Sawfly (Hym., Diprionidae) outbreaks on Scots pine: effect of stand structure, site quality and relative tree position on defoliation intensity

Forest Ecology and Management 194 (2004) 305–317

Sawfly (Hym., Diprionidae) outbreaks on Scots pine: effect of stand structure, site quality and relative tree position on defoliation intensity Bert De Somvielea,*, Pa¨ivi Lyytika¨inen-Saarenmaaa,b, Pekka Niemela¨a a

Faculty of Forestry, University of Joensuu, P.O. Box 111, 80101 Joensuu, Finland Department of Applied Biology, University of Helsinki, P.O. Box 27, FIN-00014 Helsinki, Finland

b

Received 18 January 2002; received in revised form 4 September 2003; accepted 27 February 2004

Abstract From 1997 to 2001, Finland has experienced its largest-ever documented outbreak of Diprion pini L. (500 000 ha). We investigated—in four affected areas—the effects of site quality, stand structure and relative tree position on outbreak dynamics during two consecutive years (1999–2000). Most of the variation in outbreak intensity, as measured by defoliation intensity, was accounted for by variables pertaining to tree size, indicating a preference for more mature trees. However, we also found elevated and systematic defoliation of younger stands. Tree species composition, age difference with surrounding stands, stand area, stand openness and soil characteristics are secondary to the tree size variables in explaining the defoliation intensity, and often had ambiguous effects. As for the common assumption linking outbreaks to shallow, well-drained and unproductive soils, our data only suggest such a link on the level of pine forest vulnerability on a landscape level. As for the various tree canopy strata, we found only in young stands significant differences in defoliation intensity between them, with the dominant trees most affected. This has implications for forest management, because these trees, now undergoing the highest growth losses due to defoliation, normally benefit as trees of the future from thinning activities. # 2004 Elsevier B.V. All rights reserved. Keywords: Diprionid sawfly; Diprion pini; Scots pine defoliator; Stand structure; Site quality; Relative tree position; Canopy strata

1. Introduction The Finnish pine forests have recently undergone the first large-scale outbreak of the common pine sawfly, Diprion pini (Linnaeus). First reported in * Corresponding author. Present Address: Laboratory of Forestry, University of Ghent, Geraardsbergse Steenweg 267, 9090 Gontrode-Melle, Belgium. Tel.: þ32-9-264-9049; fax: þ32-9-264-9092. E-mail address: [email protected] (B. De Somviele).

1997 to affect extensive areas in the West of the country (Anderbrant et al., 2000), the outbreak gradually covered areas dispersed over the whole breadth of Finland, in a latitudinal belt between ca. 628300 and 648300 N, with one affected area situated more to the South (Fig. 1). By 1999, the year in which the outbreak reached its peak, the infested forests covered an area of ca. 500 000 ha (Lyytika¨inen-Saarenmaa and Tomppo, 2002). Decline began in 2000, and by the end of summer 2001 the D. pini populations in most areas had dropped back to non-outbreak densities (personal observation, Martti Varama, unpublished).

0378-1127/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2004.02.023

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noscandia (Juutinen, 1967; Larsson and Tenow, 1984; Lyytika¨ inen, 1993a; Virtanen et al., 1996). Outbreaks by D. pini currently have to be regarded as a significant threat to boreal pine forests as well, not only because of the increase in affected area, but also because of their high damage potential. The ability of D. pini larvae to feed on all needle age classes of the host trees (Niemela¨ et al., 1982) leads to stands where growth losses are potentially higher, amount of dead trees larger and the secondary damages caused mostly by bark beetles more common (Lyytika¨ inen-Saarenmaa and Tomppo, 2002). D. pini feeding damage is contrasted with N. sertifer outbreaks, whose larvae can only feed on old needle classes and thus never totally defoliate a tree.

2. Aims of the study

Fig. 1. Outbreak areas in Finland of D. pini (in grey) and study areas ().

