Béla Keresztesi
BÉLA KERESZTESI is Director-General of the Forest Research Institute at Frankel Leo University in Budapest. This article is adapted from a paper he gave at the FAO Technical Consultation on Fast-growing Plantation Broad-leaved Trees for Mediterranean and Temperate Zones, Lisbon, October 1979. The meeting was held in cooperation with the International Union of Forest Research Organizations (IUFRO).
Robinia pseudoacacia, grown in many parts of the world' now rivals the poplar as the second most extensively planted genus after Eucalyptus.
In 1601, the black locust (Robinia pseudoacacia L.) became the first tree species to be brought from North America to Europe. Since that time it has become widely distributed not only in Europe but throughout temperate and Mediterranean zones of the world. It is not suited to lowland tropics. It tolerates a wide variety of soils, with the exception of extremely dry or clayey ones. Its ideal habitat is one with a temperate, moist climate and silty or sandy loams of loose structure.
According to currently available data there are now about a million hectares of man-made black locust forests. The most important cultivating countries are, in order of area, the Republic of Korea, Hungary, the USSR, Romania, France, Bulgaria, Yugoslavia, Czechoslovakia and China. On the basis of area, black locust occupies second place in the world after Eucalyptus among fast-growing broad-leaved species.
Farmers find that black locust meets nearly all the demands of rural life. It can be planted and regenerated easily, has no serious diseases, and grows well on good sites. The wood is hard, has great strength and durability, and is easily workable. The timber may soon be utilized for both high-quality fibreboard and for structures produced from glued and jointed black locust boards. Poles may be utilized by the mining industry, vineyards and orchards. Because it is abrasion-resistant and of good colour, interior architecture can use black locust for parquetry and wall panelling. Since it is very durable, black locust is also good for garden furniture and equipment for forest recreational areas, and outdoor sports activities. Because of its high caloric value and its ability to burn even when wet, it is popular also as firewood.
With oil prices rising, the importance of black locust as a nitrogen-fixing species should be underlined. The mass application of fertilizers in agriculture and forestry may become more and more expensive in the future; therefore nitrogen-fixing tree and shrub species will gain more importance in the forestry of the future.
The black locust may also be encountered in urban street plantings as a decorative tree. In addition it is one of the most important species of the forest bee pasture. It is a favoured food of forest game. In Bulgaria and the Republic of Korea the foliage is used for livestock. And finally, it is important both in erosion control and in the afforestation of gullies and of strip-mined areas. It should be noted that it has a capacity for vigorous regeneration by root suckers. This can be a useful quality where black locust is planted for such purposes as erosion control, but it can also be a bother under other conditions.
In order to determine the economic viability of black locust, the Hungarian Forest Research Institute (ERTI) carried out detailed studies of costs, revenues and profitability in 1968. These studies also show that for wood production of Robinia to be profitable it is necessary to plant only better-than-average sites. However, the calculations were restricted to the timber yield so that other natural products, such as honey, were not taken into consideration. The analyses were based on summarized data from a nationwide survey. The costs for afforestation, tending, exploitation and total prime costs were investigated separately.
The initial seedling crops are approximately 10 percent less profitable than the subsequent coppice crops from the same root stocks. Returns from intermediate cutting were not taken into consideration, as the Hungarian experience has shown that the return from sales roughly equals the respective costs in these cases.
Due to its favourable properties and manifold uses, black: locust has been. extensively planted in the United States on abandoned farmlands for timber production and erosion control. Out of the 170 million plants used for the reforestation of the Tennessee Valley, 65 million were black locust. In 1941 alone, 25 million plants were put down, covering 5 600 hectares of forest.
The species is also extensively used for afforestation of strip-mined land. In the central states region, 100 000 hectares had been strip-mined for coal by 1947, 15 000 of which were afforested. Since that time the tempo of stripping has not declined. On strip-mined land, black locust is planted as a pure stand or as an admixed species to control erosion and to improve soil quality. According to the most recent data, 200 hectares of black locust stands are planted annually.
