THE DEPENDENCY OF THE QUALITY AND SIZE STRUCTURE AND OF THE VALUE OF TIMBER ON THE ORIGIN AND AGE OF STANDS

Anna Barszcz

ABSTRACT

Standing timber assessment shows that, in the area under research, it is spruce stands of artificial origin that are characterized by the best quality and value of timber. Rebuilt stands over 50 years old also gained high assessment with respect to the features under research. A relatively low assessment of timber quality in stands of natural origin may result from their advanced age and the fact of being protected as nature reserves. Timber quality in the area under research is also affected by the bedrock type and altitude.

Key words: timber quality, timber value, the origin and age of stands, stands of natural origin, monocultures, rebuilt stands..

INTRODUCTION AND AIM

The forests of the Żywiecczyzna region are a unique object, where research on the relations between the character of forest management and the origin as well as age of stands, may yield interesting results. In the past, the composition of a considerable part of the original, natural forests was disturbed by the artificial introduction of pure, even-aged spruce stands. The image of forest ecosystems is affected by the differentiation of bedrock and, consequently, by types of soil and forest sites which were formed on them and are now undergoing the processes of degradation, as well as by large differences in altitude. Damage to stands which is caused by industrial immission has been noted for several decades.

Norway spruce is one of the species characterized by high natural variability and adaptability. Such features of this species are reflected in the quality and value of its timber, which is influenced by numerous biotic and abiotic factors. A better knowledge of these relations is not an easy task but it may have a great scientific and practical significance.

The aim of the present research was to determine the influence of the origin and age of stands, with their division into stands of natural origin, monocultures (whose age qualifies them as mature stands) and rebuilt stands (thinned stands), on the quality and size structure of wood on stem in the forests of the Żywiecczyzna region. Additionally, some analyses included the influence of the bedrock type and of the climatic and vegetation zone. Due to the dominance of Norway spruce in the area in question, the present research focused mainly on this species.

RESEARCH AREA

The research was conducted within the borders subsumed under a common name of the Żywiecczyzna region [12] and included the following 5 forest districts: Jeleśnia, Ujsoły, Węgierska Górka, part of Bielsko and Wisła as well as the Babiogórski National Park. According to the nature and forest regionalization, the Żywiecczyzna region is located in the VIII Carpathian Region, and its Provinces of the Beskid Śląski Mts and of the Beskid Żywiecki Mts [23].

The vertical zones that can be distinguished in this area are the foothill zone, the lower forest zone and the upper forest zone. The average annual temperature amounts to 6-7°C with a falling tendency at higher altitudes. The average annual rainfall exceeds 900 mm and depends on the eco-climatic zone. The length of the vegetation period largely depends on the region and altitude, amounting to, on average, 220 days [10].

The bedrock of the part of the Żywiecczyzna region called the Beskid Śląski Mts are the deposits of the Silesian nappe (godula and istebna beds). The bedrock of the part that belongs to the Beskid Żywiecki Mts are magura beds. The dominant soils are brown, with a smaller share of podzolic soils [19].

Lower locations of the lower forest zone, depending on the type of bedrock, are dominated by the sites of mountain forest, mixed mountain forest and, sometimes, mixed mountain coniferous forest. Upper locations of this zone are overgrown with mixed mountain forest and mixed mountain coniferous forest. The upper forest zone, independently of the bedrock type, is the site of high mountain coniferous forest [14].

The research area has a unique history of forest management. An increased demand for timber in the middle of the 19^th century initiated large-scale clear cutting. After their deforestation, the natural sites of beech and fir in the lower forest zone were artificially reafforested with spruce, sometimes using the local seeds, though most often the seeds came from distant, now unknown regions.

The timber of the spruce monocultures which were formed in this way in large areas was characterized by a bad health condition and low quality already in the early 20^th century. However, among these negative stands of artificial origin, there still occurred refuges of pure spruce stands of natural origin as well as mixed stands with a share of spruce, which were characterized by a good health condition and good technical quality [12].

