Forest Ecology and Management

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Effects of forest restoration treatments on the abundance of bark beetles

in Norway spruce forests of southern Finland

Tero Toivanen

a,

*

, Veli Liikanen

a

, Janne S. Kotiaho

a,b a

Department of Biological and Environmental Science, PO Box 35, 40014 University of Jyva¨skyla¨, Jyva¨skyla¨, Finland

b

Natural History Museum, PO Box 35, 40014 University of Jyva¨skyla¨, Jyva¨skyla¨, Finland

1. Introduction

Restoration-oriented approach to forest management and conservation is needed to protect the biodiversity of boreal forests

(Kouki et al., 2001; Kuuluvainen et al., 2002). Today, natural

disturbance dynamics are often used as a guideline to develop sustainable forest management practices and restoration methods that enable the persistence of natural processes, structures and

species composition in human-utilized ecosystems (Fries et al.,

1997; Angelstam, 1998; Bergeron et al., 2002;Kuuluvainen et al.,

2002). In boreal forests of Fennoscandia, the most commonly used

restoration tools have been reintroducing fire to the boreal forest

ecosystem and increasing the volume of dead wood at protected areas that have formerly been under intensive forest management. Forest restoration is likely to be effective in maintaining and increasing species diversity since it benefits a large number of saproxylic (dead wood dependent) species, and rare and red-listed

(IUCN classes CR, EN, VU and NT) species in particular (Hyva¨rinen

et al., 2006; Toivanen and Kotiaho, 2007). However, restoration

practices that create substantial volumes of dead wood can also be seen to form a contradiction with the traditional view of forest hygiene. For example, the Forest Insect and Fungal Damage Prevention Act of Finland obligates forest owners to remove wind-felled trees from the forest if the proportion of damaged trees is >10% of the total number of trees or if the damaged conifers form

groups of >20 trees (Anonym, 1991). These thresholds are not

based on data from any specific studies (Eriksson et al., 2005), but it

is nevertheless clear that the number of dead and weakened trees A R T I C L E I N F O

Article history:

Received 18 December 2007

Received in revised form 19 August 2008 Accepted 21 August 2008 Keywords: Boreal forests Controlled burning Dead wood Harvesting Pest management Scolytinae A B S T R A C T

Restoration of protected areas in boreal forests frequently includes creating substantial volumes of dead wood. While this benefits a wide range of dead wood dependent invertebrate species, some of these are regarded as forest pests. Therefore, the risk of elevated levels of tree mortality in surrounding commercial forests must be considered. In a large-scale field experiment in southern Finland, we studied the effects of restoration treatments on the abundance of bark beetles within and in the vicinity of restored areas, in particular focusing onIps typographusandPityogenes chalcographus. The treatments applied to managed Norway spruce forests were controlled burning and partial harvesting combined with retaining 5, 30 or 60 m3/ha of cut down wood. We found that the abundance of bark beetles increased by both burning and harvesting with down wood retention, being highest where burning and harvesting had been combined. The actual volume of down wood retention had no significant effect. The effect of burning on the number of bark beetles along host tree boles was negative which suggests that burnt spruces provided a less suitable resource for bark beetles than unburnt dead spruces. The abundance of bark beetles along host trees also decreased with increasing volume of down wood retention. The abundance ofP. chalographus was slightly elevated up to 50 m outside restored areas but the abundance was very low compared to that within the areas. The abundance ofI. typographuswas extremely low outside restored areas. We conclude that restoration treatments increase the abundance of bark beetles via increased availability of resources, but that the effect of burning is likely to be counteracted by decreased resource quality. Thus, burning might be the ‘‘safest’’ way to produce large quantities of dead wood. Furthermore, the fact that only few beetles were collected in adjacent areas suggests that restored areas pose little threat of serving as refugia in which bark beetle populations increase in sufficient numbers to attack live trees in adjacent forests. However, restoration actions repeated at consecutive years within a small area might enable the populations to grow to outbreak levels.