Outside of the United States, the broad site-tolerance and excellent silvicultural properties of black locust have allowed it to spread rapidly. For instance:
Figure 1. Natural range of black locust in the United States
· In Hungary black locust has the widest distribution among fast-growing tree species, occupying 19 percent of the total forest area in 1976, or 271 thousand hectares. A tree species is "fast-growing" if its mean annual increment surpasses 10 m³ per hectare (Table 1).· In the USSR there are about 144000 hectares of black locust forests. Most stands are encountered in certain areas of Moldavia and the southern Ukraine. The species is also used for planting in urban and rural settlements.
· In the Federal Republic of Germany black locust occupies about 3 000 hectares.
· About 3 000 hectares are registered in the German Democratic Republic v/here successful afforestation :has been carried out on strip-mined areas.
· In Romania a total of 100 000 hectares of black locust stands may be found. The centre of the cultivation is the excellent Calafat Sand region, which is the biggest contiguous black locust plantation area in Europe.
· In Czechoslovakia there are 28 000 hectares of black locust forest, concentrated mostly in southern Moravia, central Bohemia and Slovakia.
· In Yugoslavia black locust plantations amount to 50 000 hectares. Further plantings are planned on the Deliblat Sand Plateau.
· It is known that the Republic of Korea has areas planted to Robinia well in excess of 100 000 hectares. It is used mostly for firewood production, erosion control and animal feed. The species is therefore widely distributed in agricultural regions.
· There are 73 000 hectares of black locust stands registered in Bulgaria (mostly in southern Dobrogea), where it is the most extensively planted exotic species in afforestations, occupying 2.3 percent of the total forest area.
· In France 1 00 000 hectares were inventoried in 1979, centring in the Paris-Lyon-Belfort triangle.
· In Argentina there are about 3 000 hectares of locust forests, including some irrigated plantations.
· Spain has approximately 3 000 hectares of black locust stands.
Seeds. The annual plant demand in Hungary (5, 7) amounts to 25-30 million seedlings, a quantity which may be raised from about 2.5 tons of seed. The oldest and highest quality stands are to be found in the sandy Pusztavacs forest district, where all of the needed seeds may be collected. A machine was constructed here to screen the seed from the top 10 cm of soil layer, where seeds had accumulated over decades.
In the forest district of Pusztavacs, about 770 kg of seed per hectare may be screened from the soil under 30 year-old locust stands, a figure which equals a yield quantity of approximately 10 seed years. Suitable scarifiers are used for the preparation of hard-coated black locust seed for sowing.
The raising of black locust seedlings presents no special problems. Soil preparation, cultivation, protection and lifting are done in the same way as with other species. The minimum plant size is 4-mm stem-diameter, measured 4 cm above the root collar. It is advisable to grade the plants before planting into two size classes below and above 8 mm, and to outplant them on separated areas.
Figure 2b Machine set for screening of black locust seed from the topsoil: double tumbler
Figure 2c Machine set for screening of black locust seed from the topsoil: seed cleaner screen
Figure 3. One-year-old black locust plants in the nursery of Derecske
In Bulgaria 25 million seedlings are produced annually.
In Hungary new black locust stands are planted, whereas both planting and coppicing are practiced for regeneration. Since coppicing is cheaper, it is chosen wherever possible.
For regeneration in sandy regions, operations are concentrated on big clear-cut areas. Stump removal is done with grubbers. The stumps are pushed together with bulldozers into parallel lines to clear rows for planting. Ploughing is done at a depth of 6070 cm to prepare the soil.' A quadratic pattern is used for planting. Mechanical cultivation in both directions is continued until closure.
An up-to-date machine system is available for clear-felled plots. It includes an S- 100 caterpillar tractor, grubber type K 1-A or TK-1, plantation ploughs PP-50-PG and Nardi OODMR/F, one- and two-row planting; machines SLC-1 and ERTI, MTZ-:50 wheel and narrow-gauge "TL" cater. pillar tractors, and various disc-tillers.
The choice of coppicing is deter. mined by the age of the stand. The lifespan of black locust trees and roots is estimated to be about 90 years. Con. sequently, coppicing is possible three times on planted stands in a 20-year rotation or twice in a 30-year rotation. The application of the ERTI-type subsoil loosener proved promising for coppicing. Following slash removal this device can cut rows 80 cm wide and 70 cm deep 1.5-2.5 m apart. When encountering a stump, the blade passes over, but roots are cut with the edges of the winged loosener. This also aerates the soil, improving the conditions for coppice growth. Traction force demand of the subsoil loosener is about 90 hp for wheel tractors (type D-4K-B) or 75 hp for caterpillars (DT-75). The daily performance varies from 3 to 5 hectares.