Research conducted in the 1970s showed that some monocultures also had good mass increments and high quality of their timber. This can be explained by an accidental choice of site conditions that turned out suitable for the cultivation of spruce or by the local origin of the seeds used (the documentation reveals the existence of seed extraction stations in the area under research in the 19^th century) [16, 17, 24].

The structure of harvesting in the research area is dominated by spruce timber, which constitutes about 87% of the mass of timber, 85% of which is large-sized timber, mainly class WC0, and about 17% is middle-sized timber, mainly group S2 [20].

MATERIAL AND METHODS

One sample plot was set up in each of the 70 stands selected in the five forest districts, listed in the previous section above, and in the Babiogórski National Park. The selection of the stands for research considered the differentiation of their origin, age, bedrock as well as their vegetation and climatic zones so as to represent the variability of the forests and timber under research.

The stands which contained the sample plots were located in two vegetation and climatic zones, namely in the lower forest zone (52 plots) and in the upper forest zone (18 plots). The stands grew on 3 types of bedrock: on magura beds (24 plots), godula beds (23 plots) and istebna beds (23 plots).

The data was grouped with relation to the origin of stands, closely connected with their age, in the following way:

1. Stands of natural origin, including forest reserves, aged between 103 and over 230 years (11 plots) and the remaining stands of natural origin, aged from 80 to 201 years (10 plots);

2. Stands of artificial origin (monocultures), aged between 70 and 193 years (30 plots);

3. Rebuilt stands: older ones, aged between 51 and 80 years (10 plots) and younger ones, aged from 20 to 50 years (9 plots).

The sample plots, which constituted 5 or 10% of the area of the selected stands [25], covered altogether 46.11 ha. Altogether 28116 items of trees were measured and classified, including 16426 items of spruce, 8262 items of beech, 2377 items of fir and a number of trees of other species. The joint mass of merchantable boles amounted to 15690 m³, including 11353 m³ of spruce timber, 2967 m³ of beech timber, 1015 m³ of fir timber and a certain amount of timber of other species.

Due to a considerable share of Norway spruce in the stands under research, the characteristics of its timber were analysed separately, and the results were referred to the joint mass of timber of all species.

Trees in the sample plots underwent standing timber quality assessment [25] with the application of the Polish standards and technical conditions. The field data was fed into the computer programme for standing timber assessment Acer 2.0 and the result received was the timber mass according to quality and size classes, calculated for 1 ha of the stand area.

In the next stage, the data on the assortment structure of the timber from the sample plots was grouped according to the criterion of the origin of stands. Certain analyses also took into consideration the climatic and vegetation zone and the bedrock type. The results then constituted the basis for an analysis of the significance of differences between groups of sample plots, with relation to assortment structure. In this analysis, the non-parametric chi-square test on the significance level 0.05 was applied.

An analysis of timber value was conducted, too. Taking into consideration the average prices of timber sale in the forest districts under research in 2003, the following values were calculated: the value of timber per 1 ha and the value of 1 m³ of timber in single plots and in the created groups. When computing the average value of 1 m³ of timber for groups of plots, the values from single plots were weighed by means of the volume of timber per 1 ha in these plots.

RESEARCH RESULTS

An analysis of the quality and size structure of timber as related to the origin of stands and the bedrock type showed that, among the stands of both artificial and natural origin, belonging to older age classes, the best assortment structure of all timber and of spruce timber characterized group 4 (monocultures on magura beds). The stands in this group contained the most timber of classes WA and WB and little middle-sized timber. This group is also distinguished by the greatest volume per 1 ha, the highest value of timber per 1 ha as well as the highest value of 1 m³ of timber (Table 1, Figures 1 and 2). Good quality was also assessed in the stands of groups 3 and 6 (stands of natural origin and monocultures on istebna beds), which were also distinguished by a high value of 1 m³ of timber. Low quality assortments were found in group 2 (stands of natural origin on godula beds), which had a particularly large share of pile wood and was also characterized by the lowest volume per 1 ha. The category of older age classes in this group had the lowest average value of timber per 1 ha and the lowest value of 1 m³ of timber. Timber of all species in group 1 was characterized by a particularly low quality despite its large volume per 1 ha, resulting in a high value of timber per 1 ha. Its low quality was influenced by a large share of pile wood in the species other than spruce (which can be seen in a comparison of Figure 1 and Figure 2); hence the value of 1 m³ of timber is low in the stands of this group.