ß2008 Elsevier B.V. All rights reserved.

* Corresponding author. Tel.: +358 14 260 2292; fax: +358 14 260 2321.

E-mail address:tertoiv@cc.jyu.fi(T. Toivanen).

Contents lists available atScienceDirect

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created in restoration practices (controlled burning in particular) is so high that it might eventually result in an increase of bark beetle species that are typically regarded as forest pests capable of attac-king vigorous trees. Therefore, restoration of protected areas may cause a risk that forest damage will occur at nearby commercial forests and thus the economic risks should also be considered in planning restoration treatments and in studying the effects of restoration.

Bark beetles (Coleoptera: Curculionidae, Scolytinae) are the most abundant group colonizing recently killed and weakened trees. Bark beetles are an essential component in forest ecosystem dynamics as

they often start the decomposition of wood (Wermelinger, 2004),

provide resources for a large number of associated species (Weslien,

1992) and may even facilitate forest succession by creating

disturbances of various scales (Martikainen et al., 1999). Under

certain conditions, in particular when a surplus of breeding material

is available (Mulock and Christiansen, 1986), bark beetle

popula-tions may increase such that the species are able to attack vigorous trees and overcome host tree defenses.

The spruce bark beetle,Ips typographus(L.), is regarded as the

most significant forest pest in Europe. In Norway spruce (Picea abies

[L.] Karst) forests in Central Europe, there have been many massive

outbreaks ofI. typographusafter severe windstorms or tree deaths

caused by heavy snow load that have caused significant forest

damage during the last decades (Wermelinger, 2004). In Northern

Europe, outbreaks leading to large-scale forest damage have been

scarce (see Christiansen and Bakke, 1988; Eidmann, 1992). For

example, in Finland no extensive outbreaks have been reported (Eriksson et al., 2005). This is likely due to the fact thatI. typographus is able to produce only one generation per year at northern latitudes (Sauvard, 2004; Johansson et al., 2006). However, it is worth noting that several univoltine bark beetles (e.g., mountain pine beetle, Dendroctonus ponderosaeHopkins, in North America) are capable of

causing large-scale forest damage (Westfall and Ebata, 2008).

In Europe, Pityogenes chalcographus (L.) is sometimes

men-tioned as a potential forest pest that can occasionally attack

stands of young Norway spruce. It is unclear to what extentP.

chalcographuscan overcome the defenses of vigorous trees, but it seems unlikely to cause tree deaths in the absence of attack by

other bark beetle species (Hedgren, 2004). This may be due to the

fact that, unlikeI. typographus,P. chalcographusis not consistently

associated with tree-killing fungi (Krokene and Solheim, 1996).

In the absence of large-scale disturbances, populations of bark beetles are likely to remain at endemic levels in natural forests (Martikainen et al., 1999). It has been suggested that intensively managed forests may be more susceptible to bark beetle outbreaks

(Martikainen et al., 1999) due to the favourable microclimates

(Va¨isa¨nen et al., 1993), a lack of predators and competition

(Nuorteva, 1968; Schlyter and Lundgren, 1993), and a lack of

heterogeneity in tree genetics, age structure of trees and forest

structure and composition (Wermelinger, 2004). In addition,

managed forests may provide a relatively high amount of breeding material for bark beetles due to the abundance of logging residues

and the increased probability of windfalls at forest edges (Schlyter

and Lundgren, 1993). In managed forests,I. typographusis known to be able to kill solitary healthy trees at the margin of recently

harvested areas (Peltonen, 1999; Hedgren et al., 2003). To reduce the

amount of bark-beetle-caused tree mortality in managed forests, thinning to reduce tree competition and increase individual tree growth, shorter rotation times, and maintenance of multiple tree

species and age classes, are often suggested (Fettig et al., 2007).

