RADIAL GROWTH RESPONSE OF SCOTS PINE (PINUS SYLVESTRIS L.), BLACK PINE (PINUS NIGRA ARNOLD), AND BLACK ALDER (ALNUS GLUTINOSA (L.) GAERTN.) TO THE NUN MOTH (LYMANTRIA MONACHA L.) OUTBREAK IN THE SŁOWIŃSKI NATIONAL PARK AND THE DAMNICA FOREST DISTRICT (NORTHERN POLAND)

Bartłomiej Bednarz, Dominik Kochanowski
Department of Forest Protection and Forest Climatology, Agricultural University of Cracow, Poland

ABSTRACT

It was found that deep growth depressions which occurred between 1981 and 1983 in Scots pine (Pinus sylvestris L.) and black pine (Pinus nigra Arnold) in the Słowiński National Park and neighboring forests (Damnica Forest District) were the consequence of the nun moth (Lymantria monacha L.) outbreak. Black alder (Alnus glutinosa (L.) Gaertn.), growing in the same area, was free of mass feeding of this foliophagous insect. Mean widths of annual rings of both these pine species in a 5-year period in which the nun moth outbreak occurred were smaller than mean widths in 5-year periods before and after the outbreak. These differences were statistically significant. It was also found that the effect of the nun moth feeding on diameter growth of Scots and black pines growing on a dry coniferous forest site was lower than in the case of a fresh mixed coniferous forest site. These differences were statistically significant.

Key words: Lymantria monacha, foliophagous insects, outbreak, defoliation, growth depression, tree rings.

INTRODUCTION

The nun moth (Lymantria monacha L.) is one of the most dangerous primary insect forest pests. In Poland since World War II its outbreaks occurred five times. As a foliophagous insect, contrary to secondary insect pests which only exceptionally attack healthy stands, the nun moth breeds and feeds in weakened stands growing on poor sites as well as in healthy stands growing on fertile sites [20].

At first the nun moth mainly threatened pure Norway spruce stands. Not sooner than after World War II it became a pest of Scots pine, and to a lesser degree Norway spruce and European larch [21,20]. Apart from these species, feeding of the nun moth was observed on almost all species of broadleaf trees and shrubs. During outbreak caterpillars of this species also feed on herbaceous plants of the forest floor, especially on Vaccinium vitis-idaea L. and ferns [21].

During the outbreak, feeding of the nun moth may cause a complete or partial defoliation of tree crowns, and in consequence cause losses in diameter, height, and volume increments of trees [4,21,23,27]. During the mass occurrence of this pest usually chemical control in form of insecticide sprays is used. In the late 1970s one of the greatest nun moth outbreak in Poland, and even in Europe, began. The pest showed a tremendous dynamics in its reproduction thus expanding over the large area. Between 1978 and 1985 the nun moth invaded forests of northern and western parts of Poland. In total, 7 295 181 ha of forests were attacked, including 3 489 898 ha of a very severe attack. In the Słowiński National Park the nun moth outbreak lasted for three years (1981–1983), and chemical control treatments were carried out in 1982 and 1983 [21,22].

The purpose of this study was to determine the effect of the outbreak feeding of the nun moth on width of annual tree-rings of Scots and black pines growing on the dry coniferous forest site and the fresh mixed coniferous forest site, as well as of black alder growing on the alder swamp forest site so far considered to be a little attractive food base for Lymantria monacha L. [21,22]. The study was also concentrated on differences in response of trees growing under different site conditions to the nun moth outbreak. It was also attempted to supply documentary evidence of the progress of the nun moth outbreak in northern Poland during 1981-1983 on the basis of tree-ring width increments.

STUDY AREA

Studies were conducted on five study plots situated in northern Poland, in the Słowiński National Park (N54°43', E 17°13') and in stands of the northern part of the Damnica Forest District in a direct neighborhood of the southern boundary of the Park (N54°40', E17°24') (Fig. 1).

The plots were located 3–8 m above the sea level, several hundred meters away from the Baltic coast. From the geobotanical point of view this fragment of Pomerania is situated in the Baltic Coast Region forming a narrow strip of land along the coast [19]. According to division of Poland into natural forest regions [24] the area under investigations was situated in the Wybrzeże Słowińskie Mesoregion of the Sea Coast Subregion of the Baltic Natural Forest Region.