The massive size of the outbreak is a new and unexpected phenomenon, because D. pini outbreaks of this scale are generally confined to areas where the insect has a bivoltine life cycle (Ge´ ri, 1988), whereas in Finland D. pini is univoltine. In more southern parts of Europe, this pine defoliator is since long considered the most damaging member of the Diprionidae family (Ge´ ri, 1988). In Finland however, it is Neodiprion sertifer (Geoffroy), the European pine sawfly, that is believed to have the highest outbreak and damage potential. N. sertifer was until recently the only pine sawfly characterised by large-scale outbreaks in Fen-

Because D. pini is now a major pest species in the Fennoscandian forests, it is essential to improve our knowledge about the relations between stand structure, site quality, and outbreak intensity. Autecological inductive research of this kind on other forest pest species (Ge´ ri, 1983; Larsson and Tenow, 1984; Mitchell and Preisler, 1991; McMillin and Wagner, 1993) has often resulted in easily applicable silvicultural measures to reduce defoliation intensity and damages. At present however, scientific information on this subject is scarce for sawflies in general (McMillin and Wagner, 1993), and for D. pini in particular. A scientifically sound description of previously undetected or unexplained empirical patterns would allow us to move from specific observations to general conclusions (Oksanen, 2001) about sawfly defoliation dynamics in the Finnish environment. More specifically, the recent outbreak provided the opportunity to analyse the effect of stand structure, site quality and relative tree position on defoliation intensity by D. pini. In the discussion, we compared some of these relations with the better-documented outbreak dynamics of N. sertifer. We also made inferences about the validity of a number of the existing common assumptions about D. pini, such as its preference for mature and maturing trees (Ge´ ri and Goussard, 1986; Ge´ ri, 1988; Dajoz, 2000), and its preference for Scots pine stands on shallow, infertile and well-drained soils (Schwenke, 1964; Eichhorn, 1982; Larsson and

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Tenow, 1984; Hanski, 1987; Ge´ ri, 1988; McMillin and Wagner, 1993; Kouki et al., 1998). Finally, these analyses allowed us to make suggestions for forest management in susceptible areas.

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characterised by sandy soils of rather low forest productivity (Table 1), with a gradient of increasing productivity from Palokangas over Valkeaja¨ rvi and Rautavaara to Sonkakoski. In Valkeaja¨ rvi, Rautavaara and Sonkakoski, a proportion of the total area is composed of peatland, scattered between the larger areas of mineral soil. Relief is flat, or gently rolling (Rautavaara and parts of Valkeaja¨ rvi). The Palokangas and Valkeaja¨ rvi study areas consist mainly of stands where Scots pine is dominant, but in Sonkakoski and Rautavaara, Norway spruce (Picea abies (L.) Karsten) and birch (Betula spp.) are important as well (Table 1). In September 2000, stand inventory data were collected. They provided the eleven independent variables that were used to determine the effect of stand structure and site quality on defoliation intensity. In addition, the variable ‘‘stand attractiveness’’ was calculated, to assess the influence of the neighbouring stands’ age. Since tree age is assumed a critical factor to D. pini, we wanted to assess whether a stand surrounded by younger stands would be attractive, i.e. if it would attract adults emerging from its neighbouring stands, and therefore experience a higher

3. Material and methods 3.1. Study areas and stand characteristics The four study areas were located in the provinces of North Karelia (Palokangas and Valkeaja¨ rvi) and Northern Savo (Rautavaara and Sonkakoski) in eastern Finland, within the broad latitudinal belt to which this outbreak was almost totally confined (Fig. 1). They had all undergone significant defoliation in 1999, but to different degrees of intensity, which allowed for comparison of the results between severely (>50%), moderately (25–50%) and weakly (<25%) defoliated areas. The actual areas of investigation were located within larger outbreak areas, to avoid edge effects. The study areas are production forests characterised by even-aged and mostly small stands. They are Table 1 General characteristics of the four study areas

Valkeaja¨ rvi

Palokangas 0

0

0

Rautavaara 0

638430 N; 278300 E 123 47 16 MT: 41 VT: 59

8 0.5 1

99.5 (0.87)

97.6 (11.47)

73.2 (37.05)

88.3 (15.34)

