COMPARATIVE ANALYSIS OF THE QUALITY OF TIMBER OF NORWAY SPRUCE (PICEA ABIES L. KARST.) FROM TWO RANGES OF THIS SPECIES IN POLAND IN RELATION TO AGE, STAND QUALITY CLASS AND SITE TYPE OF FOREST

Krzysztof Michalec
Department of Forest and Wood Utilization, Agricultural University of Cracow, Poland

ABSTRACT

The research concerned two natural ranges of occurrence of Norway spruce in Poland – the south-western range (mountain stands) and the north-eastern one (lowland stands). Twelve sample plots of 1 ha were located in the region of Warmia and Mazury (12 forest districts), 8 plots in the Sudeten Mts (6 forest districts) and 12 ones in the Carpathian Mts (7 forest districts). The collected data served for comparative analyses of the quality of spruce timber from stands of different ages and stand quality classes, and growing on different site types of forest. On the basis of the criterion of age it was noted that in the mountains it is the youngest stands under research that have the best quality while in the lowlands the older stands are of better quality. Considering spruce stand quality classes in its south-western range, stands with higher quality classes were found to be characterized by better quality. In the north-eastern range, no significant differences were noted in timber quality between the compared stand quality classes.

Considering site types of forest, the best timber quality was found on sites optimal for the species under analysis: BMG in the mountains and BMw in the lowlands; very good timber quality was also found in stands growing on fertile sites. Such results suggest the necessity of research on the relations between site types of forest and spruce timber quality.

Key words: spruce, spruce timber quality, stand indexes, timber defects.

INTRODUCTION

The main natural place of occurrence of Norway spruce in Poland are the mountains; it is also found in the regions of Warmia and Mazury. Warmia and Mazury form the north-eastern range of spruce in our country whereas the mountains form its south-western range [38]. Spruce is characterized by quite large expansiveness and growth dynamics. It can grow on all sites except dry coniferous forest. It is thought that the optimal sites are fresh mixed coniferous forest and wet mixed coniferous forest. The best site for obtaining valuable assortments is mixed mountain forest [42]. The optimal conditions for the growth and development of spruce do not always correspond to the production of high quality timber. Lush spruce trees may be characterized by a long crown, thick branches and wide annual rings, which significantly influences the results of classification of such timber and its quality.

The present study is an analysis of the quality of spruce timber from the stands of different ages, growing on different site types of forest and characterized by different stand quality classes in relation to places (the Sudeten Mts, the Carpathians) and ranges (the south-western range and the north-eastern one) of the occurrence of this species in Poland. This was the way to determine the optimal conditions in which very good quality timber might be produced.

METHODS

Preliminary selection of stands for research was performed on the basis of descriptions contained in the management statements of particular forest districts. Considered were these stands in which spruce had already achieved maturity age, and at least the ones where all maintenance activity (late thinnings) had already been performed, i.e. stands aged 70 years or older, with a minimal area of over 3 ha. Such a stand offered a possibility to create a sample plot of 1 ha. Stands characterized by different values of stand indexes (age, stand quality classes, site type of forest) were chosen. Eventually, within the north-eastern range of spruce 12 sample plots were located, in the Sudeten Mts – 8 plots and in the Carpathian Mts – 12 plots. The characteristics of the sample plots are presented in Table 1.

In the present research, the directives of standing timber quality assessment which are applied in practice were modified: all trees on a sample plot were measured and assessed – not only the ones which, according to the directives, are to be felled [40]. The following measurements and assessments were taken on sample plots:

• measurement of the breast-height diameter of each tree 7 cm thick or thicker;

• measurement of the height of each tree;

• classification of standing timber including the class or the quality and size group of the butt-end of the tree;

According to the rule of classification of coniferous large-sized timber [39] in the butt-end of a tree 4 m long (measured from the stem base and identified as the bottom face of timber after felling), considered was the presence of knots and, in particular, their diameter and form of concentration (knots occurring only in verticals or also beyond them). The remaining defects, included in the Warunki Techniczne [39], were considered along the whole visible length of the stem.

If the first, butt-end part of the stem fulfilled the requirements for special timber, i.e. matchwood (WB1) [30], the length of the stem zone where the assortment occurred was assessed. If the section of large-sized timber needed shortening due to the occurrence in its upper end of a defect which did not meet the standard, this fact was noted in the fieldwork diary.