The investigated pine stands of two plots situated in the Słowiński National Park were growing on the dry coniferous forest site in the phytosociological association Empetro nigri-Pinetum. Pine stands of the other two plots situated in cultivated forests of the Damnica Forest District were growing on the fresh mixed coniferous forest site in the association Querco roboris-Pinetum. The black alder stand in the fifth plot situated in the Damnica Forest District was growing on the alder swamp forest site in the association Ribeso nigri-Alnetum [15] (Table 1).

The progress of the nun moth outbreak in the Słowiński National Park and the Damnica Forest District between 1981 and 1983

The first more numerous appearances of the nun moth in pine stands under investigations were already observed in 1978 and 1979. During 1981–1982 partial, and in some cases complete, defoliation of Scots pine trees occurred in many parts of the Słowiński National Park and the Damnica Forest District. Mass feeding, and even a complete defoliation, was also observed in crowns of black pine. During the outbreak between 1981 and 1983 the nun moth caterpillars were also feeding on herbaceous plants of forest floor, mainly on Vaccinium vitis-idea L., V. uliginosum L., and ferns. No mass feeding was observed in black alder stands, and only sporadic caterpillars were found on alder leaves.

MATERIAL AND METHODS

The study material consisted of increment cores taken at breast height with the Pressler's borer from trees of the following tree species: Scots pine (Pinus sylvestris L.), black pine (Pinus nigra Arnold), and black alder (Alnus glutinosa (L.) Gaertn.), growing on forest sites of three types: dry coniferous forest, fresh mixed coniferous forest, and swamp alder forest. In each population 30 trees were cored, 150 trees in total. These trees were devoid of external deformations and disease symptoms, and according to Kraft's classification they belonged to the main stand layer (dominant and codominant trees). Height of each tree was measured with Sunto hypsometer, and dbh with a diameter tape (Table 1). From each tree two cores reaching the pith were taken along two radii perpendicular to each other. The increment cores were used to determine tree-ring width. These measurements were done in the laboratory using the measuring set BIOTRONIK BEPD4C, exact to 0.01 mm. The tree-ring chronologies of Scots and black pines [9] were used to compare width of annual rings of pines growing on sites of two different types (dry coniferous forest and fresh mixed coniferous forest), and the significance of differences obtained was determined using the Student's t-test [13].

Also the actual mean ring widths for 5-year periods were computed. They were compared with mean annual increments from 5-year periods before and after the nun moth outbreak as well as with mean tree-ring widths from the 5-year period in which the outbreak occurred. Significance of differences between mean increments of respective periods was determined by the Sudent's t-test [13].

RESULTS

On the basis of tree-ring chronologies, including 70 and 109 years in the case of Scots pine, 80 years in the case of black pine, and 34 years in the case of black alder [9] it was found that there were differences between tree-ring widths of both pine species growing on the dry coniferous forest site and tree-ring widths of these species growing on the fresh mixed coniferous forest site. Trees of both pine species growing on the fresh mixed coniferous forest site had greater annual increments than trees growing on the dry coniferous forest site. This difference was statistically significant at the significance level p<00.5.

During the nun moth outbreak (1981–1983) width of tree-rings considerably decreased reaching its minimum in 1982, i.e. a year after the beginning of intensive feeding of caterpillars. While, after termination of the 3-year outbreak period pine trees significantly (p<0.05) increased width of their annual rings (Fig. 2). It was also found that during the outbreak period annual rings of both pine species were characterized by increased percentage of the early wood.

In both pine species during the nun moth outbreak there were statistically significant (p<0.05) differences between width of annual rings of trees growing on the dry coniferous forest site and ring width in trees growing on the fresh mixed coniferous forest site. A greater growth depression occurred in trees growing on the latter site.

In black alder growth depression did not occur during the nun moth feeding. In the tree-ring chronology of this species the decreasing tendency occurred earlier, lasting until 1981. After that year, the first year of the nun moth outbreak, a gradual increase of ring width was taking place. This increase lasted until 1984 (Fig. 2E).