Severe

Moderate

Moderate

Moderate

Severe

Moderate

Weak

Weak

62852 N; 30856 E 170 247.45 49 OmaT: 0.5 VT: 21.5 CT: 78

62851 N; 31808 E 133 297.25 113 MT: 1 VT: 88 CT: 11

Peatland area (% of total area) Artificially drained area (% of total area) Area where other tree species than Scots pine are dominant (% of total area) Volume share of Scots pine (% and standard deviation) Average defoliation at start of study; after feeding season 1999 (Defol99) Average defoliation after feeding season 2000 (Defol2000)

0 0 0

a

Sonkakoski

63833 N; 28837 E 138 118.5 27 MT: 11 VT: 47 CT: 39 ClT: 3 10 9 6.7

Geographic location (DMS co-ordinates) Altitude (m) Total area of study site (ha) Number of stands Forest site typea (% of total area)

0

0

25.1 61.4 0

In Finland, forest site type is commonly determined by soil vegetation indicator species, leading to the Cajander classification into six principal classes for productive forest (Cajander, 1926): (1) OMaT: Oxalis–Maianthemum type, with annual increment of 3–8 m3/ha; (2) OMT: Oxalis–Myrtillus type, with annual increment of 2.5–7 m3/ha; (3) MT: Myrtillus type, with annual increment of 2–6 m3/ha; (4) VT: Vaccinium type, with annual increment of 1.5–5 m3/ha; (5) CT: Calluna type, with annual increment of 1.5–4 m3/ha and (6) ClT: Cladonia type, with annual increment of 1–3 m3/ha.

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Table 2 Classification of the development stages Development stage

Finnish equivalent and codes

Description (adapted from Sevola, 2000, p.57)

Seedling stand Young stand Maturing stand Mature stand Seed tree stand

Seedling stand; T1 and T2 Young thinning stand; O2 Advanced thinning stand; O3 Mature stand; O4 Seed tree stand; S0 and Y1

Stand Stand Stand Stand Stand

defoliation intensity than a stand of similar age surrounded by older stands. Stand attractiveness for a stand A was calculated in the following way:   n X AgeA  Agei Stand attractiveness ¼ EPAi Agemax  Agemin i¼1 where n is the number of neighbouring stands; EPAi the proportion of its edge stand A has in common with stand I (%/100); AgeA the age of stand A (years); Agei the age of stand i (years); Agemax the maximum stand age in the whole study area and Agemin is the minimum stand age in the whole study area. The variable EPAi was determined from the stand maps of the study areas. This formula provided a value between 1 and 1 for each stand, with values close to 1 indicating ‘‘highly attractive’’ stands, and values close to 1 ‘‘highly unattractive’’ stands. We also compared defoliation intensity between the various stand development stages, based on the classification commonly used in commercial forest management in Finland (Table 2, after Sevola, 2000, p.57).

average average average average average

D1.3 D1.3 D1.3 D1.3 D1.3

< 8 cm, stem density > 1000 stems/ha from 10 to 17 cm; first thinning done from 17 to 25 cm; second thinning done > 25 cm, ready for final cut >25 cm; stem density < 100 stems/ha

The variable ‘‘stand density’’ is noteworthy, because it gives both a measure of tree size and of stand openness. Over the whole age range of a study area, stand density decreases as stands grow older and trees bigger, as commercial thinning operations repeatedly free up space for the remaining trees. In order to evaluate how stand openness per se, i.e. unrelated to tree size would affect defoliation intensity, we had to ‘‘filter out’’ this factor from the measured stand density. The following approach allowed us to disconnect tree size from stand openness, as measured by stand density. First, we analysed the relation between stand density and defoliation intensity for all stands of a study area pooled together. In this approach, stand density was closely correlated to tree size (Table 3). Secondly, we analysed the data, but this time within the young, maturing and mature development stages. As tree size does not vary much between different stands of the same development stage, this approach permitted us to evaluate the effect of stand openness. To further reduce the correlation with tree size, we eliminated outliers of the stand density population from this last analysis (Table 3). The Sonkakoski study area and seedling and seed tree

Table 3 Pearson correlation coefficients between stand density and stand mean tree height, for each study area overall, and within development stage, with and without outliers Location

þ Outliers

Overall

Young stands Palokangas Valkeaja¨ rvi Rautavaara *

***

0.886 0.669*** 0.797***

**

0.904 0.419* 0.705

 Outliers Maturing stands **

0.695 0.185 0.411

Significant correlations are indicated for P < 0:05. Significant correlations are indicated for P < 0:01. *** Significant correlations are indicated for P < 0:001. **