To minimize the socioeconomic risks, restoration practices are not recommended in the vicinity of privately owned lands in

Finland (Kuuluvainen et al., 2002). However, this recommendation

is not based on empirical evidence and there are practically no

studies concerning the negative effects of forest restoration on the abundance of forest pests. Here, we report the results of a study focusing on the effects of restoration on the abundance and dispersal of bark beetles. In a large-scale field experiment, the treatments applied included controlled burning and partial harvesting with down wood retention (DWR). We sought to determine: (1) whether treatments increase the abundance of bark beetles within restored areas; (2) what is the effect of burning and harvesting on the number of bark beetles along host tree boles; and (3) whether restoration treatments result in elevated abundance of bark beetles in adjacent, untreated forests and whether there is a risk of elevated tree mortality in those forests.

2. Materials and methods

2.1. Study plots

The study area was located in the vicinity of Evo, southern Finland

(618110N, 258050E, altitude 100–150 m), within the south boreal

vegetation zone. For the study, 24 two-hectare plots located within

25 km15 km area were selected. The lands were owned by

Finnish Forest and Park Service (6 plots), Finnish Forest Research Institute (4), forest product company UPM (4), Ha¨me Polytechnic University of Applied Sciences (6), and the town of Ha¨meenlinna (4). All of the plots were originally on average 80-year-old managed mesic forests. The initial volume of standing wood on the plots was

251.964.8 m3/ha (mean

S.D.) and the volume of dead wood

17.313.7 m3/ha. The volumes of living or dead wood did not differ

between the plots (Lilja et al., 2005). Dead wood consisted almost

exclusively of logging waste, i.e., small-diameter (<20 cm) logs and cut

stumps (Lilja et al., 2005). The dominant tree species of the plots (i.e.,

about 90% of the volume of standing trees) was Norway spruce with

some birch (Betulaspp.) and Scots pine (Pinus sylvestrisL.).

2.2. Experimental design

Controlled burning and partial harvesting with DWR were applied as restoration treatments at the study plots. During February and March 2002, 18 plots were harvested such that the volume of

standing retention trees was set to 50 m3/ha. On the harvested plots,

5, 30 or 60 m3/ha of cut down wood was retained (six plots of each

DWR treatment). Six study plots were left unharvested. In summer 2002, 12 study plots (9 harvested and 3 unharvested plots) were burnt. The first five burnings were conducted in mid-June, the following five in mid-July and the last two in the beginning of August

(for detailed description of the treatments, seeLilja et al., 2005). In

addition, to allow us to compare burnt and unburnt host trees, three Norway spruces were killed by mechanical girdling (stripping off a piece of bark at 1.3 m height to prevent water and nutrient flow) at each unburnt plot in the beginning of June 2002.

2.3. Sampling of beetles

To study the effect of restoration treatments on the relative abundances of bark beetles, beetles were sampled with flight-intercept traps. The traps consisted of two crosswise-set transparent plastic panes with a funnel and container below them. The traps were set hanging from a string between two trees or poles in order to collect samples independent of the effect of individual host trees. Saline water with detergent was used in the containers to preserve the beetles. Five traps were set in random locations at each study plot. The trapping period was 10 May to 10 September 2003.

To study the effect of restoration on the numbers of bark beetles along host tree boles, we used window traps attached to recently dead standing trees. The upper edge of the funnel was modified

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heart-shaped such that the length that it touched the tree trunk was about 25 cm. We selected three burnt spruces at each of the burnt plots and three girdled spruces at each of the unburnt plots. The trapping period was 10 May to 10 July 2003.

To study the effect of restoration on the abundance of bark beetles in forests adjacent to restored areas, we constructed a line of six free-hanging flight-intercept traps perpendicular to the edge of the study plots. Traps were located 25 m inside each plot, at the edge of each plot, and at 25, 50, 75 and 100 m outside each plot. The trapping period was 10 May to 10 July 2003, which covers the main

dispersal season ofI. typographusandP. chalcographusin Finland

(Helio¨vaara et al., 1998). 2.4. Statistical analyses

The analyses were performed with SPSS 13.0 for Windows software (SPSS Inc., Chicago, Illinois). In all the analyses, we analyzed the effects of the restoration treatments on the total number of bark beetles and on the abundances of the species that

are most likely to attack healthy trees (I. typographus and P.

chalcographus). The species abundance data were log10 (x+ 1)-transformed before the analyses.

