Site Requirements and Stand Establishment Techniques for Black Locust (Robinia Pseudoacacia l.) Stands in Hungary

0320-B2

E. Führer^[1] and K. Rédei

Abstract

Black locust (Robinia pseudoacacia L.) is one of the most important stand forming tree species in Hungary, covering approximately 20% of the forested land and providing 25% of the country’s annual timber cut. One third of these black locust stands is high forest (seed origin) and the rest is of coppice origin. This paper investigates the impact of site conditions and stand establishment (regeneration) techniques on wood production, the quality of stems and the health of trees in black locust stands. Our data show that under suitable site conditions for black locust, professional and careful regeneration from root suckers there is no considerable difference between the average quantity and quality of stems as well as the health of trees in stands regenerated from root suckers and seedlings. On the basis of these results and considering the economic requirements, regeneration of black locust stands from root suckers may be recommended on sites of yield classes I-III (Rédei, 1984) on a larger scale.

Introduction

Black locust was the first forest tree species to be imported from North America to Europe (to France) sometime after 1601. Its rapid spread in Europe and other parts of the world may be attributed to its adaptability to a range of conditions, favourable breeding properties, frequent and abundant seed production, excellent coppicing, fast growing and high yield, as well as its highly valuable timber.

In Hungary, black locust is the most widespread tree species occupying approximately 20 % of the forested land (350 thousand hectares) and providing 25 % of the annual timber cut of the country. The mean crop volume of all black locust forests is 125 m³/ha, with a mean volume of 190 m³/ha at the age of final cutting (31 years on average). Black locust forests in Hungary have been established on good as well as medium and poor quality sites. Establishment of black locust stands producing timber of good quality is possible only sites with adequate moisture and well aerated as well as loose structured soil that are also rich in nutrients and humus. Black locust forests on medium and poor site quality are managed for production of fuel wood, fodder, poles and props, as well as for melioration.

Locust's role in fixing atmospheric nitrogen is one of its most valuable attributes. By serving as a host for fixing and recycling nitrogen through leaf and twig litter, black locust seedlings improve site quality. Rapid cycling of nitrogen into the soil allows relatively quick improvement of the soil nutrient status, enhancing succession from locust to other, possibly more valuable tree species, and allowing rehabilitation of otherwise non-productive land. It may even be by using more effective soil management techniques or by genetically improving black locust.

Black locust can be regenerated naturally, from root suckers, or artificially, i.e., with seedlings. For establishment new black locust plantations (stands) seedlings are also used. There are some favourable plant characteristics of black locust which make both regeneration methods possible. For seedlings growing seeds are produced in a wide range of conditions, germinate rapidly, and preserve their germination capacity for a long time. Black locust cannot be regenerated by seed in natural way due to its vary hard seed-coat. On the other hand the root system is very plastic, its vegetative growth from fragments is intensive and it is hard to uproot.

Besides Hungary and other European countries where the black locust is much preferred in the forestry practice (Romania, Bulgaria) there are two bigger regions where the fast spread of this species can be expected. In Europe the Mediterranean countries (Italy, Greece, Spain and Turkey), while in Asia China and Korea may be the most prominent black locust growers. In there regions black locust has been widely valued as a tree species that performs well in reclaiming disturbed lands as well.

Site requirements for a successful black locust management

The variable nature of geographical conditions in Hungary and the large area of black locust stands have made determination of its site demands possible and the characterization of suitable sites. This task has been solved by the Hungarian system for forest site classification (JÁRÓ, 1968). We are able to tell not only the site types where black locust growing is recommended, but the expected yield and profitability. The classification of forest sites is based on four dominant factors, which are: climate, hydrologic conditions (non-precipitation water sources, like ground-water, inundation waters etc.), genetical soil type, physical make up and rootable depth.

As for the climatic demands of black locust, the climatic conditions in the Sessile oak-Turkey oak (Table 1) and forest-steppe zones of Hungary (Table 2) cover its requirements. It is susceptible to late and early frosts, therefore, it is not recommended for sites in the higher hilly zones and where frost hollows are present. Good results with this tree species can be attained in regions where the mean annual temperature is over 8°C.