Mature stands

Young stands

Maturing stands

Mature stands

0.040 0.214 0.705

0.292 0.444* 0.447

0.467 0.206 0.411

0.040 0.022 0.193

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stands in all of the other areas were excluded in this analysis, because of the limited number of cases. 3.2. Defoliation assessment Defoliation intensity was used as the measure for outbreak intensity. It has the advantage of a straightforward correlation with the outbreak consequences important for forest management, such as growth loss (Kyto¨ et al., 1999) and tree mortality. Defoliation in the four areas was assessed in March and April 2000 and 2001, before the new growing seasons started. Observation points were randomly chosen, but well inside each stand to avoid edge effect. From those points, four trees were selected in respectively northern, western, southern, and eastern direction, in that way avoiding subjective selection of the trees. Looking at the chosen tree’s canopy from an angle of ca. 458, defoliation was visually assessed, and expressed in relative needle loss (%) compared to a reference, imaginary tree with full, healthy foliage. The details of this method are described by Eichhorn (1998). Our approach differed from his guidelines in the sense that defoliation was classified in 10% classes, and that also trees from the intermediate and understory canopy strata were assessed. On-line pictures provided for this purpose by the Forest Information Service of Metla (The Finnish Forest Research Institute) increased the reliability of the assessments. The average of the four observations then provided the mean defoliation for the assessment point. For stands smaller than 1 ha, one point assessment was carried out; for stands between 1 and 5 ha, we did two point assessments; and for every additional 5 ha, we added one point assessment. These point observations pooled together provided the measured average defoliation for a stand. In total, the defoliation assessment yielded average stand defoliation data after feeding season 1999 (Defol99) and 2000 (Defol2000) for 205 stands. 3.3. Relative tree position To compare defoliation between the canopy strata, we assessed defoliation on 201 individual trees in Palokangas and 36 in Valkeaja¨ rvi, in early 2001. The trees were located well within the stands, to avoid edge effects. We classified the trees into three canopy strata: dominant, intermediate and understory. Stand

309

defoliation Defol2000 was then subtracted from the individual tree defoliation, providing the new parameter ‘‘defoliation difference from average stand defoliation’’ that allowed comparison of the defoliation differences between the canopy strata, with positive values indicating higher than average defoliation of a canopy stratum, and negative values the opposite. We did not carry out defoliation assessments in seedling nor in seed tree stands, because these stand development classes are not relevant in this analysis, since neither has a closed canopy cover. 3.4. Statistics SPSS 10.0 for Windows (Norusis, 2000) was used for the statistical tests. Kolmogorov–Smirnov tests- or Shapiro–Wilk tests when there were less than 50 cases in a population-were performed to assess whether the populations are normally distributed. For each study area, average Defol99 and Defol2000 were compared using the paired samples t-test, while Pearson’s correlation coefficient yielded a measure of linear correlation between Defol99 and Defol2000. In each study area and for each feeding season, an analysis of variance was then performed, to see whether significant differences existed between the defoliation intensity of the various development stages in an area. The significant differences were pinpointed using Tukey’s or Dunnett’s T3-test, depending on whether or not the variances of the groups were equal. Within each development class and for each study area, a paired samples t-test was done to compare Defol99 and Defol2000. Linear multiple stepwise regression analyses were carried out, for each study area and per feeding season, to assess how much of the stand defoliation was explained by the stand structure variables. For this purpose, the categorical dependent variables were recoded into dummy variables. Pearson correlation coefficients gave an indication of the correlation between defoliation and stand openness, within each development class and for each feeding season. Comparisons of the parameter ‘‘defoliation difference from stand defoliation’’ for each of the study areas were made between dominant, intermediate and understory strata, using analysis of variance. This test was repeated to compare the canopy strata within each investigated development stage.