• determination of the kinds and varieties of defects and their measurement;

Defects located on the stem up to the height of 2 m were measured according to the rules of the standard [23]. Sizes of defects on the stem above the height of 2 m from the ground were, according to the requirements of standing timber quality assessment, assessed. For the classification of standing timber, the significant defects were considered, i.e. the ones which are included in the standards and decisive of the results of the quality and size classification.

• taking increment cores by means of Pressler’s borer from 15 selected spruce trees on each sample plot;

The aim of this examination was to determine the presence of internal rot, its kind and size and to find out the width of annual rings. The trees were selected for taking cores by means of Draudt’s method [10], which consists in choosing the number of sample trees in proportion to the number of trees in degrees of breast-height diameter.

In the course of work on the data, each tree growing on a sample plot underwent a simulated division into quality and size classes which could be theoretically identified over the whole length of a given tree up to its top. The procedure started from the quality and size class of timber determined in the butt-end of a log (section 1) during field work and made use of the Radwański tables [31], which allow for defining the size and volume of particular sections of spruce stems on the basis of the breast-height diameter and height of the trees.

On the basis of the standards [24,27,30,39], the following border values were assumed for particular sections over the length of the stem: for large-sized timber – minimum 14 cm in the upper end under bark, for middle-sized timber – minimum 5 cm in the upper end under bark, in the bottom end – without a size limit, and for small-sized timber – less than 5 cm under bark in the bottom end. Therefore, if the first section from the stem bottom was subsumed under class WA, then the next (second) section of large-sized timber was classified as WC. When the length of the second section was too small, i.e. it did not meet the required length for large-sized timber (minimum 2.7 m) or when it had a defect which violated the requirements of the standard [39], then this section was classified as S2.

A similar simulated manipulation was performed when the first stem section was classified as WB1, whose minimal upper diameter cannot be smaller than 20 cm under bark, or as large-sized timber of class WA or WB, whose possible maximal length was shortened due to the occurrence – in its upper part – of a defect which disqualifies this piece of timber. In this case, the second section was classified as class WC, group S2 or S4.

The next section of a merchantable bole, whose size corresponded exclusively to middle-sized timber (thinner than 14 cm under bark), was subsumed under group S2. The thinnest section, i.e. the top one, was classified as small-sized timber, group M.

The stage of work described above allowed for obtaining the following data for each section of the stem: length and diameter in the middle of the length (under bark), volume (under bark); for the whole stem: diameter (under bark) in its mid-length and its volume (under bark). These data formed the basis which served for further calculations and comparisons for both groups of the material created according to selected stand indexes in the locations and ranges of the occurrence of spruce in Poland.

Next, the increment cores taken from model trees during fieldwork were analysed. The presence and kind of rot was noted. There are two kinds of rot: hard and soft [23]. In the case of the occurrence of both kinds of rot on an increment core, the rot was qualified as the kind which dominated. The width of the zone on the radius of the stem cross-section affected by rot was also noted [20].

The width of annual rings was also determined on the increment cores [20]. In accordance with the European standard concerning the requirements for spruce timber [21], a division of the average annual width into three groups was assumed. The sample trees were classified on the basis of their ring width into the following groups: 1 (width of up to 4 mm), 2 (width 4–7 mm) or 3 (width over 7 mm).

The next stage of research consisted in grouping the measurement and assessment data obtained on particular sample plots, in the aspect of the selected stand indexes, according to the following criteria:

• Age – 3 ranges: 75–100 years, 101–120 years and over 120 years. The age of spruce was considered here. In the case of the occurrence of spruce of varying ages in a single stand (as indicated in the data in forest district management plans), age was calculated as weighed by timber volume according to Block’s equation [10].

• Stand quality classes – two groupings: I and II, III and IV. During the grouping in mixed stands, spruce stand quality classes were taken into consideration. Middle values were rounded down, e.g. II.5 was treated as stand quality class II. In all-aged stands, in which each age group of spruce had a different stand quality class, this class was calculated as weighed by the volume of timber of this species.

• Site type of forest – 5 groups in mountain regions: BWG, BG, BMG, LMG, LG and 4 groups in the lowlands: BMśw, BMw, LMśw, Lśw. The site of BG was considered jointly in the stands of the whole south-western range because there was one sample plot in the Carpathians and one in the Sudeten Mts (without repetitions).

The first stage consisted in juxtaposing the average volumes (m³·ha^-1) of the timber quality and size classes and groups in the analysed groups of the materials and their percentages in the overall mass of timber in a group. This was done using the input data obtained from the sums of volume of single trees according to Radwański’s tables [31].