Rebuilt stands were dealt with separately in the analyses of quality and value of timber, taking into consideration their lower age and special management performed there. Among the older rebuilt stands, group 7 (on magura beds) was the best one with respect to the assortment structure of all species together as well as of spruce timber and with respect to the two indexes of value. Group 8 turned out to be the weakest in quality, with a low value of 1 m³ of timber. The smallest volume per 1 ha and the lowest value of timber per 1 ha was found in group 9 (on istebna beds). In the category of younger rebuilt stands, the largest volume per 1 ha, the best quality and the highest value of timber per 1 ha characterized group 10 (on magura beds). The highest value of 1 m³ of timber was found in group 12 (on istebna beds). Group 11 (on godula beds) turned out to be the weakest, with the dominance of pile wood, the smallest volume per 1 ha and the lowest value indexes.

The division of the analysed data into 4 groups according to the criteria of altitude and the origin of stands (Table 2, Figures 3 and 4) allows for a better assessment with respect to the quality and size structure of the stands of artificial origin as compared with the stands of natural origin. In the stands of natural origin situated in the lower forest zone, spruce timber had a much better quality than the timber of all species considered together. Because it co-occurred with other tree species, spruce showed smaller volume per 1 ha in the stands of natural origin than in monocultures. A relatively large volume per 1 ha and, above all, the best quality timber affected the highest value of 1 m³ of timber and a high value of timber per 1 ha in the stands of artificial origin. In the upper forest zone, the stands of natural origin had a slightly larger volume per 1 ha and a higher value of timber than the stands of ar tificial origin. The assortment structure of both groups in the upper forest zone was similar. In comparison with the stands in the lower forest zone, the upper forest zone was dominated by large-sized timber of the average quality (class WC), irrespectively of the origin of these stands.

The division of sample plots into 4 groups according to a single criterion of the origin of stands (Table 3, Figures 5 and 6) allowed for conclusion that the stands of natural origin, the ones of artificial origin and the older rebuilt ones had a similar volume per 1 ha of the timber of all species considered jointly. Differences occurred with respect to the volume per 1 ha of spruce, which was the largest in monocultures due to a small share of other tree species. The smallest volume per 1 ha was found in the younger rebuilt stands, which is due to their young age. The best assortment structure and the highest value indexes in this grouping of the data characterized the stands of artificial origin, which had a very similar quality of spruce timber and of all species timber (jointly). The natural stands were characterized by a small share of the best quality classes. Moreover, considering the timber of all species, large amounts of pile wood were found in these stands. The high value of timber per 1 ha in this group was influenced by a large volume per 1 ha in these stands. The youngest stands of group 4 were characterized by a large share of middle-sized timber, which revealed an evident differentiation of the quality structure, where the quality of spruce was better than of the timber of all species considered jointly. This group had small volume per 1 ha and low value of timber.

The significance of differences between the quality and size structure of timber in the groups of plots which were formed on the basis of the origin of stands was analysed using the chi-square test. The test proved the existence of statistically significant differences between the following groups of stands: between the stands of natural origin and the ones of artificial origin (timber of all species: chi-square = 47.70 and p = 0.0000; spruce timber: chi-square = 26.28 and p = 0.0018), between the stands of natural origin and the rebuilt stands (timber of all species: chi-square = 49.89 and p = 0.0000; spruce timber: chi-square = 35.86 and p = 0.0000), and between the stands of artificial origin and the rebuilt ones (timber of all species: chi-square = 44.63 and p = 0.0000; spruce timber: chi-square = 24.85 and p = 0.0031). The test results proved that all pairs of sample plots compared, when the grouping criterion was the origin of stands, differ from each other significantly both with res pect to the quality and size structure of timber of all tree species considered jointly and with respect to the structure of spruce timber.