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Table 4 Average Defol99 and average Defol2000 for the four study areas, and Pearson correlation between Defol99 and Defol2000 Location

N

Defol99 (%)

Defol2000 (%)

Correlation coefficient of successive stand defoliation

Palokangas Valkeaja¨ rvi Rautavaara Sonkakoski

49 113 27 16

57.9 49.2 46.0 34.9

54.6 25.9 24.1 15.9

0.858 0.636 0.730 0.839

  

Significant differences in successive average defoliation for the whole study area, and Pearson correlations of successive stand defoliation intensities are indicated by ().

4. Results 4.1. Stand defoliation The measured stand defoliation in successive years shows a highly positive and highly significant linear correlation, and this in all study areas (Pearson correlation in Table 4). This indicates that the defoliation of the stands was highly conservative, i.e. stands that exhibited relatively high defoliation in 1999 continued to do so in 2000. 4.2. Overall defoliation All areas exhibited a decrease in overall defoliation in the 2 years of observation (average Defol99 and

Defol2000 in Table 4). However, the reduction was only small and not statistically significant in Palokangas, the area with severe defoliation. The decline is larger—20–25%—and highly significant in the other study areas, where Defol99 was only moderate. 4.3. Stand structure, site quality and defoliation Defoliation intensity increases as stands become more mature (Fig. 2), as indicated by the significant increases in defoliation between development classes. The results of the multiple stepwise regressions (Table 5) are similar, and show that most of the variation in defoliation within an area is explained by stand variables pertaining to the stand tree size, such as stand age, stand mean D1.3, stand mean height,

Table 5 Results of the multiple stepwise regressions (P Enter ¼ 0:05; P Remove ¼ 0:10) for the four study areas and for Defol99 and Defol2000 Location

Dependent variable

Independent variables

Palokangas (n ¼ 49)

Defol99

Included Excluded (0.05 < P < 0.1)

R2

R2 change

Standardised coefficient b

P

Stand mean D1.3 Scots pine proportion Forest site type

0.723 0.768 –

0.723 0.044 –

0.829 0.212 0.164

0.000 0.010 0.051

Defol2000

Included

Stand mean height Scots pine proportion

0.700 0.850

0.700 0.150

0.809 0.388

0.000 0.000

Valkeaja¨ rvi (n ¼ 113)

Defol99

Included

Stand age Forest site type Stand density Scots pine proportion Basal area

0.589 0.615 0.640 0.668 0.793

0.589 0.027 0.025 0.027 0.025

0.324 0.179 0.420 0.188 0.214

0.018 0.010 0.000 0.010 0.018

Defol2000

Included

Stand age

0.541

0.541

0.736

0.000

Rautavaara (n ¼ 27)

Defol99 Defol2000

Included Included

Stand density Stand density

0.475 0.380

0.475 0.380

0.689 0.616

0.000 0.001

Sonkakoski (n ¼ 16)

Defol99 Defol2000

Included Included

Stand mean height Stand age

0.708 0.656

0.708 0.656

0.841 0.810

0.009 0.015

Grey rows indicate the ‘‘stand tree size variables’’.

B. De Somviele et al. / Forest Ecology and Management 194 (2004) 305–317 Fig. 2. Defol99 and Defol2000 of the successive stand development stages in the four study areas. The letters in the boxes below indicate significant differences (P < 0:05) between the development stages, for Defol99 and Defol2000, respectively: development classes that have a letter in common exhibited no significantly different defoliation intensity. The full arrows (P < 0:05) and the grey arrow (P < 0:1) indicate significant differences between Defol99 and Defol2000 within a development stage. The question marks in Sonkakoski mark the cases where no statistical test was performed, because n ¼ 1. 311

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Fig. 3. Scatterplots of stand defoliation vs. stand density, after feeding seasons 1999 and 2000, for young, maturing and mature stands in Palokangas, Valkeaja¨ rvi and Rautavaara. Significant correlations (P < 0:05) between stand defoliation and stand openness, as measured by stand density (trees/ha), are indicated by the black trendlines; non-significant trends are in grey.

