THE RANGE OF CROWN DAMAGE CAUSED BY THE PINE SHOOT BEETLES TOMICUS SPP. (COL. SCOLYTIDAE) IN SCOTS PINE STANDS SITUATED IN THE NEIGHBORHOOD OF STACKED WOOD YARDS IN SOUTHEASTERN POLAND

Piotr Bilański
Department of Forest Protection and Forest Climatology, Agricultural University of Cracow, Poland

ABSTRACT

The location of stacked wood yards in outskirts of the town of Mielec (southeastern Poland) in 1998, near forests of the Mielec Forest District, caused the occurrence of severe damage to crowns of Scots pine (Pinus sylvestris L.) by the pine shoot beetles (Tomicus piniperda L. and Tomicus minor Hartig). A study was carried out during 1999 – 2002, and included felling of trees and determination of the crown damage index in 19 sample plots situated along 4 lines running in the interior of stands situated in the neighborhood of stacked wood yards. The range of crown damage caused by adult beetles during their maturation and regeneration feedings was determined on the basis of results of the proportion test. In the fifth year since establishment of wood yards the crown damage reached as far as 800 m inside stands situated east of the yards, i.e. 1800 m from the places of wood storage.

Key words: Tomicus piniperda, Tomicus minor, Pinus sylvestris, timber yards, crown damage.

INTRODUCTION

A wrong management of timber yards, especially those neighboring with the forest, may lead to a mass reproduction of insects, which subsequently cause damages in near stands.

This mainly concerns Scots pine stands endangered by the pine shoot beetles (Tomicus piniperda L. and Tomicus minor Hartig) breeding in timber yards. Their young adults migrate to crowns of pine trees for their maturation feeding.

Repeated damages caused by Tomicus spp. in the crowns lead to their deformation in such a way that they become conical in shape [16, 13, 20].

Injuries to shoots caused by adult beetles force the trees to use up their stored foods to produce new shoots, and this leads to decrease in their volume increment [16, 13, 14, 15, 5]. In addition, due to shoot deformation, the silvicultural quality of younger Scots pine stands becomes degraded [6]. Also in stands damaged by the pine shoot beetles there is less seed produced, because cones fall together with damaged shoots before they are ripe [17, 18].

Research on the effect of Tomicus spp. on Scots pine stands in the neighborhood of timber yards was carried out in Poland by Michalski and Witkowski [13, 14, 15], and then by Łęgowski [12] and Borkowski [1, 2, 3, 4]. In Scandinavia this problem was studied by Långström and Hellqvist [10].

In 1998 the “Kronwood” enterprise has established three large yards of unbarked stacked wood in outskirts of Mielec, and close to the forest. A year later considerable damages caused by feeding of adults of the pine shoot beetles in shoots were observed in crowns of trees in near Scots pine stands.

The purpose of this study was to determine the range of crown damage caused by maturation and regeneration feedings of adults of the pine shoot beetles in Scots pine stands of the Mielec Forest District.

MATERIALS AND METHODS

The study area was located in the Mosciska Forest Section of the Mielec Forest District. The western part of this forest section is situated in the neighborhood of the stacked wood yards of the “Kronowood” firm. The yards are located in a Special Economic Zone Euro-Park Mielec, situated in outskirts of the town of Mielec.

In the Mosciska Forest Section, as well as in the whole Mielec Forest District, Scots pine is the main forest tree species constituting 86.2% of the total volume of all tree species [19]. The investigated stands belonged to age classes III and IV (i.e. 40–80 years of age), and were growing on sites of a fresh coniferous forest, a fresh mixed coniferous forest, and a moist coniferous forest. All these stands were situated in zone II of industrial damages [19].

Stands threatened by insect pests migrating from wood yards were situated at different distances from places of wood storage due to irregular line of the forest edge (Fig. 1). This distance varied from 500 m to 1000 m, and more. The space between the yards and the forest was partly cultivated, partly occupied by farm buildings, or left as an area reserved for the Special Economic Zone.

The estimation of shoot damage in pine crowns caused by the pine shoot beetles during their maturation and regeneration feedings was carried out during the period from 1999 to 2002.