DISCUSSION

The stands under analysis were differentiated with respect to the quality and value of timber, which was considered with relation to their origin and related to the bedrock and altitude. The opinion expressed many times in the literature emphasizes the poor quality of spruce monocultures in the lower forest zone of the Beskid Żywiecki and Beskid Śląski Mts, especially the ones that were introduced into the former sites of the original beech stands of natural origin or came from seeds of unknown origin. They are vulnerable to damage caused by wind and snow as well as the harmful biotic influence of fungi, insects and deer [7, 8, 9, 15]. However, the results of the present research indicate a high average volume per 1 ha as well as a high quality and value of the timber from monocultures of the lower forest zone. Also Myczkowski [16, 17] notes the presence of high quality spruce stands of artificial origin in the lower forest zone of the Żywiecczyzna region, which occur besides the negative s tands. He explains this fact by the artificial introduction of spruce into e.g. its natural sites in the lower forest zone in the past. According to him, in such cases, even unsuitable genetic material (of unknown origin) could bring good production results thanks to advantageous ecological factors. Better results were achieved when introducing spruce into the former natural sites of fir; worse results – if these were the former sites of beech. Also according to Bobrowicz [6], it is in the stands of artificial origin that large amounts of spruce resonance timber, regarded as the highest quality assortment, used to be harvested in the area in question.

According to Sabor [21, 22], the world-famous population of Istebna spruce, characterized by excellent mass increments and the highest quality, and distinguished by considerable adaptability to the local conditions, may have been introduced artificially. The famous stand in the forest district of Wisła, the forestry of Bukowiec, division 149h, where the artificially introduced spruce found suitable site conditions, may serve as an example. However, this now 170-year-old stand reveals some processes of wood depreciation due to its advanced age. Twaróg [24] notes that in the 19^th century, there used to exist a seed extraction station in this area and it produced seeds of local origin. The author associates the good quality of spruce monocultures in this area with this fact.

Timber quality in the lower-forest-zone stands of natural origin, with large volume per 1 ha, was assessed lower than in monocultures. The reason was that these stands of natural origin included stands with advanced age (on average older than monocultures), which was the cause of a more frequent occurrence of defects, resulting in wood depreciation. Moreover, this category was largely constituted by forest reserves, where the forester's involvement is not aimed at improving the quality of wood, unlike in economy-oriented stands. The fact that wood defects, mainly rot, increases with the age of stands was previously observed in spruce growing in the lower forest zone of the Tatrzański National Park [13], in the mature stands of the lower forest zone of the Beskidy Mts [4] and in the north of Finland [18].

A noticeable influence of age on the quality of timber was shown in the present research in the forest reserves of Śrubita and Pod Rysianką as well as in the sample plots set up in the Babiogórski National Park. Jaworski and Karczmarski [11], who analysed changes which occurred in the period of 10 years in one of the stands of the Babiogórski National Park (in which one sample plot was set up in the present research, too), noted that the stand is in the stage of decomposition accompanied by an advanced stage of renewal. Good quality timber was found in the reserve of Mahodora, which is younger than other reserves, with trees aged 103 to 130 years, where spruce (mixed with fir and beech) had about 30% high quality assortments (classes WA and WB) and only 1% fuel wood.

In the upper forest zone, the stands of natural and artificial origin were characterized by similar volume per 1 ha and a very similar quality structure of timber. It must be concluded, then, that in this climatic and vegetation zone, it is the difficult climatic conditions that have a dominant role in forming timber quality and that the origin of stands has a smaller significance. It was shown that volume per 1 ha, quality and value of timber were worse here than in the lower forest zone. Within the research area, examples of upper-forest-zone stands, of both natural and artificial origin, with low quality timber, can be found on eastern slopes of Barania Góra Mt and near the top of Skrzyczne Mt. Trees in plots located in the upper forest zone were mostly characterized by considerable taper and a large share of knots and curvatures (my own observations), which affected the results of standing timber quality assessment. More frequently than in the lower forest zone, the trees here had trace s of broken tops with the resulting occurrence of knee curvature, formed during subsequent years. Earlier research, conducted in the mature spruce stands of the Beskidy Mts, showed significant relations between the taper of spruce stems and the factor of diameter and the occurrence of knots [1]. In the Karkonosze Mts, Barzdajn [5] noted that the length of the zone of knots in spruce stems significantly increases with altitude. Despite their low technical quality, the upper-forest-zone stands in the Żywiecczyzna region were considered to be more stable in quality than the lower-forest-zone stands. The reason was a smaller share of trees with rot, which is a very serious defect, leading directly to timber depreciation after some time [2].