The second stage included a number of statistical analyses. First, the significance of differences in the quality and size structure of timber (in m³·ha^-1) was analysed in relation to selected stand indexes. Due to a lack of accordance of the data with the normal distribution, the chi² test was applied.

The third stage of research consisted in analyzing the occurrence of timber defects. First, the share of trees with the defects which lower timber quality was listed in relation to particular stand indexes. Due to a small number of certain defects (insect damage, multiple cores), they were subsumed under one heading: “others”. The material was analysed considering the structure of kinds of defects. The number of trees where a given kind of defect occurred was each time referred to the number of all trees under analysis in a given location or range. Hence the sum of percentages of trees with particular kinds of defects sometimes reaches a value of over 100% (when there were several kinds of defects in a single tree).

The next stage of research was analysis of hidden defects, which became revealed on increment cores taken by means of Pressler’s borer. According to the methods assumed, two kinds of analysis focused on: internal rot and the width of annual rings. In the analysis of rot, the trees were divided into: ones without rot, ones with hard rot and ones with soft rot. On the basis of annual ring width, the trees were divided into the three groups described above.

RESULTS

Age of spruce
The highest values of average volumes·ha^-1 of timber class WA in the Sudeten and Carpathian Mts as well as in the whole south-western range occurred in the stands where spruce was aged 101–120 years (Table 2). This age group, in these locations, was also generally characterized by the highest volume/ha of the stands. The highest average volume/ha of classes WB and WB1 was noted in younger mountain stands, aged 75–100 years (locations: the Sudeten and Carpathian Mts and the whole south-western range), with the exception of class WB1 in the Sudeten, where the highest volume/ha was noted in the age group of 101–120 years. This was related to a considerable number of trees that did not yet exceed the breast-height diameter of 35 cm (required for timber class WA), which, however, showed very good technical quality of their timber. The oldest stands, aged over 120 years, were characterized by the highest average volumes/ha of their middle-sized timber (groups S1, S2, S3 and S4). The high volume of groups S1 and S3 was related to the presence of a young generation of trees, which did not exceed the breast-height diameter of 18 cm (the minimum breast-height diameter for large-sized timber) and which showed good quality of stems while a considerable number of timber noted in group S4 and partly S2 was related to the depreciation of the timber of the oldest trees.

In the north-eastern range, the highest average volume of classes WA and WB1 was found in the age group of 101–120 years. The youngest stands (75–100 years old) were characterized by the highest average volume in class WB. This was connected with the occurrence of a large number of trees whose breast-height diameter was not larger than 35 cm but where the presence of knots between verticils ruled out the qualification of this timber to class WB1. In the age group in question, the highest average volume/ha of group S4 as well as the highest volume/ha were noted.

The statistical chi² test, applied in order to examine the significance of differences between the quality structure of spruce timber (in m³·ha^-1) grouped according to age groups, on the basis of the data in Table 3, showed statistically significant differences (shown in red) in most comparisons. Only a few of the compared stand groups in similar age groups, in the mountains, did not have significant differences (in black) in their timber structure. A lack of significant differences between the age groups from the Sudeten and Carpathian Mts and the stands from the whole south–western range was due to the fact that they were part of the range in question.

In the mountain stands (the Sudeten and Carpathian Mts and the whole south-western range), the joint share of classes WA and WB clearly diminished in stands of increasing age while class WC increased (Fig. 1). In the Carpathian Mts and the whole south-western range, in the oldest stands, the share of class WB1 decreased whereas WD increased. The oldest mountain stands had the largest share of groups S1, S2 and S4.

In the north-eastern range, the share of timber class WA in older groups of stands increased, reaching the maximum in the age group of 101–120 years and then (at an older age) decreased. In the older groups of stands, the share of timber class WB decreased. The trees of this class “passed” to class WA as they became thicker, thus enlarging its share. The joint share of both classes in the first two age groups remained on a similar level (about 22%); it decreased at the oldest age. The share of class WB1 and group S1 systematically grew with the age of stand groups. This was connected with the appearance of a young generation of trees, unfortunately not good quality, in the oldest stands.

As in the analyses of timber quality, also in the examination of timber defects the stands were divided into three groups according to their age. Then, in the groups of material thus created, analyses for locations and ranges of the occurrence of spruce were carried out. In the case of both locations and the south-western range, in subsequent age groups of stands, a higher share of trees with defects was noted. The maximum of this share was in the age group of 101–120 years (Fig. 2). At older ages, this share decreased. In the Sudeten, in all age groups, smaller shares of trees with defects were noted than in the Carpathians. In subsequent age groups of stands of the north-eastern range, smaller shares of trees with defects were noted. Each time these stands also showed smaller shares of trees with defects in comparison with the stands of the south-western range.