Each year shoot damage in sample plots was estimated in October, after felling of trees. In total, nineteen 5-are sample plots were established along lines running into the forest interior. Plots were situated at different distances from the stacked wood yards (Fig. 2). Each sample plot was designated by a symbol composed of a letter and a number. Successive letters (A, B, C, D) indicated respective lines, while numbers indicated the distance between the plot and the forest edge (e.g. 1 – 100 m, 2 – 200 m etc.). Lines A and B began at the distance of 1000 m from wood yards and run approximately eastwardly, while lines C and D began at the distance of 500 m and 800 m respectively, and run approximately southeastwardly. Sample plots on lines A, C, and D were situated 100 m apart, while those on line B 200 m apart, due to different environmental conditions. There was no plot A0 on line A because its establishment was impossible due to configuration of terrain and nearness of buildings. Sample plot B4’ was established 400 m away from the forest edge (similarly as plot B4), not on line B but at the road running from the wood yard eastwardly and perpendicularly to the forest edge (Fig. 2). This plot was established at that place in order to determine the effect of created opening on extent of crown damage caused by the pine shoot beetles. The control plot B43 was situated on the continuation of line B in compartment 186c, 4.3 km away from the forest edge, and 5.3 km from wood yards (Figs. 1 and 2). Trees of Kraft’s classes I and II were selected for this study. Each year 5 trees were felled in each plot. On each tree 100 shoots of 5 highest whorls were examined. Shoots with symptoms of maturation or regeneration feeding of adults of the pine shoot beetles were accepted as damaged ones, while shoots grown as a result of regeneration of damaged shoots were called the secondary shoots.

The degree of damage of shoots of individual trees was expressed by the index of crown damage in the form of fraction W_u or percentage of damaged shoots W_u%.

The index of crown damage for larger number of trees was determined by the mean value of fraction of injured shoots – or by the mean percentage of damaged shoots.– .

The mean indexes of crown damage () determined for respective plots were compared with those obtained for the control plot (B43). The boundary of crown damage caused by adults of the pine shoot beetles migrating from wood yards to forest stands was determined using a statistical method, i.e. the test for the difference between two proportions [9]. The plot was accepted as damaged by adult beetles when its value , at the significance level alfa=0.01, was statistically different from the value obtained for the control plot.

RESULTS

The proportion test permitted to determine the depth of beetle attack into the forest interior in directions represented by respective lines of sample plots during successive years of investigations.

In the first year of the study (1999) the proportion test showed that beetles migrated into stand’s interior for the distance of up to 300 m in the direction represented by line A. In the direction represented by line B the damage level statistically different from the control one was recorded as far as 400 m from the forest edge, while in the case of line C such a damage level was recorded not further than 200 m from the forest edge. In the case of the direction represented by line D results of the test showed that statistically significant differences in crown damage occurred only in the plot situated at the forest edge (Table).

The values of the damage index in plots of line A obtained in 2000, in spite of the fact that they were higher than in the previous year, showed that the range of a significant damage remained the same, reaching the distance of up to 300 m from the forest edge. Similarly the range of a significant damage on line B remained the same as in 1999, i.e. 400 m into stand interior. All plots situated on line C had in 2000 the damage index statistically higher than that calculated for the control plot. Thus the distance of 400 m from the forest edge may be assumed to be the distance of a significant effect of beetles migrating from wood yards in the direction represented by line C. In the case of line D such a distance in 2000 was 200 m (Table).

Table. Results of the proportion test for the difference between values of the crown damage index ()

In 2001 the distance of beetle migration from wood yards into the forest interior was at least 400 m on line A, 800 m on line B, 400 m on line C, and 100 m on line D (Table).

During the last year of the study (2002) values of obtained for respective plots generally were higher than during the previous two years. Results of the test on significance of differences between sample plots and the control plot showed that boundary of the range of beetle activity was not moved in comparison with the previous year, with the exception of line D, where, similarly as in 2000, trees of all plots were significantly more severely damaged than those in the control plot. Thus the distance of at least 200 m may be assumed as the distance of a significant effect of migrating beetles on line D (Table).

