GROWTH OF TREES USED IN REFORESTATION OF A NORTHERN SLOPE OF THE EXTERNAL SPOIL BANK OF THE "BEŁCHATÓW" BROWN COAL MINE

Marek Pająk, Marcin Forgiel, Wojciech Krzaklewski

ABSTRACT

The purpose of this study was to assess the growth of tree species planted 20 years ago on a northern slope of the external spoil bank of the “Bełchatów” Brown Coal Mine. The results showed that black poplars and black locust were characterized by the highest mean volume of merchantable timber of a single tree. The lowest value of this characteristic was found for black alder. The ecological characteristics of tree species being introduced during reforestation of the postmining areas should be well adjusted to existing site conditions. In order to assess the value of individual tree species (in respect of their growth and productivity) for the reforestation of such areas the similar studies should also be undertaken in older stands growing in postindustrial grounds. Total results of such studies would make a prerequisite for formulation of general conclusions and recommendations for the reforestation projects.

Key words: forest reclamation, growth of trees, spoil bank, brown coal.

INTRODUCTION

The strip mining of brown coal causes various changes in natural environment. The geomechanical changes are the most evident ones, as exemplified by spoil banks (external or internal) created at the mines. Such lands require reclamation. In Polish brown coal mining the land reclamation is being attained by either reforestation (predominating in the Turoszów and Bełchatów basins) or improvement of lands to make them suitable for production of cultivated crops (predominating in the Konin-Turek basin). Reclamation of the external spoil bank of the “Bełchatów” Brown Coal Mine, where this study was carried out, was accomplished by reforestation.

The purpose of the study was to assess the growth of tree species planted on a northern slope of this spoil bank.

MATERIAL AND METHODS

The Bełchatów basin is situated in the central part of Poland, in the Małopolska Region, Łódź-Opoczno Province, Sieradz-Łódź Mesoregion [14]. The climate there shows transitional characteristics between oceanic and continental climatic types [15].

The study was conducted on a northern slope of the external spoil bank of the “Bełchatów” Brown Coal Mine. It is the second largest, next to Turoszów, external spoil bank in Poland. Its relative height varies from 126 to 180 m, and its total area is about 1500 ha [10]. This spoil bank was designed by “Poltegor” from Wrocław, and formation of its surface took place during 1977–1993. Its biological reclamation was carried out according to the directions of the Department of Formation and Protection of Environment, University of Mining and Metalurgy of Cracow [7]. The spoil bank is built of the Quaternary and Tertiary formations. The Quaternary is represented mainly by fluvioglacial sands, silts, varved clays, boulder clays, lacustrine chalk, peat, and gyttja, while the Tertiary by the loose rocks (sands and gravels) alternately with compact ones (clays, silts, and lacustrine chalk). The investigated northern slope is formed of the mixture of Quaternary formations, mainly sands, clays and silt s [8].

The field investigations were conducted in 2000 and 2001. There were 55 circular sample plots established in nodes of the network of squares 200 m x 200 m. Each plot was composed of two concentric circles. The area of a large circle was 2.5 ares, while that of a small circle 1 are [12]. In each sample plot the diameter breast high (at 1.3 m above the ground) of all trees was measured in two directions perpendicular to each other (N-S and W-E). The measurement of tree height was limited to trees in a small circle. Also from trees in a small circle the cores were taken at breast height with the Pressler’s increment borer.

On cores the 5-year diameter increment was measured, and this made the calculation of the current diameter increment in individual plots possible.

In addition, soil pits were dug in each sample plot, and soil samples were taken in order to assess the soil conditions. Depending on the silt and clay fraction (ø<0.02 mm) the sample plots were classified into three groups:

A – plots built of light formations, where in a 100-cm initial layer of soil the formations containing up to 10 % of silt and clay prevail,

B – plots built of medium-heavy formations, where in a 100-cm initial layer of soil the formations containing from 10 to 20 % of silt and clay prevail,

C – plots built of heavy formations, where in a 100-cm initial layer of soil the formations containing over 20 % of silt and clay prevail.