To analyze the effect of treatments on the abundance of bark beetles, we used the pooled data of the five free-hanging traps of each study plot. We used two-way ANOVA in which the factors were burning (burnt or unburnt) and harvesting with DWR

(unharvested, harvested with 5, 30, and 60 m3/ha DWR,

respec-tively) followed by Tukey’s pairwise comparisons. If there was an interaction between factors, the ANOVA was followed by simple effects tests and pairwise comparisons.

The effect of treatments on the number of bark beetles along host trees was analyzed with two-way ANOVA in which the factors were burning (burnt or unburnt [girdled] spruce) and harvesting with DWR. Data from three spruces on each plot was pooled for the analysis. If there was an interaction between factors, the ANOVA was followed by simple effects tests and pairwise comparisons.

To analyse how the abundance of bark beetles changed with the distance from restored areas, the treatments were grouped in four classes (burnt harvested, burnt unharvested, unburnt harvested and unburnt unharvested). The volume of DWR on the harvested plots was not included since it had only minor effect on the abundance of bark beetles within restored areas (see results). The effects of distance and treatment on the abundance of bark beetles were analyzed with repeated measures factorial ANOVA followed by simple effects tests and pairwise comparisons.

3. Results

3.1. The effect of restoration treatments on the abundance of bark beetles

In total, we collected 21,695 bark beetles representing 33 species,

including 421I. typographusand 9487P. chalcographus(which was

the most numerous species), in flight-intercept traps (Table 1). In

Table 1

The total number of bark beetle individuals collected in flight-intercept traps within restored plots (N= 120) in 10 May to 10 September 2003, in traps attached to boles of dead Norway spruces (N= 72) in 10 May to 10 July 2003, and in traps along lines perpendicular to the edge of restored plots (N= 144) in 10 May to 10 July 2003

Species Flight-intercept traps Traps attached to host trees Trap lines

Cryphalus saltuariusWeise 7 11 12

Crypturgus cinereus(Herbst) 93 539 66

Crypturgus hispidulusThomson 19 1 48

Crypturgus pusillus(Gyllenhal) 26 21 4

Crypturgus subcribrosusEggers 295 2,893 79

Dendroctonus micans(Kugelann) 0 1 0

Dryocoetes autographus(Ratzeburg) 3,368 5,586 1553

Dryocoetes hectographusReitter 9 0 7

Hylastes brunneusErichson 329 3,307 110

Hylastes cuniculariusErichson 4,017 26,754 3096

Hylastes opacusErichson 263 2,276 50

Hylurgops glabratus(Zetterstedt) 6 0 4

Hylurgops palliatus(Gyllenhal) 280 1,009 144

Ips amitinus(Eichoff) 85 410 18

Ips typographus(Linnaeus) 421 1,941 82

Orthotomicus laricis(Fabricius) 37 251 14

Orthotomicus proximus(Eichoff) 2 1 1

Orthotomicus suturalis(Gyllenhal) 301 1,049 44

Phloeotribus spinulosus(Rey) 0 0 16

Pityogenes bidentatus(Herbst) 37 9 26

Pityogenes chalcographus(Linnaeus) 9,478 23,585 3182

Pityogenes quadridens(Hartig) 18 13 12

Pityophthorus lichtensteini(Ratzeburg) 4 6 1

Pityophthorus micrographus(Linnaeus) 94 545 41

Polygraphus poligraphus(Linnaeus) 512 766 104

Polygraphus punctifronsThomson 9 0 0

Polygraphus subopacusThomson 28 355 2

Scolytus ratzeburgiJanson 14 0 0

Tomicus minor(Hartig) 0 0 2

Tomicus piniperda(Linnaeus) 0 7 0

Trypodendron domesticum(Linnaeus) 4 1 9

Trypodendron lineatum(Olivier) 1,550 3,319 420

Trypodendron laeveEggers 1 0 14

Trypodendron signatum(Fabricius) 227 108 160

Trypophloeus bispinulusEggers 1 0 0

Xyleborus dispar(Fabricius) 130 6 49

Xylechinus pilosus(Ratzeburg) 30 340 71

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general, the total number of bark beetles collected was positively