Black locust is also planted in mixture. It thrives especially on the oligotropic dry calcareous sandy soils of the sand-area between the Danube and Tisza, where during the last decades about 30 000 hectares of Scots and Austrian pine stands mixed with black locust were established. In pure pine stands on these sites dry leaf-litter is formed, slowing the start of the decomposition of needles. In pine stands mixed with black locust, however, the decomposition is accelerated, reviving the cycling of nutrients and thereby fixing nitrogen into the soil. According to the experience of forest engineers, pine stands mixed with black locust produce healthier, better growing pines.
Figure 4. Collection of legumes in the seed orchard of Albertirsa
Figure 5a. Rooting of green cuttings: collecting green cuttings
Figure 5b Rooting of green cuttings: mist propagation with soil heating in foil house
Figure 5c Rooting of green cuttings: rooted cuttings
Figure 6a. Plant production from root cuttings: preparation of root cuttings
Figure 6b. Plant production from root cuttings: root cutting
Figure 6c. Plant production from root cuttings: cuttings taking root in foil bags
Establishment and tending of forest stands are regulated in Hungary by so-called "cultivation models" (8). A tree cultivation model describes numerically the combined functioning of biological processes and programmed silvicultural activities in terms of time and stand parameters.
The standardized black locust cultivation model (Table 1), drawn up in 1976, contains data on the most important stand-structure and yield parameters for tending, intermediate and final felling in four yield potential groups-A, B. C and D-(8).
A silvicultural model comprises the up-to-date, rational principles of black locust cultivation in a uniform system. The stand treatments are reduced to a minimum. The model tables contain in general two tending operations and three, two, one or zero intermediate cuttings, depending on the yield potential group. Accordingly, the cuttings are heavy. In groups A and B the final stand stem numbers are much lower than usual, as sawlog dimensions were set as ultimate production goal, following the demand in the sawmill industry. Therefore felling age was significantly increased in the productive yield classes. Through the consequent application of the model a substantial increase in industrial timber output may already be anticipated in 15 to 20 years. In group A, for example, an average diameter of 32 cm may be achieved at age 40, with 350 stems per hectare and a volume of 400 m³/ha. In order to achieve these goals, the selection of the best stems must be started together with the tending operations and pruning.
Different results must be expected in different areas. In the Republic of Korea, for example, the growth of black locust is remarkable, as shown by Table 2.
In Hungary, by contrast, it is assumed that the growth of locust de creases so much after 35 years of age that maintaining the stand is no longer advisable.
Frost is the most important abiotic damage in Hungary to which black locust, as an exotic species, is susceptible. :Frost-damaged shoots and cankerous growth are frequent. In frost pockets or on unsuitable sites trees become dwarfed as a consequence of repeated frost damage. Height and diameter growth decrease and various secondary pests and diseases attack. the trees.
In addition, because black locust branches are fragile and the wood splits easily, the tree suffers frequently from windbreak, glazebreak and snow-break.
Table 1. Black locust compared to poplars in Hungary (Current annual increment at the average age of 10 years)
|
|
m³/ha |
Metric tons/ha |
|
Euramerican poplars |
17.09 |
7.01 |
|
Black locust, high forests |
8.76 |
6.39 |
|
Black locust, coppice forests |
6.80 |
4.96 |
Biotic enemies, mostly insects and mammals, are most active in nurseries and afforestations. Owing to the high regenerative potential of the species, root damages (cockchafer and other larvae) as well as stem and leaf damages (hemp-seed beetle, turnip moth, rabbits) have no lasting effect on its growth.
The locust shield scale (Lecanium corni Bache) is a secondary pest, which appears in large numbers in stands which suffer from other, mostly abiotic, damages such as frost, unsuitable site conditions or inadequate light.
The main wood-destroying diseases are Fomes fraxineus (Fr.) Cooke, Grifola sulphurea (Bull)/Pil. and Phellinus robustus (Karat.) B. & G. The first occurs sporadically, but may cause considerable damage while the other two affect primarily already damaged roadside trees. The best prevention against such fungal diseases is to avoid mechanical damage to stems and roots. In coppice stands, in grazed forests and on pruned stems such wounds are frequent.