Using the measurement data (d.b.h. and height) the volume of merchantable timber of trees growing in a small circle was read from tables for volume of standing trees of CZURAJ [4]. Then the volume curves (so called local tariffs) were drawn for individual tree species. The tariffs made the determination of volume of merchantable timber of all trees at the beginning and end of a 5-year increment period possible. Thus, it was possible to determine the volume increment during that period. Further calculations included determination of the species composition by volume, the merchantable volume per hectare (m³ha^-1), and the volume increment (m³ha^-1year^-1). Because of a very large number of tree species and their strong mixing it was impossible to make a growth analysis of whole stands. Therefore the analysis was limited to the mean volume of merchantable timber (being a function of dimensions of a tree, i.e. its height and diameter) of a single tree of a given species. Only the tree species with volume higher than 2.1 % of the total stand volume were analysed. Also the average tree height was calculated and the stand quality determined.

RESULTS AND DISCUSSION

In 2001 the mean volume of merchantable timber of the stand growing on a northern slope of the spoil bank, as determined on the basis of measurements carried out in 55 circular sample plots, was 65.67 m³ha^-1, while the mean volume increment of merchantable timber was 6.33 m³ha^-1year^-1. High values of the coefficient of variation of volume of merchantable timber (83.5 %), and also of volume increment of merchantable timber (80.4 %) may be explained by a young age of stands (about 20 years old) and a diversified species composition. For this reason standard errors of estimation of the mean volume and mean volume increment of merchantable timber were high, 7.39 m³ha^-1 (11.3 %) and 0.69 m³ha^-1year^-1 (10.8 %) respectively. Total volume of merchantable timber on a northern slope of the external spoil bank investigated (about 160 ha) was estimated to be 10 659 m³ (ą 1205 m³ ), and total volume increment of merchantable timber 1027 m³year^-1 (ą 111 m³year^-1).

According to recommendations of the Department of Formation and Protection of Environment, University of Mining and Metalurgy of Cracow, the northern slopes of the external spoil bank of the “Bełchatów” Brown Coal Mine should be reforested with stands of a high percentage of the site improving and final crop tree species [7]. Such a species composition is characteristic for the biodynamic method of reforestation. However, the species composition on slopes investigated during this study is different from the recommended one, and it is quite similar to the composition characteristic for the so called method of pioneer species, where the species used for a quick protection against erosion dominate (e.g. black locust), while the site improving species (e.g. black alder) and final crop species (e.g. pedunculate and red oaks) form only a supplementary admixture [9]. Thus the investigated stands were characterized by a much diversified species composition, and it would be difficult to find thei r equivalent in natural as well as managed forests. In total, there were 25 tree species present in the circular sample plots. Their percentages by volume, in three groups of sample plots, and also for all plot groups together, are given in Table 1. Black locust reached the highest percentage (about 25 %), followed by common birch and black alder (about 20 % each), and the remaining species comprised less than 10 % each (Table 1). The percentage of individual species was different in each of the three groups of plots. In the group “A”, on light rock formations, black locust was more frequent (over 30 %), while the percentages of common birch and black alder (about 15 % each) were lower than average percentages of these species for the entire plantation. In this group also the percentage of willows (about 12 %) was higher than the average. In plots of the group “B” (medium-heavy formations) common birch (over 30 %) and black alder (about 25 %) were the most abundant tree species. Also th e percentages of poplars (about 15 %) and aspen (about 10 %) were relatively high, while that of black locust was only 12 %. In plots included in the group “C” (heavy formations) black alder was the most abundant species (almost 30 %). Also black locust and common birch were quite numerous (about 20 % each). The percentage of black cherry was relatively high (over 10 %), while that of poplars was low (only 4 %).

Poplars reached the highest mean volume of merchantable timber of a single tree in all three groups of sample plots, while black locust was only a little inferior in this respect. Aspen, black cherry, willows, and common birch were ranked in the middle, and black alder was characterized by the lowest mean volume (Fig. 1).

In sample plots of the group “A”, where sands dominated, the highest mean volumes were found for poplars, black locust and aspen. A relatively high mean volume of merchantable timber of a single tree was reached by willows (Fig. 2). It was mainly sharpleaf willow.

In sample plots of the group “B” also poplars reached the highest mean volume of merchantable timber, while willows and black cherry were characterized by the lowest values (Fig. 3).