affected by burning (F1,16= 65.03,p<0.001) and by harvesting with

down wood retention (DWR) (F1,16= 6.01,p= 0.006) but there was

an interaction between the factors (F1,16= 5.34,p= 0.010). The effect

of burning was not significant among harvested with 60 m3/ha DWR

treatment, and the effect of harvesting with DWR was significant

among unburnt treatment but not among burnt treatment (Table 2).

Among unburnt treatment, the number of bark beetles collected was increased by harvesting but the volume of DWR had no effect (Table 3).

The number ofI. typographuscollected was positively affected

by burning (F1,16= 36.31,p<0.001) and by harvesting with DWR

(F1,16= 4.90,p= 0.013) but there was an interaction between the

factors (F1,16= 4.76, p= 0.015) (Fig. 1). Burning increased the

number ofI. typographusamong unharvested and harvested with

5 m3/ha DWR treatments but not among harvested with 30 and

60 m3/ha DWR treatments (Table 2). Among unburnt treatment,

the number ofI. typographuscollected was increased by harvesting

but the volume of DWR had no effect (Table 3). Among burnt

treatment, moreI. typographuswere collected on harvested with

5 m3/ha DWR treatment than on other treatments (Table 3).

The number of P. chalcographus collected was positively

affected by burning (F1,16= 44.25,p<0.001) and by harvesting

with DWR (F1,16= 20.39,p<0.001) but there was an interaction

between the factors (F1,16= 14.18,p<0.001) (Fig. 2). The effect of

burning was not significant among harvested with 60 m3/ha DWR

treatment, and the effect of harvesting with DWR was significant among unburnt treatment but not among burnt treatment

(Table 2). Among unburnt treatment, the number of P.

chalco-graphuscollected was increased by harvesting but the volume of

DWR had no effect (Table 3).

3.2. The effect of fire, harvesting and DWR on the number of bark beetles along host trees

In total, we collected 75,110 bark beetles representing 29

species, including 1941I. typographusand 23,585P. chalcographus,

along the boles of recently killed Norway spruce (Table 1). The total

Table 2

The simple effects tests for the effects of burning and harvesting with DWR on the abundance of bark beetles

Species Factor Simple effects test F p

All Burning Among unharvested F1,16= 50.500 <0.001 Among DWR5 F1,16= 19.591 <0.001 Among DWR30 F1,16= 7.541 0.014 Among DWR60 F1,16= 3.422 0.083 Harvesting Among burnt F3,16= 1.052 0.397 with DWR Among unburnt F3,16= 10.300 0.001

I. typographus Burning Among unharvested F1,16= 24.900 <0.001 Among DWR5 F1,16= 22.351 <0.001 Among DWR30 F1,16= 0.377 0.548 Among DWR60 F1,16= 2.958 0.105 Harvesting Among burnt F3,16= 5.879 0.007 with DWR Among unburnt F3,16= 3.781 0.032

P. chalcographus Burning Among unharvested F1,16= 79.068 <0.001 Among DWR5 F1,16= 3.143 0.095 Among DWR30 F1,16= 4.234 0.056 Among DWR60 F1,16= 0.339 0.569 Harvesting Among burnt F3,16= 2.363 0.110 with DWR Among unburnt F3,16= 32.206 <0.001

Fig. 1.The effect of burning and harvesting with DWR on the number ofI. typographuscollected in five flight-intercept traps in 10 May to 10 September 2003 (meanS.E.,N= 3 in each treatment).