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stand basal area (positive coefficients) and stand density (negative coefficient). In all study areas, the first and foremost proportion of the goodness-of-fit measure R2 is given by one of these ‘‘stand tree size variables’’ (Table 5). Which of them was retained by the regression as the principal explanatory variable varied from place to place and sometimes also between successive years. But the ‘‘stand tree size variables’’ are so closely correlated (P < 0:001) that the statistical test’s choice for one or the other can be attributed mainly to chance in the sampling process. The other independent variables tend to be greatly secondary to the ‘‘stand tree size variables’’ in explaining the variation in defoliation. The age difference with surrounding stands, as calculated in the dependent variable ‘‘stand attractiveness’’, did not have a significant effect on defoliation in any of our study areas (Table 5). The tree species composition of a stand can indeed have a significant effect on defoliation. This is shown by the recurrence of the variable ‘‘Scots pine propor-

313

tion’’ in the regressions in Valkeaja¨ rvi and Palokangas, indicating that the purer the Scots pine stand in these areas, the higher the observed defoliation intensity was. A similar effect could not be noticed in Rautavaara or Sonkakoski, although Scots pine is less dominant there, and heterogeneity in terms of tree species composition is higher (Table 5). Stand area did not play a significant explanatory role in any of our study areas during the assessment years (Table 5). ‘‘Forest site type’’ is negatively correlated to defoliation in Valkeaja¨ rvi, Defol99 (P < 0:05) and—to a lesser degree—in Palokangas, Defol99 (P < 0:1). In these areas, and for that feeding season, defoliation was lower in the stands of more productive forest site type. Nothing similar could be discerned in Rautavaara or in Sonkakoski, although these areas exhibit a higher variation in forest site type (Table 5). The drainage situation within the affected areas did not have any significant explanatory power in the average defoliation of the stands, as indicated by

Fig. 4. Tree defoliation difference from stand defoliation, overall and within the relevant development classes, for the different canopy strata in Palokangas and Valkeaja¨ rvi. Significant differences between canopy strata within a development class are indicated by ().

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the exclusion of the independent variables ‘‘artificial drainage’’ and ‘‘soil humidity’’ in all regressions (Table 5). In the three areas with massive declines in defoliation between 1999 and 2000 (Valkeaja¨ rvi, Rautavaara and Sonkakoski), we notice a corresponding decline in goodness-of-fit measure R2: less of the defoliation in a stand is explained by stand structure and soil quality in 2000 than in 1999. On the contrary, in Palokangas the R2-value increases (Table 5). 4.4. Stand density as a measure for stand openness There was significant correlation between stand density and stand defoliation in the Valkeaja¨ rvi mature stands for Defol99, and in the Palokangas maturing stands for Defol2000 (Fig. 3). But the trends of these two significant cases are opposite. Moreover, it was impossible to discern any consistency in the trends of all analysed cases. 4.5. Relative tree position When comparing ‘‘defoliation difference from average stand defoliation’’ between the various canopy strata, one will—for all data pooled together—observe higher defoliation in the dominant canopy stratum (Fig. 4). However, these differences are not statistically significant for our dataset. The same comparison within each of the relevant development stages reveals that the dominant stratum is most defoliated only in the young stands, but certainly not in the maturing and mature stands (Fig. 4). In Palokangas, the difference between the dominant and the other strata in the young stands is statistically significant (P ¼ 0:006). Probably due to the small number of observations, the Valkeaja¨ rvi analysis did not result in statistically significant differences. However, the young stands exhibit the same pattern as in Palokangas, with a higher defoliation of the dominant stratum.

5. Discussion Our observations corroborate the common assumption that in D. pini outbreaks—stands with mature trees will be the most defoliated ones. This could be an