DISCUSSION

The methods assumed in this study, concerning the determination of intensity of shoot damage in Scots pine crowns caused by the pine shoot beetles, makes the comparison of results with results of other studies difficult. The earlier papers describing a spatial diversification of crown damage caused by the pine shoot beetles were based on two methods. The most popular one was to estimate the damage through determination of the number of damaged shoots fallen to the ground on a definite area under the trees. Such authors as Łęgowski [12], Borkowski [1], Långström and Hellqvist [10] used this method. The second method was based on observation of deformations of shoots in crowns of Scots pine trees. This method, besides the first one, was used by Michalski and Witkowski [13, 14, 15].

In spite of differences in methods the results of this study confirmed tendencies observed by other authors in a spatial distribution of shoot damage caused by the pine shoot beetles. During this study a severe damage occurred in crowns of trees nearest to wood yards. Similar results were obtained by such authors as Michalski and Witkowski [13, 14, 15], Łęgowski [12], Långström and Hellqvist [10], and Borkowski [1].

The crown damage in Scots pine stands caused by the pine shoot beetles migrating from wood yards was changing with passage of time, and depended on location of sample plots in relation to wood yards.

The mean index of crown damage () reached the highest values at the forest edge. The damage decreased as the distance from the forest edge increased. This decrease was rapid in the forest edge belt, 300 – 400 m wide, and then it was slower, gradually reaching low values nearing those obtained in the control plot situated 4300 m away from the forest edge. The range of damage caused by beetles migrating from wood yards was diversified in individual years and on respective plot lines. A similar pattern of damage occurrence was obtained by Långström and Hellqvist [10].

The highest values of the mean index of crown damage () in the first year of studies were registered in the forest edge plots. During the next two years there was a decrease of damage in these plots, while the values of the index increased in some plots situated deeper in the stands.

The crown damage in the fourth year of studies, especially in the forest edge plots, reached the level similar to that observed during the first year. At the same time there was a farther increase in the mean crown damage index () in plots situated in the stand’s interior.

The level of crown damage found in the control plot, situated 4300 m away from the forest edge, may be considered as unchanged by the attack of beetles migrating from wood yards because in some plots situated at smaller distances from yards the values of the crown damage index were lower or similar. During successive years the index of crown damage varied in respective plots. This variation decreased as the distance from the forest edge increased. Values of the index were most even in the control plot.

The highest values of the mean index of crown damage () were found in plots situated 500 m and 1000 m away from wood yards, amounting to 79.8 % (plot C0) and 75.4% (plot B0) respectively. In the same year the values of the damage index calculated for the forest edge plot (D0) situated on line D running southeastwardly was by a half smaller that the values calculated for plots mentioned above. This showed that beetles damaged trees situated close to the place of their reproduction, but at the same time the modification of the direction of their migration by wind is possible.

Michalski and Witkowski [13] supposed that the difference in extent of shoot damage between two young Scots pine stands was caused by the difference in the distance of their edges from the timber yard. Beetles during their maturation and regeneration feedings penetrated deeper the stand situated a little closer to the timber yard, the main place of their reproduction.

In stands investigated during the present study a reversed phenomenon was observed, because the most distant damages caused by beetles migrating from wood yards were recorded in the stand situated in zone II, i.e. 1000 m away from wood yards, while during initial years of a study in stands of zone I the damages occurred closer to the forest edge and wood yards. Most likely this was caused by the effect of wind on flying beetles. Westerly winds prevailed in the investigated area.

During successive years a decrease in intensity of crown damage was observed at a simultaneous increase of the distance of beetle migration, and increase of the area endangered by the pine shoot beetles.

The size of pine shoots may be one of the factors making them less attractive to beetles. For example secondary shoots, due to their dimensions, are not suitable for beetle feeding in their pith.

The adult of the lesser pine shoot beetle damages 3 to 5 shoots when feeding on pith section about 7 cm long [8]. Taking into account that diameter of secondary shoots is as a rule smaller than that of normal shoots, the number of secondary shoots damaged by a single beetle must be considerably larger, because in the case of many of them beetles were biting through them as they were too thin for beetle feeding in the pith.