Table 1. Site types suitable for black locust in the sessile oak-Turkey oak climate zone

Table 2. Black locust site types in the forest steppe climate region

The genetical soil type, rootable depth and physical make up in the soil are the factors which must be regarded when planting black locust. From this point of view soils of shallow rootable depth, of poor water regime and coarse sand or with many stones, are unfavourable for black locust growing. Clay texture is also unfavourable due to poor aeration and compact condition. Fine sands and light loamy soil types are good for black locust, provided the rootable depth is enough.

Stand establishment techniques

When making semi-natural or man-made afforestation or reforestation with black locust the following basic technologies and operation groups are:

• Black locust afforestation with deep loosening: soil preparation (without trenching) by deep loosening of soil, planting by planting-machine or tractor drawn pit-drilling machine, manual soil cultivation in the rows, in interrows by machine.

• Black locust afforestation with trenching or deep- ploughing: trenching or deep ploughing, planting by planting machine or tractor drawn pit-drilling machine, manual soil cultivation in the rows, in the interrows by machine.

• Semi-natural reforestation by root-suckers: slash removal from the cut-area, bush-cutting, root-ripping, knocking down of coppice shoots, singling of clumps of shoots.

• Man-made reforestation of black locust stand by deep loosening: salch removal, bush cutting, chemical treatment against sprouting, deep loosening, planting by machine or tractor drawn pit-borer, knocking down of coppice shoots, manual soil cultivation in the row and mechanized in the interrow.

• Man-made reforestation of black locust stands by complete soil preparation: slash removal, bush cutting, stump removal (stump-lifting, removal and terrain leveling), trenching, planting by machine or tractor mounted pit-borer, manual soil cultivation in the rows and mechanized in the interrow.

The best for planting is the spring. The most popular spacing for planting is 2.4 m between rows and 0.8-1.0 m within rows (4000-5000 seedlings/ha), age of planting stock: 1 year, of seedbed quality. Planting may be by machine into split or in a pit manually prepared or tractor mounted borer. Coppicing by root ripping provides abundant root suckers, when the roots have been wounded. This operation is made with a winged deep-loosening machine working at a depth of 35-40 cm.

Criteria for accomplished afforestation: at least 3500 plants/ha alive when planting was carried out with seedlings, in coppiced young stands at least 5000 suckers/ha which must be of not less than 3 m height and consist of non-forked healthy trees, regularly distributed.

The impact of regeneration on yield of black locust stands

Our trial on black locust regeneration methods is found in the Danube-Tisza Interflow region, in central part of Hungary (subcompartment Pusztavacs 224C). Regeneration of the former black locust stand of coppice origin was carried out by the following methods:

• from suckers developed from large roots after felling trees with stump extraction (I);

• from suckers developed from small roots after felling with stump extraction and removal of large roots (II);

• with seedlings planted into deep-ploughed soil, after trees had been removed with stump extraction (III).

Regeneration and tending cuttings were carried out in conformity with the practices of intensive silviculture of that time. A precommercial thinning was carried out at age of 5 and 10, a selective thinning at the age of 15 and an increment thinning at the age of 20. The stand was harvested at the age of 34 and regenerated with seedlings. There was no replication in the trial.

According to the Hungarian classification of site types, the main ecological characteristics of the study area the following (JÁRÓ, 1968):

• forest steppe climate zone: the humidity is less than 50 % in July at 2 pm, the annual precipitation is less than 600 mm,

• hydrology: free draining,

• genetic soil type: combination of humic sand soils.

According to the yield table for black locust stands (RÉDEI, 1984) the yield class of the investigated stands is II, the second best out of VI.

The following parameters were measured and calculated at the age if 6, 17, 24, 29 and 34 years: stem number, d.b.h., basal area, tree height, stem volume, stand volume and mean tree volume. Stem volume was estimated using the volume function (SOPP, 1974):

V=10^-8 d² h¹(h/[h-1,3])^p° (p₁d h+p₂ d+p₃ h+p₄),

where d is diameter at breast height (cm), h is tee height (m), p_o=4,000, p₁=-0,6326, p₂=20.23, p₃=0,00 and p₄=3034.

To characterize stem quality for calculating the stand-value index, four quality classes (1-4) were used at the age of final cutting:

1 - trees providing high quality industrial,
2 - trees providing lower quality industrial wood,
3 - tree suitable for short logs of poor, quality,
4 - trees suitable for firewood only.

Stand-value index: weighted arithmetical mean of stem quality values.

The most important stand parameters of the plots are included in Table 3.