illustration of what Wagner (1991, in McMillin and Wagner, 1993) calls ‘‘the partitioning by closely related species of food resources to avoid competition’’, since the other important Fennoscandian pine sawfly pest, N. sertifer, feeds predominantly on young Scots pine trees, and three minor outbreak species (Hanski, 1987), Gilpinia pallida (Klug), Microdiprion pallipes (Falle´ n) and Diprion similis (Hartig) cause damages only in young stands (Lyytika¨ inen, 1993a). However, there are important overlaps: N. sertifer larvae often feed on pole stage stands of Scots pine (Larsson and Tenow, 1984), and the D. pini damages we observed are definitely not strictly limited to mature stands, with moderate and even severe defoliation in young and maturing stands as well. In Valkeaja¨ rvi and Rautavaara, the mature stands have been far from totally defoliated all through the outbreak, so food for the D. pini larvae was still available in these stands. Yet we did observe elevated defoliation in younger stands too. This indicates that the defoliation observed in the young and maturing stands is not characterised by a real starvation-driven dispersal of individuals from the mature stands, where food would have totally run out, towards younger, socalled ‘‘less preferable’’ stands. Another explanation for the relatively high defoliation in young and maturing stands could be the mechanism of induced resistance, which could force sawflies out of earlier preferred stands, because the host trees there have become less palatable. But we do not agree with this hypothesis; first of all because the defoliation intensity over an affected area is very conservative throughout the outbreak, i.e. the defoliation intensity in a stand will remain high if it was already high in the previous year. This is demonstrated by the very high and significant positive correlations between stand defoliation measured after successive feeding seasons (Table 4), and by the fact that defoliation does not systematically decrease more in the previously most defoliated stands (Fig. 3). It is also corroborated by observations of Ge´ ri and Goussard (1986) during an outbreak in France. Moreover, the hypothesis of—delayed—induced resistance has been rejected repeatedly for pine sawfly outbreak species (Niemela¨ et al., 1991; Lyytika¨ inen, 1993b). We found yet another indication that migration between differently aged stands in an affected area in search of more preferable host trees is not a major

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factor in the population dynamics of D. pini. The age difference with surrounding stands, as measured by the variable ‘‘stand attractiveness’’ does not play a significant role in stand defoliation. While it is very plausible that the age difference with neighbouring stands has its effect on defoliation in the edges of a stand, as observed by Larsson and Tenow (1984) in a N. sertifer outbreak, in D. pini outbreaks it does not seem to affect the central part of stands. Rather than the hypotheses mentioned above, we conclude that D. pini larvae commonly, and throughout the outbreak, feed on far from mature Scots pines. We hypothesize the existence of a critical tree size: while only reluctantly feeding on trees below this critical value, we suspect that D. pini feeding behaviour becomes rather indifferent to tree size, once this critical value has been surpassed. This hypothesis would explain why only in the young stands the dominant trees are more defoliated. We found no unambiguous proof that tree species composition has a significant impact on the variation in defoliation, within an affected area. Ge´ ri and Goussard (1986) concur, and even found cases of Scots pine monocultures up to the pole stage that were less affected than pole stage stands mixed with other tree species. Rather surprisingly, we have some indications that in Palokangas and Valkeaja¨ rvi, areas with very homogenous Scots pine stands, the more mixed stands are less affected, whereas in the more heterogeneous study areas, we could not observe a similar trend. The majority of sawfly species in North America has been observed to prefer open-grown stands (McMillin and Wagner, 1993). Neodiprion species in North America show a preference for open-grown stands, although the information is sometimes conflicting (McMillin and Wagner, 1993; McMillin et al., 1996). Closer to Finland, N. sertifer was found to prefer open-grown over dense stands in Scotland (Britton, 1988). Our results however, after removal of the correlation between stand openness and tree size, do not support these observations for D. pini. Stand defoliation has an inconsistent, if any, relation with stand openness, and we suspect that an observed preference for a certain stand density can often be brought back to a preference for a certain tree size per se. We conclude that within an outbreak area, D. pini is indifferent to stand openness.