The increase in the damage level during the fourth year of investigations to the level found during the first year may indicate a cyclic character of this phenomenon. This was probably caused by regeneration of crown shoots which took place after initial beetle feedings. After three years the damaged crowns produced secondary shoots in amount and size suitable for beetle feeding.

The decrease in the crown damage intensity in the forest edge zone during the second and third year most likely was connected with a lack of the adequate number of shoots suitable for beetle feeding. Twigs damaged by beetles undergo different deformations producing secondary shoots, which due to their dimensions may not be suitable for requirements of the pine shoot beetles [14, 10]. Michalski and Witkowski [13, 14, 15] concluded that the phenomenon of moving of crown damage into forest interior with passage of time is caused by shoot damage near timber yards in previous years, which forced beetles to look for shoots in the interior of stands. Lutyk [11] was of the opinion that a small number of undamaged shoots in the Scots pine stand adjoining a sawmill timber yard forced adults of the pine shoot beetles to look for shoots of silver fir and Norway spruce growing up to 200 m away from the forest edge.

Also other factors, such as the behavior of adult beetles after leaving their nests under the bark, may affect their migration and extent of damage caused. According to Eidmann [7] damage of pine shoots by young adults of the pine shoot beetles indicated that not more than 30% of population leaving nests under the bark conducted maturation feeding in the direct neighborhood of the colonized material. This may point out that in the case of a lack of shoots suitable for maturation feeding adult beetles migrate to more distant places. Also a natural tendency of insect adults to spread may be one of the causes of this kind of behavior.

In successive years the distance of beetle migration into forest interior was increasing. In the first study year statistically significant differences in intensity of crown damage occurred up to 400 m into forest interior, which was less than reported by Långström and Hellqvist [10]. According to results of their studies the range of damage caused by the pine shoot beetles in stands situated in the neighborhood of a large pulp mill timber yard was up to about 600 m in the first year of the yard’s functioning. In the present study in Mielec forests an increased crown damage occurred in a distance of 800 m from the forest edge.

In Scandinavia the distance of over 1000 m into forest interior was considered by Långström and Hellqvist [10] to be the greatest range of increased damage of shoots caused by adults of the pine shoot beetles migrating from timber yards. Borkowski [1] on the basis of statistical analyses of fallen shoots concluded that forest in a zone up to 300 m from a sawmill timber yard was affected by beetles migrating from the yard, while when using a method proposed by Łęgowski [12] determined the effect of beetles, similarly as this author, to be significant in a zone reaching as far as 600 m into interior of stands surrounding the sawmill. Taking into account study results of other authors, as mentioned above, as well as results of this study it may be concluded that the extent of damage and its dynamics depend on the kind and amount of timber stored in a yard, and also on frequency of its delivery and length of its storage. Similar conclusions were drawn by Långström and Hellqvist [10] pointing out that the amount of fallen shoots damaged by beetles, and therefore the level of damage, may depend on factors mentioned above.

CONCLUSIONS

1. Location of yards with unbarked wood near Scots pine stands in the Mielec Forest District already during first four years of their functioning caused migration of some species of cambio- and xylophagous insects to the near forest causing severe damage in tree crowns, and in consequence an increased mortality of trees.

2. A spatial range of crown damage caused by adults of the pine shoot beetles depended on location of stands in relation to wood yards. The damage intensity decreased as the distance from wood yards increased.

3. The range of migration of adults of the pine shoot beetles to conduct their maturation feeding increased during successive years.

4. In the fifth year since establishment of wood yards the damage in stands situated east of them ranged as far as 800 m away from the forest edge into its interior, i.e. 1800 m away from places of wood storage.

5. The distance of migration of adults of the pine shoot beetles for maturation feeding was the greatest in stands situated east of wood yards, i.e. in direction of prevailing winds.

6. Almost 80% of shoots were damaged on the average in crowns of Scots pine trees growing at the edge of stands situated 500 m away from wood yards.

7. In the tree crowns attacked by the pine shoot beetles the secondary shoots were formed, but due to their dimensions, they were less attractive to beetles in the next year. It was probably one of the causes of increasing range of crown damage during successive years.

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Accepted for print: 04.11.2006

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