Table 3. Stand characteristics of experiment Pusztavacs 224 C

On the basis of the data, there is no considerable difference in stem number between plot I, coppiced from suckers developed from large roots, and plot III, regenerated with seedlings. In plot II, regenerated from suckers sprouted from small roots, a number of sprouts died, beating-up was not effected, the missing seedlings were replaced by sprouts again. Note that at the age of six, the difference in stem number was 1188 between the plot regenerated from suckers from large and the plot regenerated from suckers from small roots.

Mean height as a function of age (Figure 1.) shows that the tendency of height growth patterns in black locust stands of seed and coppice origin are very similar. At the age of final cutting, the mean height of crop regenerated from suckers developed from large roots is just as large as that of the crop regenerated from suckers developed from small roots. The height growth rate of crop regenerated with seedlings is somewhat lower than that of the crop regenerated from suckers.

Figure 1. Mean height values of the crops

In spite of the difference in stem number between the plots regenerated from suckers sprouted from large and small roots, the DBH growth rates are similar, while the crop regenerated with seedlings produces the lowest DBH values after the age of 20 (Figure 2.).

To compare the yield of crops regenerated by different methods at the age of final harvesting, mean tree volume is used (Figure 3). Taking the arithmetic mean of tree volume as 100 % for the whole experiment (478.3 dm³), plot II, regenerated from suckers developed from small roots had the highest mean (105.9 %). It is followed by the crop regenerated from suckers sprouted from large roots (plot I, 103.3 %) and by the crop regenerated with seedlings (plot III, 90.8 %). These data show that the crops raised from suckers produced trees of a bit higher dimension in both cases than the crop of seed origin.

Figure 2. DBH values of the crops

Figure 3. Mean tree volume of the crops at the age of final cutting

As for stem quality of the experimental stands, the calculated stand-value index was 2.2 in plot I (large roots), 2.0 in plot II (small roots) and 2.1 in plot III (seedling). The differences between the plots are not significant, in other words, stem quality does not depend essentially on the method of regeneration.

After harvesting, the degree of butt rotting caused by Fomes fraxineus Cooke, was less than 50 cm in 72 %, and 50-100 cm in 28 % of the investigated trees. Butt rot over 100 cm was not found in any tree. The butt rotting degree (ratio of damaged and healthy tree in the plot) was 46 % in plot I (large roots), 43 % in plot II (small roots) and 40 % in plot III (seedlings). From these data it is evident that the infection caused by the butt-rotting fungus does not depend essentially on the method of regeneration.

Conclusions

The investigations at Pusztavacs experiment showed that under suitable site conditions for black locust the mean tree volume of the stands established by various coppicing methods was higher by 12-15 % at the age of final harvesting than that of stand regenerated with seedlings. The stand of seed origin did not produce better stem quality than stands of coppice origin, i.e., the coppice stands did not produce less valuable timber assortments for industry than stands of seed origin.

No information is available as to how many times the stand in question was coppiced. Therefore, the question whether the stand is to be regenerated by coppicing or not should be decided on the basis of its growing rate and health. Our experiences from investigations in many stands showed that black locust stands of good and medium quality (yield classes I, II and III) may in general be regenerated from suckers until their growth rate attains, or exceeds the volume increment of yield tables for the yield class in question. A stand can be declared as healthy if only less than 50 % of the stumps are butt rotted and rotting in trunks does not reach 1 m.

References

1. Führer, E., 1998. Characterization of black locust from ecological aspects. In: Rédei.K. Black locust growing in Hungary. Issues of Hungarian Forest Research Institute. No. 11., Budapest.

2. Járó. Z. 1968. The funfamental principles of the site-typological systematisation. Kísérleti Közlemények, 61/D, 1-3:11-25.

3. Keresztesi, B. 1987. The evaluation and results of the trial carried out on black locust regeneration. Erdészeti Kutatások 79:7-17, Budapest.

4. Rédei, K. 1984. Yield of black locust stands. Research report. FRI, Kecskemét.

5. Rédei, K. 1987. Yield study relations of regeneration of black locust stands. Erdészeti Kutatások, 79:63-69., Budapest.

6. Rédei, K. edit. 1998. Black locust growing in Hungary. Issues of Hungarian Forest Research Institute No. 11., Budapest.

7. Sopp, L. edit. 1974. Volume tables. Mezõgazdasági Kiadó, Budapest, pp. 89-93.