Table 3

The pairwise comparisons following the simple effects tests for the effects of harvesting with DWR on the abundance of bark beetles

Species Comparison Unburnt plots MDa

p Burnt plots MDa p All Unharvested vs. DWR5 1.281 0.001 0.404 0.234 Unharvested vs. DWR30 1.544 <0.001 0.118 0.724 Unharvested vs. DWR60 1.562 <0.001 0.157 0.637 DWR5 vs. DWR30 0.263 0.434 0.287 0.394 DWR5 vs. DWR60 0.281 0.403 0.562 0.105 DWR30 vs. DWR60 0.018 0.956 0.275 0.413 I. typographus Unharvested vs. DWR5 1.500 0.019 1.349 0.032 Unharvested vs. DWR30 1.804 0.006 0.706 0.236 Unharvested vs. DWR60 1.111 0.071 0.765 0.201 DWR5 vs. DWR30 0.304 0.603 2.056 0.002 DWR5 vs. DWR60 0.389 0.507 2.114 0.002 DWR30 vs. DWR60 0.693 0.244 0.059 0.920 P. chalcographus Unharvested vs. DWR5 3.406 <0.001 0.303 0.498 Unharvested vs. DWR30 3.748 <0.001 0.769 0.097 Unharvested vs. DWR60 3.274 <0.001 0.349 0.435 DWR5 vs. DWR30 0.342 0.444 0.466 0.301 DWR5 vs. DWR60 0.142 0.765 0.652 0.154 DWR30 vs. DWR60 0.474 0.293 1.118 0.021 a

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number of bark beetles in traps attached to dead spruces was

decreased by burning (F1,16= 23.76,p<0.001), affected by

harvest-ing with DWR (F3,16= 4.42,p= 0.019) and there was no interaction

between the factors (F3,16= 0.12,p= 0.948). The number of bark

beetles collected on harvested with 5 m3/ha DWR treatment was

higher than on unharvested treatment (Tukey’s pairwise

compar-ison,p= 0.028), and it tended to be higher than on harvested with

60 m3/ha DWR treatment (p= 0.062).

The number ofI. typographusin traps attached to dead spruces

was decreased by burning (F1,16= 25.52,p<0.001) and increased

by harvesting with DWR (F3,16= 4.33, p= 0.020) (Fig. 3). There

was no significant interaction between the factors (F3,16= 2.13,

p= 0.137), but nevertheless the simple effects test revealed that

the effect of burning was not significant among unharvested treatment and that harvesting with DWR did not affect the number ofI. typographusamong traps attached to burnt spruces (Table 4).

Among traps attached to unburnt spruces, the number of I.

typographus was higher on harvested than on unharvested

treatment but the volume of DWR had no effect (Table 5).

The number of P. chalcographus in traps attached to dead

spruces was decreased by burning (F1,16= 19.10, p<0.001),

affected by harvesting with DWR (F3,16= 11.27, p<0.001) and

there was an interaction between the factors (F3,16= 13.30,

p<0.001) (Fig. 4). Among unharvested treatment, more P.

chalcographus was collected in traps attached to burnt spruces than in those attached to unburnt spruces, but among harvested

with DWR treatments, moreP. chalcographuswas collected in traps

attached to unburnt spruces than in those attached to burnt

spruces (Table 4). Among traps attached to unburnt spruces, more

P. chalcographuswere collected on harvested with DWR treatments

than on unharvested treatment, and the number of P.

chalco-graphuswas higher on harvested with 5 m3/ha than on harvested

with 60 m3/ha DWR treatment (Table 5). Among traps attached to

burnt spruces, lessP. chalcographuswere collected on harvested

Fig. 2.The effect of burning and harvesting with DWR on the number ofP. chalcographuscollected in five flight-intercept traps in 10 May to 10 September 2003 (meanS.E.,N= 3 in each treatment).

Fig. 3.The effect of burning and harvesting with DWR on the number ofI. typographuscollected in three traps attached to dead Norway spruces in 10 May to 10 July 2003 (meanS.E.,N= 3 in each treatment).