In sample plots of the group “C” the mean volume of merchantable timber of a single tree was decidedly the highest in the case of poplars. Also trees of black locust and aspen were characterized by large dimensions. The mean volume of black cherry trees in this group of plots was considerably higher than in the remaining two groups (Fig. 4).

The mean height of trees growing on a northern slope of the external spoil bank of the “Bełchatów” Brown Coal Mine is shown in Table 2. Common birch, black alder, and aspen reached the greatest heights in plots built of medium-heavy formations. Sharpleaf willow reached the greatest mean height in plots of the group “A” (10.1 m), and the smallest in plots of the group “C” (6.5 m). Contrary to this, the greatest mean heights for the ‘Robusta’ poplar and black cherry were calculated in sample plots of the group “C”, 14.0 m and 13.0 m respectively, and the smallest in sample plots of the group “A”, 12.5 m and 11.5 m respectively (Table 2). On the basis of the mean height the stand quality class I was estimated for common birch, and class III for black alder.

The analysis of growth of individual tree species in different groups of sample plots revealed various relationships. The average tree dimensions of oaks, black cherry and poplars were increasing as the fertility of soil was increasing. This relationship was reversed in the case of black alder and willows. The average dimensions of black alder were decreasing as the soil fertility was increasing, while willows reached the greatest volume of merchantable timber in sample plots of the A and B groups. The greatest mean volume of black locust was reached in plots of the group B, while its dimensions in plots built of loose sands and loams were only slightly smaller.

Most of the growth tendencies of individual species, observed on different soil formations during this study, may be explained by their ecological characteristics [6] and their specific growth pattern [2]. For example, the sharpleaf willow, the most abundant species among introduced willows, is often used to stabilize sand dunes due to its low soil requirements [3]. Therefore its good performance on barren soils is a natural phenomenon. According to literature also black alder trees attain the greatest dimensions not on their most favourable sites, from which they are forced out by more shade-tolerant broadleaf species, but on poorer sites, and this is associated with their ability to assimilate the atmospheric nitrogen thank to actinomycetes living in simbiosis with alder roots [11]. This characteristic of alders may explain the drop in mean volume of black alder trees along with increasing soil fertility. The large sizes attained by oaks on fertile soil formations are associated with thei r high soil requirements [13], and the equated growth of common birch on different soil formations is associated with ecological tolerance of this species [6].

A good growth of common birch on spoil banks of brown coal mining (stand quality class I) found during this study confirmed the results of WÓJCIK [16], who also arrived to stand quality class I for common birch growing on a spoil bank of Adamów. Also in other reforested postindustrial grounds this tree species was characterized by a good growth [5]. BÖCKER AT ALL. [1] reported common birch together with Scotch pine to be the species dominating on spoil banks in the Lower Lusatia Brown Coal Mining District in Germany. There, both these species attained the stand quality class I-II.

Thus, the mean volume of merchantable timber of a single tree of the species investigated during this study was the result of the degree of agreement between their ecological requirements and the physical and chemical properties of soil formations of sample plots. This indicates that it is necessary in reforestation programs to make a proper choice of tree species according to definite soil formations, in other words to definite site conditions.

It should be remembered, however, that results of this study concern only young stands (age class I). To obtain a more conclusive estimation of value of individual tree species for the reforestation of spoil banks, in respect of growth and productivity, similar studies should be undertaken in older stands, taking into account not only the average size of a single tree, but also the characteristics of whole stands.

SUMMARY OF RESULTS AND CONCLUSIONS

1. Mean volume of tree communities established 20 years ago on a northern slope of the external spoil bank of the “Bełchatów” Brown Coal Mine was 65.67 m³ha^-1, while volume increment of merchantable timber 6.33 m³h-¹year-¹.

2. Poplars reached the highest mean volume of merchantable timber of a single tree, and the next was black locust, while black alder had the lowest volume.

3. Common birch and aspen were characterized by the stand quality class I.

4. Ecological properties of tree species used in reforestation of the strip mining spoil banks should be adjusted to existing site conditions.

5. To obtain a more conclusive results in respect of growth and productivity of stands established on the investigated spoil bank a similar study should be undertaken in future when trees are older, and in respect of the entire stands. Results of the present and future studies would make a prerequisite for drawing general conclusions and making recommendations for reforestation of such lands.

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