In the analysed data from the Sudeten, two tendencies were noted: a decrease, in subsequent age groups of stands, in the share of trees with open knots and a clear increase in the share of trees with internal rot (Fig. 3). In the age group of 101–120 years, there was a noticeable influence of open injuries on spruce timber quality; in the oldest stands (over 120 years old) – of external rot. In the Carpathians, there also were a few tendencies: a decrease, in subsequent age groups of stands, of the share of trees with internal rot and an increase in the number of trees with curvatures and external rot. In stands aged 101–120 years, the highest share was noted of trees with open knots, revealing symptoms of overgrown injuries, and of trees with open injuries.

In the stands of the whole south-western range jointly, in older and older age groups of stands, there was a decrease in the share of trees with overgrown injuries and an increase in the share of trees with external rot and all kinds of curvatures. In the material from the age group of 101–120 years, the maximum shares were noted of trees with: open knots, internal rot and open injuries. The stands in the north-eastern range were characterized, in subsequent age groups, by a decrease in the share of trees with symptoms of the presence of overgrown injuries (as in the mountains) and with open knots. The oldest age group (of over 120 years) had a considerable share of trees with internal rot and curvatures (also analogously to the mountains).

On the basis of analysis of the research material, it was noted that in the corresponding age groups, the timber from the Carpathians was characterized by a larger number of knots (a larger share of trees with open knots) in comparison with the Sudeten Mts, whereas in the case of overgrown injuries the relation was inverse. The north-eastern range showed smaller shares of trees with: open knots, overgrown injuries, burls, internal and external rot in relation to the stands of the same age groups in the south-western range. Stands aged 75–100 years in the north-eastern range were an exception as their share of trees with overgrown injuries and external rot was slightly higher than in the south-western range.

In the mountain stands, subsequent age groups showed larger shares of trees with hard rot (Fig. 4). In the Sudeten Mts, in the youngest age group, no trees with soft rot were noted; the largest share of trees with this defect was found in the age group of 101–120 years; in the next age group it decreased. In the Carpathians, the frequency of occurrence of trees with soft rot decreased in subsequent age groups of stands. In the material from the south–western range, the share of trees with hard rot increased in subsequent age groups and the frequency of occurrence of trees with soft rot was variable in subsequent age groups. The largest number of trees with this defect was noted in the age group of 101–120 years. In the north-eastern range, there was no clear differentiation of stands in relation to rot. The stands aged 101–120 years were slightly healthier (with the largest share of trees without rot) than the two other age groups.

The research allows for stating that in the stands of the Sudeten Mts, the share of trees with the average annual ring width of 4–7 mm increased with age (Fig. 5). In the Carpathians, in the first two age groups, the share of trees in the first and second annual ring group was the same while in the oldest stands there occurred only trees with annual ring width of up to 4 mm. The Carpathian stands in the age groups of 86–100 and 101–120 years had much higher shares of trees with annual ring width of 4–7 mm than the stands of the same age in the Sudeten Mts.

In the mountain stands under research, treated jointly, the share of trees with annual ring width of 4–7 mm in the stands aged 101–120 years was slightly higher than in the younger group. In this age group, also the smallest share of trees with annual ring width of up to 4 mm was noted. The narrowest rings in the Carpathians and in the whole south–western range were found in the timber of the oldest stands. The youngest stands in the north-eastern range have wider annual rings than older stands. For example, in the youngest group there were trees with rings of over 7 mm.

Spruce stand quality classes
In all stands under analysis, along with the deterioration of their quality classes, there occurred a decrease in the volume/ha of large-sized timber of classes: WA, WB, WC and WB1 and a decrease in the volume/ha of the stands (Table 4). In the Sudeten Mts, however, these differences were not so distinct, e.g. the average volume/ha in class WB1 in both stand quality classes was almost the same. In the mountain stands (the Sudeten, the Carpathians and the whole south-western range), along with the deterioration of spruce stand quality classes, there was noted an increase in the volume/ha of middle-sized timber of all groups. In the north-eastern range, the relation was reverse.

In the Sudeten, the Carpathians and the whole south-western range, the stands of the same quality classes did not show any statistically significant differences in the quality and size structure of their timber (Table 5). The other groups of the data compared showed statistically significant differences.