Table 2. Growth performance of black locust in the Republic of Korea
|
Age |
Mean DBH |
Mean height |
Single tree volume |
Stem number per ha |
Volume per ha |
Mean annual increment |
|
cm |
m |
m³ |
|
|
m³ |
|
|
12 |
15 |
16.4 |
0.15 |
1 143 |
165.89 |
13.82 |
|
36 |
30 |
25.8 |
0.91 |
508 |
463.30 |
12.87 |
|
44 |
40 |
26.8 |
1.68 |
508 |
855.66 |
19.45 |
Source: Kwangnung Experimental Forest, Republic of Korea.
Figure 7a. Regeneration of black locust forests by coppicing: ERTI-type subsoil loosener
Figure 8a. "Ulloi" black locust variety: selected stand
Figure 8b "Ulloi" black locust variety: control stand
Figure 9a. Black locust variety "Szajki" is greatly needed: Selected plus trees
Figure 9b. Black locust variety "Szajki" is greatly needed: Selected plus trees
Figure 10a. Propagation of selected plus trees by grafting: understock and scion
Figure 10b. Propagation of selected plus trees by grafting: flushing grafts
Figure 11a. Clone test at Gödöllö, Hungary: R.p. cv. Jászkiséri, 15 years old
Figure 11b. Clone test at Gödöllö, Hungary: R.p. cv. Pénzesdombi, 14 years old
Figure 11c. Clone test at Gödöllö, Hungary: R. ambigua decaisneana cv. rose-AC, 12 years old
Figure 11d. Clone test at Gödöllö, Hungary: Control plot 15 years old
In 1975, 1 386 3000 m³ of black locust timber were felled in Hungary.
The average DBH (with bark) of final fellings was 17.6 cm, while thinnings came to 11.9 cm. Uses are shown in Table 3.
Wood-quality investigations show that black locust is very hard and of medium weight. It is stable, and durable even under extreme conditions of chemical application, humidity and heat. Industrial utilization is improved by positive mechanical features, but hindered by logs of small diameter and frequently crooked stems, as well as by branch stubs and rotten heartwood.
The colour of black locust wood is yellowish green, but may be altered by normal and high pressure steaming. The possible colour shades are: yellowish green, golden yellow, yellowish brown, light brown (oak colour) and deep brown. Heat treatment improves workability and decreases shrinkage, but reduces strength. The wood can be seasoned well.
Healthy logs of sufficient diameter may be utilized for veneer production. Steamed at 1-2 atmospheres, yellow, brown or nearly black shades of quite uniform quality can be achieved.
In the fibreboard industry black locust is used mostly in dry technology. In a semi-operational experiment in Hungary, at the Mohacs fibreboard factory, fibreboards of good quality were produced with a slightly altered wet technology out of pure black locust chips produced with a Morbark TL-22 chipper. The costs were the same as for traditional tree species. The use of black locust as fibreboard raw material makes it possible to utilize the medium- and poor-quality stands industrially. It :may be mixed into particle boards as well.
Pulping of black locust is not economical in the cellulose industry. On the other hand, it is suitable for charcoal burning and dry distillation.
Up-to-date architectural structures can be produced, using glued and finger-jointed black locust boards which were first applied. in agricultural buildings (2). Black :locust posts are easily marketable for use in vineyards and fruit orchards. Transport wine barrels can also be made.
In the furniture industry, the wood is used in natural or damped quality for various elements. It is gaining popularity in the sports-articles industry because of its excellent strength values.
Table 3. Uses of black locust wood in Hungary
|
Assortment |
Thousand m³ |
Percentage |
|
Sawlog and veneer log |
146.3 |
10.5 |
|
Other timber for sawmilling |
169.6 |
12.2 |
|
Pit-props and posts |
73.7 |
5.3 |
|
Pulpwood |
69.4 |
5.0 |
|
Fibre- and particle board |
32.6 |
2.4 |
|
Other industrial wood |
164.7 |
11.9 |
|
Total, industrial wood |
656.3 |
47.3 |
|
Firewood, thick |
604.1 |
43.6 |
|
Firewood, thin |
125.9 |
9.1 |
|
Total, felled volume without bark |
1386.3 |
100.0 |
Note: Average DBH with bark of final fellings was 17.6 cm. Average for thinnings was 11.9 an. Total of all black locust harvested in Hungary came to 1 386.3 in 1975, the year of these figures.