There is very little comparative research on timber quality and value in thinned stands (which correspond to the category of rebuilt stands in the present research) as compared with mature stands (in the present research: stands of natural origin and monocultures). Spruce in older rebuilt stands (over 50 years old) in the Żywiecczyzna region had a better assortment structure, i.e. a larger joint share of classes WA and WB with a similar share of middle-sized timber, in comparison with stands of natural origin. The structure of timber of all species treated jointly was similar in the compared groups of stands but there was a larger share of middle-sized timber. Therefore it must be concluded that the rebuilding of the negative spruce monocultures is performed correctly in the forest districts under analysis, which is expressed by good technical quality of timber despite the young age of these stands. However, in order to maintain this favourable tendency, foresters should intensify their act ivities aimed at protection against parasitic fungi in the stands of this category because, as was observed in field during the present research, there are frequent cases of rot in the timber (especially spruce timber) from these stands [3].

As far as the quality of the rebuilt stands and monocultures is concerned, reference must be made to the opinion expressed by Myczkowski [16, 17] and Twaróg [24]. According to these authors, there is no need to rebuild the good quality stands of artificial origin in those sites where spruce has found suitable conditions and is characterized by high quality timber. The decision to rebuild a monoculture stand can be justified only by its bad condition and its resulting negative assessment.

SUMMING UP AND CONCLUSIONS

1. An analysis of the quality and size structure of timber, of volume per 1 ha and of value of timber in the forests of the Żywiecczyzna region reveals a differentiation of these features which depends on the origin and age of stands, with an additional factor of altitude and bedrock type. The differences in the assortment structure of the groups of stands distinguished according to the criterion of their origin proved to be statistically significant.

2. Among the stands of older age classes, of both natural and artificial origin, the stands of artificial origin which grow on magura beds had the best quality, the best volume per 1 ha and the highest value of timber. The stands of natural origin on godula beds were the worst with respect to the features under analysis.

3. In the category of rebuilt stands of both age subgroups, the stands growing on magura beds received positive assessment, while the groups of stands growing on godula beds, which had a bad quality and size structure, were assessed the lowest.

4. In the lower forest zone, it is the stands of artificial origin that had very good quality and size structure as well as high indexes of timber value. Large volume per 1 ha of the stands of natural origin in this zone had an effect on a higher value of timber per 1 ha than in the stands of artificial origin. The stands in the upper forest zone received lower assessment of the quality and value of timber and of volume per 1 ha than the stands in the lower forest zone. In the upper forest zone, the qualitative features of the trees indicate a significant influence of climatic conditions on the formation of timber quality.

5. In the final assessment, taking into consideration only the influence of the origin and age of stands, it is the mature monocultures in the area under research that had the highest quality and value of timber as well as the largest volume per 1 ha. Research done by other authors, mentioned in this study, also notes that, apart from vast areas of the Żywiecczzna region which are covered with negative monocultures, there occur numerous spruce stands of artificial origin which are adapted to their sites and come from the local seeds, yielding very good quality timber and large increments of mass.

6. Bad assessment of the stands of natural origin in the present research may result from their old age and advancing processes of decomposition. A relatively high value of timber in this group is due to large volume per 1 ha in these stands. Most stands in this group are forest reserves, in which there is either no human interference or human activity is limited, which affects the value of timber.

7. The older rebuilt stands showed large volume per 1 ha (comparable with the stands of natural origin and with monocultures) and a larger share of the best quality and size classes of timber than the stands of natural origin. Field observation made during the present research allows for a conclusion that forest protection in these stands, which are threatened with rot, must be intensified.

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