Table 4

The simple effects tests for the effects of burning and harvesting with DWR on the number of bark beetles along host tree boles

Species Factor Simple effects test F p I. typographus Burning Among unharvested F1,16= 0.138 0.715

Among DWR5 F1,16= 11.670 0.004 Among DWR30 F1,16= 8.290 0.011 Among DWR60 F1,16= 11.804 0.003 Harvesting Among burnt F3,16= 0.303 0.823 with DWR Among unburnt F3,16= 6.156 0.006

P. chalcographus Burning Among unharvested F1,16= 8.793 0.009 Among DWR5 F1,16= 26.605 <0.001 Among DWR30 F1,16= 5.000 0.040 Among DWR60 F1,16= 18.598 0.001 Harvesting Among burnt F3,16= 3.979 0.027 with DWR Among unburnt F3,16= 20.596 <0.001

Fig. 4.The effect of burning and harvesting with DWR on the number ofP. chalcographuscollected in three traps attached to dead Norway spruces in 10 May to 10 July 2003 (meanS.E.,N= 3 in each treatment).

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with 60 m3/ha DWR treatment than on other harvested with DWR

treatments (Table 5).

3.3. The abundance of bark beetles in forests adjacent to restored areas

In total, we collected 9441 bark beetle individuals representing

32 species, including 82I. typographusand 3182P. chalcographus,

in traps along the lines leading to adjacent forests (Table 1). The

number of bark beetles collected was affected by distance from the plot (F5,100= 29.68, p<0.001) and treatment (F3,20= 6.20,

p= 0.004) and there was interaction between treatment and

distance (F15,100= 2.89,p= 0.001) (Fig. 5). Inside the plots (simple

effects test:F3,20= 17.68,p<0.001), the number of bark beetles

was higher on restored than on control treatments (pairwise

comparisons: burnt harvested vs. control, p<0.001; burnt

unharvested vs. control,p<0.001; unburnt harvested vs. control,

p= 0.001). At the edge of the plots (simple effects test:F3,20= 6.75,

p= 0.003), restoration treatments still increased the number of

bark beetles (burnt harvested vs. control, p<0.001; burnt

unharvested vs. control,p= 0.043; unburnt harvested vs. control,

p= 0.002). At 25 m outside the plots, there was no general

treatment effect (simple effects test, F3,20= 2.34, p= 0.105) but

burnt harvested treatment differed from control (pairwise

comparison, p= 0.048). At 50, 75 and 100 m outside the plots,

there were no significant treatment effects (p>0.21, all cases).

The abundance of I. typographus was affected by distance

(F5,100= 2.57,p= 0.031) and treatment (F3,20= 3.78,p= 0.027) but there was no interaction between distance and treatment (F15,100= 0.74, p= 0.742). No differences were found between restoration treatments and controls at any distance along the line

(p>0.08, all cases), but analyzing the abundance ofI. typographus

within the lines revealed some differences. Within burnt harvested

treatment, the abundance ofI. typographuswas higher inside the

plots than at 25, 50, 75, and 100 m outside the plots (pairwise

comparisons,p<0.01 in all comparisons). At the edge of the burnt

harvested plots, there were moreI. typographus than at 100 m

outside the plots (pairwise comparison, p= 0.047) and there

tended to be moreI. typographusthan at 25–75 m outside the plots

(pairwise comparisons, 0.05<p<0.10 in all comparisons). Within

unburnt harvested treatment, the abundance ofI. typographuswas

higher inside the plots (pairwise comparison,p= 0.045) and at the

edge of the plots (pairwise comparison,p= 0.037) than at 50 m

outside the plots, and the abundances inside the plots and at the Table 5

The pairwise comparisons following the simple effects tests for the effects of harvesting with DWR on the number of bark beetles along host tree boles