Figure 13a. Black locust exploitation: felling with motor saw
Figure 13b. Black locust exploitation: hauling
Figure 14a. Construction from finger-jointed and glued black locust boards
Figure 14b. Construction from finger-jointed and glued black locust boards
Its durability and pleasing appearance make it suitable for parquetry and wall covers as well. In the mining industry the species is utilized for pit-props and boards. Black locust firewood is popular for its high calorific value and for its good combustibility, even in wet condition.
Honey is the most important minor product associated with black locust. In fact, it provides the basis for Hungary's commercial honey production. Between 1885 and 1970, as the land area occupied by black locust increased, the trees became the principal resources for bee-keeping. Without them no commercial apiculture could exist, given present large-scale, mechanized and chemical technologies in agriculture.
In favourable years of black locust powering, two thirds of the annual honey production of 10 000 tons is black honey. It is characterized by yellow colour and a mild flavour. Crystallization takes place very slowly, sometimes only after years.
The honey yield is the highest in 10-to 20-year-old stands and lower in younger or older ones. The value of the honey yield of even a low-producing 40-year-old stand at an interest rate of 3 percent amounts to 30 622 forints per ha, which means that the honey production may cover the costly planting with seedlings. (As a point of reference, the tourist exchange rate for the forint was 20.31 to the US dollar in 1979.)
The primary benefit of bee-keeping, however, is not the honey itself but the indirect gain through the pollination activity of bees. In fruit and seed production, the value of the additional yield was estimated to reach 3 000 million forints annually. It must be underlined that this gain can be reached only as long as black locust forests are available for bean pastures.
In the Republic of Korea black locust leaves are used for forage (1,6). Pulverized leaves, at a 30 percent ratio, are mixed into rice bran as forage for pigs. In chicken feed, they can replace alfalfa. For this purpose a tetraploid black locust, the clone Robinia pseudoacacia 'Gigas', is used, having leaves three times bigger and with 1.4 times more protein content than normal diploid black locusts.
Since its introduction to Hungary in the eighteenth century, black locust has been used not only as a forest plant but also for establishing forest patches, forest belts and shelterbelts and for greening the surroundings of farms and rural communities. In the beginning large estates introduced the species for planting extensive quicksand areas, but black locust soon became the favourite tree species of the Hungarian peasants. The people of the nearly treeless Hungarian plain discovered arboriculture through the black locust.
In Bulgaria there is practically no village or town without black locust in roadside plantings, not only because of its decorative flowers but also because this species tolerates pruning easily.
Black locust was first planted in China on the Shantung Peninsula, between 1900 and 1918. Since then the species has found broad distribution along the Huanghe river in the north and in the Changjiang valley in the south. In these regions locust may be encountered everywhere in shelterbelts and forest patches amid agricultural lands. The well-acclimatized black locust plays an important rode in protection forestry and in erosion control.
Since black locust is relatively easy to introduce and since it has such a wide range of economic, aesthetic and ecological uses, it seems an almost ideal tree to introduce into developing countries with temperate or Mediterranean climates and appropriate soils. It is particularly well suited for rural forestry. It is indeed a tree with a history of many uses and, therefore, with a future of uselfulness as well for many parts of the world.
Figure 16. Production of black locust vineyard poles
Figure 17. Week-end house, made from black locust
Figure 18a. Black locust mill of the Felsótisza Forest Company at Hajduhadház, Hungary: log yard
Figure 18b. Black locust mill of the Felsótisza Forest Company at Hajduhadház, Hungary: sawmill hall
Figure 19a. Products of the Hajduhadház mill: parquets
Figure 19b. Products of the Hajduhadház mill: pallets
Figure 19c. Products of the Hajduhadház mill: structural members for agricultural buildings
All photos by ERTI photographers István Michalovszky and René Jerome, except 15a and b, by Jánosné Molnar of FKI.
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