In the Carpathians and the whole south-western range, along with the deterioration of spruce stand quality classes, there was a decrease in the share of timber of classes WA, WB and WB1 while the share of timber of class WD and of middle-sized timber groups (S1, S2 and S4) increased (Fig. 6). In the Sudeten the findings were the same as in the Carpathians except timber classes WB1 and WD. In the north-eastern range, as in the mountain stands, the deterioration of spruce stand quality classes was accompanied by a decrease in the share of timber classes WA and WB whereas the share of class WB1 and group S1 increased.

The data presented in Fig. 7 show that in the Sudeten the share of trees with defects (in the light of the standards) slightly increased along with the deterioration of spruce stand quality classes. In the Carpathians and in both ranges, the share of trees with defects affecting timber quality decreased along with the deterioration of spruce stand quality classes.

In the Sudeten Mts, the stands of lower quality classes had a slightly higher share of trees with open knots, a much higher share of trees with overgrown injuries and rot (especially internal rot), but a lower share of trees with open injuries and burls (Fig. 8). In the Carpathians, along with the deterioration of spruce stand quality classes, the share of trees with open knots and injuries (both overgrown and open ones) decreased, while trees with curvatures (unilateral, bilateral and multilateral ones) and rot (both external and internal one) occurred more often.

In both ranges of the occurrence of spruce, in the stands with lower quality classes, there was a decrease in the share of trees with knots and open injuries and an increase in the share of trees with internal rot in comparison with better quality stands. In the south-western range, the frequency of occurrence of trees with overgrown injuries and internal rot was higher in the stands of lower quality classes; in the north-eastern range this relation was reverse.

In all groups of material (all locations and ranges), the share of healthy trees without internal rot decreased along with the deterioration of spruce stand quality classes, while the share of trees with rot (soft and hard) increased (Fig. 9). The share of trees with soft rot was an exception in the north-eastern range, where it decreased on the sample plots in the stands of lower quality classes.

In the Carpathians and the whole south-western range, the share of trees with annual ring width of up to 7 mm increased along with the deterioration of spruce stand quality classes (Fig. 10). In the Sudeten, this relation was reverse. In the north-eastern range, in the stand quality classes from I to II, there were fewer trees with annual ring width of 4–7 mm than in classes from III to IV but there occurred trees with the widest annual rings (over 7 mm).

Site type of forest

For research purposes, 9 groups of stands were distinguished corresponding to the site types occurring on the sample plots. In the north-eastern range, the stands under analysis grew on 4 types of lowland sites; in the south-western range – on 5 types of mountain sites.

In the Sudeten Mts, the highest volume/ha of high quality spruce timber, i.e. classes WA, WB and WB1 (jointly), was found on the LMG site (altogether 133 m³·ha^-1); in the Carpathians – on the BMG site (about 123 m³·ha^-1); and in the whole south-western range – on the LMG site (about 120 m³·ha^-1) (Table 6). The largest joint volume/ha of middle-sized timber of poor quality (groups S2 and S4) was noted, in the Carpathians and the whole south-western range, on the BWG site (respectively about 38 m³·ha^-1 and 33 m³·ha^-1). In the north-eastern range, the largest joint volume of timber of the best classes (WA, WB and WB1) was found in the Lśw site (about 86 m³·ha^-1). The largest average volume of middle-sized timber of groups S2 and S4 treated jointly was noted there on the BMśw site (about 19 m³·ha^-1).

The comparison of the quality structure of timber (in m³·ha^-1) from the analysed site types of forest by means of the chi², statistically significant differences were noted only in 5 cases (distinguished in Table 7).

Due to a large number of data, the shares of classes and groups of timber according to locations and ranges were presented in Table 8, and not as a diagram.

In the Sudeten Mts, the Carpathians and the whole south-western range, the highest share of high quality spruce timber, i.e. classes WA, WB and WB1 (jointly), increased with an increase in site fertility, reaching the maximum in the LMG site; in the LG site the share of these classes diminished. The largest joint share of timber of groups S2 and S4 in the Carpathians and the whole south-western range was found on the BWG site (respectively: about 12.6% and about 10.3%) (Table 8).

In the north-western range, site fertility did not affect the joint share of timber of the best quality classes (WA, WB and WB1) so largely as in the mountains. The largest share of these classes was noted on the Lśw site (about 29%) and the smallest – on the LMśw site (about 17%). The joint share of timber of groups S2 and S4 on particular lowland sites, distinguished for the purpose of the present research, was similar and each time amounted to about 6%.

