OF BLACK LOCUST

Black locust is a nitrogen-fixing legume (Hanover and Mebrahtu 1991). When mature, it can reach 35 m in height and 1.0 m in diameter and such trees are usually found on upland sites in hardwood forests with black oak, red oak, chestnut oak, pignut hickory, yellow poplar, maple, and ashes. It tolerates a wide range of soil pH (4.6 to 8.2), but grows best in calcareous, well-drained loams. The species is intolerant of water logged soils and shading. Black locust reproduces prolifically from root sprouts and dominates early forest regeneration in native forest stands and areas disturbed by man. Black locust possesses virtually all of the characteristics that define a typical weed (Hanover 1992; Keeler 1989).

Other important attributes of black locust are its extremely rapid early growth rate, very high density wood, high resistance to wood decay fungi, tolerance to low fertility sites, drought resistance, abundance of natural product chemicals in its wood, bark and leaves, and large amount of genetic variation in most attributes (Barrett et al. 1988). On the negative side, black locust has been plagued by the locust borer, Megacyllene robiniae Forster, which attacks the stem and causes deformation or breakage. This single factor has been primarily responsible for the lack of attention to the species for lumber production. In other areas of the world where black locust has been introduced and the borer does not exist, the species is attracting wide interest and utilization (Keresztesi 1988). The tree also tends to have a crooked stem but this defect has been remedied by selection and breeding (Keresztesi 1983; Hanover et al. 1989). For the fiber production systems proposed here both borer attack and crooked stems would be of little consequence due to the short rotations used.

BLACK LOCUST FOR FIBER AND BIOMASS

There are at least six ways in which black locust can be used as a fiber crop or to generate large amounts of biomass at relatively low energy inputs. These include pulp for paper, leaves and young stems for fodder, leaves and young stems for solid, liquid or gaseous fuels, and extraction of specialty chemicals such as natural wood preservatives. There now exists a substantial natural resource of black locust timber and poles of fence post size growing throughout its natural range and now appearing in colonized areas outside that range. The species is prized for making fencing and post products and paper companies harvest it for pulp. Due to its dispersion or lack of concentration in pure stands and usually poor stem form, there is virtually no black locust lumber used in commerce. This situation could change when new, improved genetic materials become widely planted and commercially available because the wood is of very high quality; in many respects superior to species like teakwood or black walnut.

What are the specific characteristics of black locust which make it a desirable fiber source? Black locust wood has a large portion of uniformly distributed libriform fibers which confers great strength to wood. It has an average specific gravity of 0.68 compared to other North American hardwoods which average 0.51. Its average fiber length is 1.05 mm, slightly shorter than other hardwoods which average 1.13 mm. The central stem or pith of young black locust has a fiber length of 0.75 mm and specific gravity of 0.57, still well above the average mature hardwood values.

Some other important characteristics of the wood of black locust are listed in Table 1 and 2. Of particular interest are: (1) very young plant material contains no heartwood with all of its associated extractive chemicals; (2) the caloric content of young material is high and unchanged with age; and (3) the moisture content is very low relative to other species.

Less information is available regarding the chemical and physical characteristics of black locust leaves compared with the wood. Leaves are very high in nitrogen and have been used in animal feeding trials with mixed success (Baertsche et al. 1986; Cheeke et al. 1983). Baertsche et al. (1986) compared leave-stem mixtures of ten woody plants and alfalfa for chemical compositions and found black locust to be superior to all species in crude protein content (22.3% of dry matter).

Black locust also has potential to serve as a source of energy. According to Abelson (1991) there is great potential for energy crops in the United States in the future, and this species should be considered along with a wide array of other species. The wood is also a veritable chemical storehouse with extractives comprising 11% of dry weight (Table 1). Thus, black locust should be considered as a potential source of natural products just as other crops are being developed for their unique extractable natural products (Hanover 1990; Simon et al. 1990; Turick et al. 1991).

A critical consideration in evaluating the potential of a new crop for fiber production and commercial utilization of its components is the efficiency and economy of producing the new crop usually under quite different or non-conventional cultural conditions. It is in this context that we are focusing on black locust, i.e., development of very efficient, large scale cultural systems for furnishing the raw material to be used for any of the purposes stated above.

CULTURAL SYSTEMS

Black locust lends itself admirably to direct seeding much as is now done conventionally with agricultural row crops. The seed must be pretreated with acid (H2SO4 for 50 min.) to allow it to germinate and germination rates are very high. We have successfully drill-sown several plantations and achieved good stands rather rapidly. Because black locust is one of the fastest growing species in North America, it literally appears to outcompete weed competition early in development. Individual trees can reach 3 m in height in one year, but the average is closer to 1 to 2 m, depending on soils and other conditions. Genetic selection and breeding efforts now underway should further enhance yields (Hanover et al. 1989).

Because black locust sprouts readily from the roots and regrows from cut stems, we have several options for regenerating another crop either in one growing season or over several seasons. Thus, the need to reestablish by seed is eliminated at considerable reduction in energy inputs for many years. A drill-sown, vigorous stand of closely spaced (30 cm) black locust can be harvested in July, in Michigan, and by early September another crop will have regrown. Alternatively, the initial crop can be allowed to grow the entire season and be harvested before or after leaf fall in October, depending upon the product to be extracted. Each of the six potential uses of the material generated now need to be closely examined to determine the best methods for harvesting and overall economic feasibility.

SUMMARY

Black locust has physical and biological characteristics that make it a prime candidate for fiber production. These include: very rapid early growth; the ability to fix N2; reproduction from root sprouts and coppice; wide climatic and edaphic adaptation; good fiber quality; high density wood; high caloric content; low moisture content; high protein content; high genetic variation; potentially useful chemical extractives; amenable to intensive culture management. Research should now focus on the genetic improvement and cloning, the utilization of leaves and stems, and the development of high yield production systems.

REFERENCES

*This research was supported by the Michigan State University/USDA/CSRS Eastern Hardwood Utilization Research Special Grant Program (Grant No. 91-34158-5895).
Table 1. Main-stem wood properties of ten- to twelve-year-old black locust treesz.

Property Average Range
Stem volume (m3) 0.043 0.03-0.05
Wood (%) 84.7 80.0-86.3
Heartwood (%) 54.1 34.8-60.2
Specific gravity 0.68 0.65-0.71
Ash content (% dry mass) 0.62 0.47-0.74
Fiber length (mm) 1.05 0.94-1.11
Extractives (% dry mass)
Benzene-EtOH 3.5 2.7-3.9
EtOH 1.1 0.7-1.6
H2O (hot) 2.8 2.4-3.1
Total 7.4 6.2-8.3
zModified from Stringer and Olson (1987). Main-stem defined from groundline to 80% total tree height. Mean dbh (diameter at breast height) and height of the 10 trees sampled was 12.5 cm and 10.5 m, respectively.

Table 2. Stem diameter variation in wood propertiesz.

Mean±SE
Diameter class
(cm)		 Specific
gravityy		 Caloric content
(cal/g)x		 Moisture content
(%)x,w		 Heartwood content
(%)y
0.1-2.5 0.549 4641±52 41.1±16.5 absent
2.6-5.0 0.588 4644±58 38.0±16.1 3.4
5.1-7.5 0.644 4637±34 33.2±9.7 28.2
7.6-10.0 0.658 4665±42 26.7±6.4 38.0
Mean 0.609 33.1
zIncludes mainstem and branch material from 2- to 10-year-old black locust trees.
yFrom Stringer (1981).
xFrom Stringer and Carpenter (1986).
w% wet-weight basis.

Last update April 23, 1997 aw