Mean tree-ring widths in pines during the 5-year period including the nun moth outbreak were smaller than during periods before and after the outbreak (Fig. 3). In Scots and black pines in the fresh mixed coniferous forest and Scots pine in the dry coniferous forest the mean increment in the outbreak period was the lowest one among all mean 5-year tree-ring increments (Fig. 3).

The value of radial increment during feeding of the nun moth made from 79% (in Scots pine in the fresh mixed coniferous forest) to 99% (in Scots pine from the dry coniferous forest) of the 5-year increment for the period before the outbreak. Mean increment during the outbreak was also smaller than that in the 5-year period after the outbreak, by 40% in black pine in the dry coniferous forest, and by 5% in Scots pine in the dry coniferous forest. Differences between mean tree-ring widths from the period including the outbreak (1980–1984) and mean widths from 5-year periods before and after the outbreak were statistically significant (p<0.05 and p<0.001 respectively) only for Scots pine growing on the fresh mixed coniferous forest site. In the case of black pine population in the fresh mixed and dry coniferous forests, statistically significant differences only occurred between increment of the 5-year period after the outbreak and the period with the outbreak (p<0.001). While for Scots pine growing on the dry coniferous forest site differences between mean tree-ring widths of respective 5-year periods were statistically insignificant.

Black alder was characterized by greater annual increments in the 5-year period before the outbreak, by 12% in relation to the 5-year outbreak period, and by 25% in relation to the 5-year period after the outbreak (1980–1984) (Fig. 3). However these differences were statistically insignificant.

DISCUSSION

In the Słowiński National Park, during the nun moth outbreak, its caterpillars were feeding not only on Scots pine but also on Vaccinium uliginosum L., Ledum palustre L., and Myrica gale L. [28,7]. While Śliwa [21,22] reported that so far no nun moth feeding was found on Pinus nigra Arnold, Pinus mugo Turra, Alnus glutinosa (L.) Gaertn., Fraxinus excelsior L., Aesculus hippocastanu L., Robinia pseudoacacia L., and Prunus serotina Ehrh. These observations were confirmed by forest managers of the Smołdzinski Forest Protection Unit in the Słowiński National Park and the Główczyce Forest Range in the Damnica Forest District who also did not observe feeding of the nun moth on black pine and black alder during its outbreak (personal information). Different observations were made by Cykowski and Grzelak [7] and Ziemniewicz [28] who reported that in the Słowiński National Park the nun moth feeding also occurred on Pinus nigra and P. mugo.

Beker [1] found significant relationship between mean annual dbh increment and mean defoliation of pine crowns. According to this author dbh increment could be more commonly used as a criterion in estimation of the degree of tree damage. Also Pawlik [17], Brubaker and Greene [5] found that during caterpillar feeding current height increment as well as diameter increment of Scots pine are very small or absent. Similar growth depression occurred in Norway spruce after defoliation caused by Cephalcia falleni Dalm. in the Świętokrzyski National Park [12].

This study showed that during the nun moth outbreak in the Słowiński National Park and the Damnica Forest District the deepest growth depression in Scots pine occurred during 1982-1983 with minimum in 1983. Such growth depression was also found in black pine, a species so far considered to be unattractive to Lymantria monacha L. In the case of black alder dendrochronological analyses carried out during this study confirmed absence of mass feeding of nun moth caterpillars in crowns of this tree species, in spite of the fact that continuous decreasing tendency in radial growth was observed since 1978. A lack of the occurrence of intensive nun moth feeding on black alder in the Damnica Forest District during 1981-1983 also was shown by a lack of statistically significant differences between mean tree-ring widths computed for 5-year periods before, during, and after the outbreak. Results obtained during this study have confirmed observations of other authors that nun moth caterpillars do not feed on black alder [21,22].

A complete or a considerable defoliation, caused by a foliophagous insect, results in deficiency of assimilates and also makes the synthesis of growth hormones activating the cambium to cell division impossible. This is why in pines affected by intensive feeding of foliophagous insects the diameter growth is inhibited [2]. The investigated pine trees during the nun moth outbreak had very narrow annual rings with a high percentage of early wood. Similar observations were made by Muzika and Liebhold [16].

In all investigated pine populations the deepest growth depression occurred in 1982, and in pines growing on the fresh mixed coniferous forest site a considerable restriction of diameter growth also occurred during the next year (1983).