In the mountain stands, the share of trees whose quality was lowered by defects on particular site types and in the areas of occurrence of spruce was very differentiated (Fig. 11). Differences in the share of trees with defects that occurred between the Sudeten and the Carpathians on the same site types of forest were very large. The smallest difference (10.4%) was noted on the LG site. Within the ranges of occurrence of spruce, the maximal shares of trees with defects were found on the most fertile sites. In the south-western range this was the LG site and in the north-eastern range – the Lśw site.

In the south-western range, spruce stands growing on the BWG site were characterized by the highest – of all sites considered – frequency of occurrence of trees with curvatures and external rot (Table 9). On the BG site more often than on the others there occurred trees with overgrown injuries, on the BMG site – trees with internal rot and on LMG – trees with open injuries. On the most fertile site, i.e. LG, the share of trees with open knots and with burls was the largest in comparison with the other groups. In the north-eastern range, on the BMśw site, the share of trees with curvatures was the largest of all sites under analysis while on the BMw site the largest was the share of trees with open injuries. Trees affected by internal rot were the most frequent on the LMśw site; the Lśw site, which was the most fertile one, was characterized by the maximal share of trees with open knots, overgrown injuries and internal rot.

Analysis of the stands of both ranges showed that on more fertile sites the trees with knots and burls were more frequent while on less fertile ones – the trees with curvatures. Stands growing on poor sites in the south-western range had large shares of trees with rot, especially internal rot.

The stands growing on more fertile sites (LMG and LG) were healthier in the Sudeten than in the Carpathians as far as the occurrence of internal rot is concerned (Fig. 12). The highest shares of healthy trees as well as a lack of trees with soft rot were noted in these stands in comparison with the other sites in this range. For technical reasons, no increment cores were taken in the stands growing on the BMG site in the Carpathians; therefore, they were disregarded in the present research. The Carpathian stands growing on different sites showed relatively similar shares of trees without rot; however, there was a tendency towards increasing the frequency of occurrence of trees with soft rot on more fertile sites. In the whole south-western range, the stands growing on more fertile sites were healthier. The image of the whole range was affected by the stands in the Sudeten, which had considerable shares of healthy trees on fertile sites. In the north-eastern range, the best healthiness characterized the stands growing on the BMw site, followed by the Lśw and BMśw ones. The weakest in this respect were the stands on the LMśw site.

In the Sudeten, in the stands on the BG and LMG sites, there occurred only trees with the average annual ring width of up to 4 mm (Fig. 13). In the Carpathians, the smallest annual ring width characterized the stands on the BWG and BG sites; the share of trees with annual ring width of 4–7 mm was noted in the BWG and LMG sites (in the Sudeten and in the Carpathians, respectively).

In the mountains, there is a tendency towards forming timber with narrower average annual rings in the stands growing on less fertile sites (BWG, BG and BMG). Also in the north-eastern range, the share of trees with annual ring width of up to 4 mm decreased along with increasing fertility of sites; the share of trees with annual ring width of 4–7 mm increased steadily. In the stands growing on the most fertile sites, LMśw and Lśw, there occurred trees with the widest annual rings: of over 7 mm. In may be noted that in the spruce timber under analysis, annual ring width increased with increasing fertility of a site. Such a tendency was the most noticeable in the north-eastern range.

DISCUSSION

In the south-western range, in subsequent, older and older age groups, the joint share of trees of better quality (WA, WB and WB1) decreased. Their share in the age group of 75–100 years amounted to 29.5% and in the age group of over 120 years only about 10% (Fig. 1). In the stands of the first two age groups of the two ranges, there is a tendency towards decreasing the share of class WB and increasing the share of class WA in older stand groups. This may be explained by the rules of coniferous timber classification [39], where the main criterion to differentiate between timber class WA and WB is the size of the nominal diameter. For WA the minimum is 35 cm and for WB it is 25 cm. Quality requirements for both classes are similar. In older stands under research, the share of timber class WB decreased as the trees “passed” to class WA once they reached a larger size. A decrease in the share of timber of the best classes, WA and WB, in the oldest age group (of over 120 years) was related to the depreciation of standing timber.

In the oldest age group of the stands under analysis in both ranges of occurrence of spruce, the quality of spruce timber decreased, which was expressed by a low percentage of timber of high quality classes treated jointly (Fig. 1).