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Defoliation intensity is higher in affected Scots pine forests where the soil is more shallow and unproductive, as overall defoliation intensifies from Sonkakoski, with a high groundwater table and of relatively productive forest site types, to Palokangas, characterised by a poor and very well-drained soil. Our results therefore support the common assumption linking sawfly outbreaks to infertile, shallow, welldrained soils, in the sense that the vulnerability of forest areas on these soils increases. In terms of susceptibility however, this assumption is challenged by the fact that Sonkakoski and Rautavaara experienced defoliation at all: these study areas are located on badly drained and relatively productive forest site type. The least heterogeneous areas in terms of forest site type showed, after feeding season 1999, a significantly lower defoliation where the soil is more productive, whereas more heterogeneous areas did not show this trend. One possible explanation could be that Valkeaja¨ rvi and Palokangas are areas that could be described as ‘‘oceans’’ of homogeneous Scots pine stands on rather unproductive soils, with here and there a small ‘‘island’’ of higher fertility. For D. pini, it would therefore be very easy to avoid these ‘‘less preferable’’ islands. On the contrary, Rautavaara and Sonkakoski are characterised by a more mixed patchwork of stands, in terms of forest site type. This probably makes it harder for D. pini, assumed to be a bad disperser (Ge´ ri, 1988), to find its preferred type of stands, and the insect may have to settle for less than ideal conditions. A much-needed better understanding of the dispersal and oviposition behaviour of D. pini (Ge´ ri, 1988) might shed more light on this outbreak pattern. We could not detect any significant difference in defoliation between well-drained and badly drained stands. Both for susceptibility and vulnerability within an affected forest area, the common assumption linking defoliation intensity to well-drained soils was not backed up by our study. We therefore conclude that the common assumption linking sawfly outbreaks to shallow, well-drained and unproductive soils only applies to vulnerability on a landscape level, i.e. infested Scots pine forests will experience higher defoliation when the soil is shallow, dry and unproductive. As for susceptibility, we have found that areas other than the ones described in the above assumption do

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undergo observable sawfly outbreaks. Also within an affected area, outbreak susceptibility and vulnerability were rather indifferent to the soil characteristics of the stand. Stands on all types of soils were susceptible to sawfly infestation, and defoliation was never correlated to the drainage situation of the stand, and only twice to stand forest site type. Concerning the decreasing explanatory power of our regressions, one could assume that, as the outbreak progresses and food becomes scarcer, the sawflies become more indifferent in their choice of host plants. But this does not seem to be the case, first of all because in Palokangas, where defoliation is highest and therefore food scarcest, the explanatory R2-value of our regressions still increases from 1999 to 2000. In the other areas, food does not become so rare as to drive the sawflies out to less preferable places. We believe that this proves that D. pini stays highly discriminatory in its choice of host plant during the whole duration of the outbreak. The decrease in R2 in the other areas is mainly due to the collapse of the outbreak, which makes that the outbreak dynamics become mixed with those of the stand restoration. These stands undoubtedly exhibit a differential refoliation pattern, as host plants in the various development stages have varying reserves to restore their foliage. We did not observe defoliation differences between the various canopy strata, except for young stands, where the dominant canopy stratum tends to be more defoliated. While Gref and Tenow’s theory (1987), stating that sun needles, which have higher resin acid contents, are more resistant to herbivorous insects than shade needles, is corroborated by Niemela¨ et al. (1982) for N. sertifer, these results for D. pini go against it. It adds to the idea that D. pini is indeed a very robust feeder, who is fairly indifferent to resin acid contents of the needles, as suggested by Buratti et al. (1987). 5.1. Management implications Forest and pest managers have to reckon with several years of considerable defoliation in stands other than mature ones as well. This is especially important in young stands, because here we found the dominant canopy stratum to be the most affected one. Growth losses will therefore be concentrated in this stratum. For forest managers, this poses a pro-

blem, since the dominant stratum contains in normal conditions the ‘‘trees of the future’’ that benefit most from the thinning operations. However, the substantial differences in defoliation between individual trees of a stand might allow for selective thinning after the outbreak to promote the least defoliated trees and hereby reduce growth losses. The potential of this silvicultural strategy should be studied further. Since no straightforward proof was found of a correlation between tree species composition and defoliation intensity, it seems a doubtful strategy to enrich vulnerable stands with other tree species in order to reduce D. pini damages. As higher stand openness does not decrease defoliation intensity, we suggest avoiding thinning operations during D. pini outbreaks. They will cause the remaining trees to suffer from increased defoliation pressure, while no beneficial effects from such silvicultural practices will be obtained.

Acknowledgements Many thanks to the forest owners, Stora-Enso and the family Mustonen, for allowing us to work on their property. A kind word of thanks also to Prof. Noe¨ l Lust of the University of Ghent, Belgium, who kindly provided the working facilities to complete the last phases of this article. This study has been funded by CIMO-scholarships MM/HA9 245 and MM/HA0 158, by the Niemela¨ Group of the Centre of Excellence for Forest Ecology and Management, University of Joensuu, and by the Faculty of Forestry, University of Joensuu.

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