It was found in this study that during the nun moth outbreak in the case of both pine species investigated, there was a difference in width of annual rings between trees growing on the dry coniferous forest site and trees growing on the fresh mixed coniferous forest site. Most likely this was caused by difference in tolerance to stress factors between trees growing on poor and those growing on fertile sites. Pine stands growing on the dry coniferous forest site after deep growth depression in 1982 faster responded with increased tree-ring width during the next year than stands growing on the fresh mixed coniferous forest site in which the period of growth depression lasted for two years. Also later pine response characterized by decreasing width of tree-rings not sooner than during the second year (1982) of the outbreak was observed. Similar relationship was found in Scotland by Straw [18] in stands attacked by Bupalus piniaria L. Many other authors are of the same opinion [3,14,10]. A lack of negative growth response during the first year of defoliation may be explained by the fact that trees during previous years had accumulated enough nutrients to continue their growth [18,14,10,11,8].

In three investigated pine populations the increase of tree-ring width occurred in the first year of the nun moth feeding (1981), while growth depression occurred in the next year. Creighead (1925) in his study of Picea glauca (Moench) Voss and Abies balsamea (L.) Mill. found that there was a considerable reduction in tree-ring width during the first year of defoliation, but only in the upper parts of the trunk, while at 1.3 m above the ground rings were wider. This occurred because there was a certain amount of nutrients stored and trees took advantage of them. Defoliations lasting for several years, strongly reducing assimilative abilities of trees, formation of buds, and storage of reserve materials, caused reduction of tree-ring width during the next year after the outbreak [6].

In stands under investigations, as it has been already mentioned, the nun moth outbreak lasted for 3 years, and it was artificially stopped by a chemical treatment. It is supposed that growth depressions observed in tree-rings would be more distinct and longer if chemical treatments were not applied.

After stress created by defoliation, trees usually come back to physiological equilibrium and increase their annual wood increments [26]. Also in the study presented in this paper similar observation was made. It may be supposed that this process was also aided by soil fertilization with large amount of caterpillar excrements. The observed regularities were distinctly more evident in trees growing on a poor site of the dry coniferous forest than on a more fertile site of the fresh mixed coniferous forest. Also caterpillars falling from trees after the chemical treatment formed organic matter enriching the nutritional base of trees. According to Witkowski [25, 26] in pine stands in which the nun moth was controlled using a chemical treatment the tree diameter increments were by 20 to 30 percent greater than in control stands with no chemical treatments.

Outbreaks of foliophagous insects often occur in forests, and they are usually the result of insect breeding dynamics. They most often occur in even aged pure stands growing on sites of low fertility, especially during years of low precipitation or directly after such years [22]. Similar observations were made by Kochanowski and Bednarz [9]. They found that during the nun moth outbreak (1982–1983) in the Słowiński National Park and neighboring forests total monthly precipitation in July and August was 5 and 12 mm respectively, and this was much lower than long-term means amounting to about 80 mm.

In the presented study, in order to show the effect of the nun moth feeding on annual diameter increments of Scots and black pines the reference was made to periods before the outbreak. Authors are aware that the results of this study would be more complete if it was possible to conduct comparative studies in stands of similar area but with no insect outbreak. Unfortunately the nun moth outbreak between 1981 and 1983 was so widely spread that almost all pine stands of the Baltic coast in Poland were attacked by this insect species.

CONCLUSIONS

1. This study has confirmed the fact that the nun moth outbreaks have a negative effect on diameter growth of Scots pine growing on poor (dry coniferous forest) as well as more fertile (fresh mixed coniferous forest) sites.

2. Observed mass feeding of this foliofagous insect species in crowns of black pine, a species so far without reported nun moth outbreak, is an important founding. Growth depressions during 1981-1983 in this tree species resulted from defoliation of tree crowns.

3. The differences in growth observed in trees growing on poor and more fertile soils showed a greater tolerance to biotic stress factors of trees growing on less fertile sites.

4. A drastic drop in radial increments in both pine species in stands with the nun moth outbreak may be compensated by increased wood increments during years after the outbreak retreat.

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Accepted for print: 16.04.2010

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