In subsequent, older stand groups, timber depreciation occurred due to large shares of trees affected by rot. The phenomenon of an increased frequency of occurrence of trees with rot in spruce stands of increasing age was noted in Germany by Schlenker [33] and in Great Britain by Pratt [28]. Research conducted in southern Poland by Barszcz and Rutkowska [6] and by Krzan [12] confirmed this dependence. Research carried out in the former Soviet Union by Storozenko and Igolkina [36] showed that an increase in the share of trees affected by rot accompanied the passing of stands to older age groups but that the number of affected trees fell when they exceeded the age of 80–100 years. The authors explain that the affected trees are the first to become broken by strong winds while the healthy ones remain in stands. A similar phenomenon was observed in Switzerland by Graber [8], who stated that in spruce stands of artificial origin the share of trees with rot increased until the age of about 110 years, after which it decreased – while stands of natural origin were characterized by the highest frequency of trees affected by rot in the oldest age groups (of over 130 years).

Apart from rot, knots are a defect which significantly affected the quality of the spruce timber under analysis. Research conducted by Barszcz [3,5] in the Beskidy Mts show that with age the influence of knots on the results of the quality and size classification of the butt-end part of the spruce stem diminishes. Such a result was also obtained in the present research, particularly in the case of the oldest age group, i.e. the one over 120 years. Research conducted by Arlauskas and Tjabera [1] in the Baltic regions of the former Soviet Union and consisting in measuring all knots over two-metre sections of the whole stem indicated that the number of knots on the stem increases with an increase in the diameter and age of spruce trees.

According to research on spruce stands conducted in Germany by Poller [22], annual ring width decreases with an increasing age of stands. Polish research carried out by Ochał [17] as well as Orzeł and Forgiel [18] in the Beskid Sądecki Mts confirm this tendency. In the present research, it was confirmed for the Carpathian Mts.

In the north-eastern range, more often than in the mountains, there occurred two–layer, uneven-aged stands. Trees growing in the lower layer usually had wider annual rings, which may have influenced the image of the structure of annual rings in he stands of this range.

Spruce stand quality classes
As results from the present research, both the overall volume/ha of spruce, the volume/ha of the best classes (WA, WB and WB1) (Table 4) and timber quality expressed as the percentage of timber classes WA, WB and WB1 decreased along with decreasing stand quality classes.

The influence of knots on the results of quality classification, considering the lowering of the stand quality classes, decreased in the stands of both ranges. The results of research by Barszcz [4], carried out in the mature spruce stands of the Beskidy Mts, prove that in the stands of better quality classes, the length of the stem section without knots and the height of location of the first knots increase.

Analysis of increment cores obtained from the stands under the present research confirms the results of research by Barszcz and Rutkowska [6], conducted in the Beskidy Zachodnie Mts. Similarly to the present research, the authors mentioned above showed that the share of trees affected by internal rot increases in the stands with lower quality classes.

Site type of forest
As results from the present research, spruce timber from the north-eastern range had the best quality, expressed as the joint share of high quality timber (WA, WB and WB1) both in the stands growing on the optimal site (BMw) and in the ones on the most fertile site (Lśw). The Zasady Hodowli Lasu [40] advise the introduction of spruce in the lowlands, within the areas of its natural occurrence, in the proportion of up to 50% on e.g. the BMw site whereas on the Lśw site it should be introduced as an additional species.

The mountain spruce stands under analysis, similarly to the lowland ones, had the best quality, expressed as the average volume/ha of timber of good quality classes (WA, WB and WB1) (Table 6) and the percentage of timber of these classes (Table 8) on optimal sites (BMG) and fertile sites (LMG). The results obtained confirm, among others, Szymański’s view [37], who maintains that the optimal site for the acquisition of the most valuable spruce timber, including resonance timber, is mixed mountain forest in the south-western range. In the mountains (the VII and VIII Regions), The Zasady Hodowli Lasu [40] advise the introduction of spruce in a considerable share: on the BMG site up to 80% and on the LMG site up to 60%; spruce may also be introduced on the BWG site even up to 90%. However, this is not dictated by good quality of its timber but by the resilience of this species to unfavourable climatic conditions. The existing general opinion about poor quality of spruce timber growing on the BWG site in higher mountain locations was also confirmed by the present research.

According to the present research, the share of trees with knots which negatively affect the results of the quality and size classification was, in both ranges of occurrence, the highest on the most fertile sites (LG and Lśw) (Table 9). These results confirm the research conducted earlier by Arlauskas and Tjabera [1,2], mentioned above, who noted that an increase in the number of knots on spruce stems accompanied an increase in soil fertility. The view of many researchers [16,7,12] that on fertile sites spruce reveals the symptoms of rot more often than on poorer sites was not fully confirmed in the present research because in the stands of the south-western range it is the stands growing on the LMG and LG sites that were the healthiest (analysis of increment cores). In the north-eastern range the highest frequency of occurrence of internal rot was noted on a relatively fertile site LMśw.

Numerous authors [32,16,11] maintain that the growth and development of spruce stands is more strongly affected by a degree of moisture in the soil than its fertility. Leibundgut [13] proved the existence of a strong relation between stem cracks, which appear in spruce during drought, and a kind of site. The drier the site, the more frequent the cracks are. The results of research by Przezbórski [29] show that a degree of the presence of rot in spruce increases after periods of long-term droughts, which may explain a smaller frequency of occurrence of trees with rot on wet sites. This was also confirmed by the present research, which shows that, in the north-eastern range, the stands growing on the BMw site are the healthiest (Fig. 12). Schönhar [34] points out that on soils with a variable moisture content the share of spruce affected by rot may reach even 30%. The influence of moisture conditions is particularly visible near the borders of the natural range of spruce, where soil fertility has only a slight influence on the existence of spruce [15]. Some of the analysed stands in the north–eastern range were located near the borders of the natural range of spruce, which may have improved the health condition of stands on wetter sites (BMw).

In the north-eastern range, along with an increase in site fertility, the frequency of occurrence of trees with the narrowest annual rings decreased while the share of trees with rings of 4–7 mm increased and, in the case of the LMśw and Lśw sites, there occurred the widest rings, of over 7 mm (Fig. 13). In the analysed mountain stands on the LMG and LG sites, which are the among the more fertile ones, relatively large shares of trees with rings 4–7 mm wide were also found. Annual ring width is not taken into account in the Polish standards applied in the classification of coniferous timber; hence they have no effect on its results. However, this defect is considered in the European Standard [21] and may significantly influence the result of timber quality classification. Annual ring width is directly related to the physical and mechanical properties of spruce timber [9], which is why many clients prefer timber with narrow rings.

In the Sudeten Mts, the stands aged over 120 years, the ones with quality classes III–IV and the ones growing on the BWG site were characterized by the highest shares of trees with annual rings of 4–7 mm. Most of these stands grew in high locations in the mountains; and the paradox consisting in an increase in annual ring width along with increasing altitude was described by Modrzyński [14] after Blanckmeister. According to him, at the turn of the 20^th c., the stands of the lower forest zone grew out of very dense sowing. Maintenance cutting was performed there for the first time at the age of about 40 years and these measures were not very intensive. The stands in higher mountain locations became renewed out of self–seeding and they were thinned naturally, which might have caused the formation of wider annual rings in them, despite more severe climatic conditions.

CONCLUSIONS

1. In the analysed mature mountain stands (in the Sudeten and the Carpathian Mts), in older and older age groups, timber quality deteriorated, which was expressed as the percentage of timber of better quality classes. In the stands of the whole south–western range, a decrease in the quality of spruce aged 101–120 years was compensated by still large volume/ha and, as a result, by a large amount of timber in m³·ha^-1. In older stands, despite their relatively large volume/ha, a distinct decrease in timber quality was noted. In the analysed stands of the north–eastern range, the best quality of spruce timber was found in the age group of 101–120 years but a better average volume/ha characterized younger stands. The oldest stands were of good quality there but their volume/ha was low.

2. The oldest age group (in both locations and ranges of occurrence of spruce) was characterized by relatively low shares of trees with knots but an increased occurrence of trees affected by rot.

3. On the basis of the results obtained, it is advised that the age of stand felling, which expresses the average age of reaching the technical production purpose, should be adjusted to stand technical maturity. For example, in the mountains, in the case of the focusing the economy on producing good quality spruce timber, this age should be assumed to be about 100 years. For the purpose of the production of a large mass of timber, this age may be increased by about 20 years, provided that timber quality is relatively good.

4. In the analysed stand groups, a decrease in spruce timber quality was noted along with the lowering of stand quality classes. This tendency was noticeable in volume/ha and in the share of timber classes of high quality. In the Carpathians and in both ranges of occurrence of spruce, a decrease in the share of trees with defects was noted in stands of weaker quality classes. There was also noted a smaller frequency of occurrence of trees with knots and a higher frequency of occurrence of trees with internal rot.

5. The analysed stands had high quality timber not only on optimal sites, i.e. BMG (mountains) and BMw (lowlands), but also on fertile sites (respectively LMG and Lśw). Although the forest management rules do not advise introducing spruce in large numbers on fertile sites, the results obtained in the present research suggest the need to undertake, for cognitive reasons, further studies on the relations between site types of forest and spruce timber quality.

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Accepted for print: 1.02.2008