Species Comparison Unburnt plots MDa

p Burnt plots MDa p I. typographus Unharvested vs. DWR5 3.018 0.001 0.634 0.430 Unharvested vs. DWR30 2.534 0.005 0.571 0.477 Unharvested vs. DWR60 2.587 0.004 0.188 0.813 DWR5 vs. DWR30 0.484 0.545 0.064 0.936 DWR5 vs. DWR60 0.431 0.589 0.446 0.576 DWR30 vs. DWR60 0.053 0.947 0.383 0.632 P. chalcographus Unharvested vs. DWR5 4.173 <0.001 0.348 0.540 Unharvested vs. DWR30 3.219 <0.001 0.324 0.569 Unharvested vs. DWR60 2.572 <0.001 1.579 0.017 DWR5 vs. DWR30 0.954 0.106 0.672 0.245 DWR5 vs. DWR60 1.601 0.011 1.131 0.059 DWR30 vs. DWR60 0.647 0.262 1.803 0.005 a

Mean difference between treatments. The abundances have been log-transformed.

Fig. 5.The effect of restoration treatments on the number of all bark beetles collected in flight-intercept traps along a line perpendicular to the edge of the study plots (one trap per distance) in 10 May to 10 July 2003. (meanS.E.,N= 9 for harvested andN= 3 for unharvested treatments).

Fig. 6.The effect of restoration treatments on the number ofP. chalcographus

collected in flight-intercept traps along a line perpendicular to the edge of the study plots (one trap per distance) in 10 May to 10 July 2003 (meanS.E.,N= 9 for harvested andN= 3 for unharvested treatments).

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edge of the plots tended to be higher than at 25, 75 and 100 m

outside the plots (pairwise comparisons, 0.05<p<0.10 in all

comparisons). Within burnt unharvested treatment, there were no

significant differences among distances (p>0.66, all cases).

The abundance ofP. chalcographus was affected by distance

(F5,100= 63.60,p<0.001) and treatment (F3,20= 11.57,p<0.001) and there was an interaction between distance and treatment (F15,100= 4.58,p<0.001) (Fig. 6). Inside the plots, the abundance of P. chalcographuswas higher on restored than on control treatments

(simple effects test:F3,20= 21.02,p<0.001; pairwise comparisons:

p<0.001 in all comparisons). At the plot edge, significantly moreP.

chalcographuswere collected on restored than on control

treat-ments (simple effects test: F3,20= 9.27, p<0.001, pairwise

comparisons,p<0.001 in all comparisons). The abundance ofP.

chalcographuswas also elevated at 25 m outside the plots (simple

effects test: F3,20= 3.32, p= 0.041; pairwise comparisons: burnt

harvested vs. control, p<0.005; burnt unharvested vs. control,

p= 0.107; unburnt unharvested vs. control,p= 0.019) and at 50 m

outside the plots (simple effects test: F3,20= 6.82, p= 0.002;

pairwise comparisons: burnt harvested vs. control, p= 0.001,

burnt unharvested vs. control,p= 0.058; unburnt unharvested vs.

control,p= 0.055) At 75 and 100 m, no significant treatment effects

were observed (p>0.14, all cases).

4. Discussion

4.1. The effect of restoration on the abundance of bark beetles The abundance of bark beetles in flight-intercept traps increased by both burning and harvesting with DWR, being highest where burning and harvesting had been combined. On burnt harvested plots, mortality among the standing retention trees was very high, the majority of spruces dying immediately

after the fire (Lilja et al., 2005). Therefore, the increase in the

abundance of bark beetles was most likely caused by increase in the availability of resources and attraction of beetles to host volatiles (e.g., monoterpenes) released from weakened trees. However, the effect of burning was dependent on the volume of DWR such that burning increased the number of bark beetles at low or intermediate volume of DWR but not at high volume of DWR. This is likely due to the fact that the intensity of fire

increased with the volume of DWR (Lilja et al., 2005) which may

have decreased the quality of resources (discussed below). In a concurrent study that was conducted on the same restored areas, it

was shown that the breeding success of I. typographus and P.

chalcographuswas generally low in burnt logs, and that it further

decreased with increasing volume of DWR (Eriksson et al., 2006).

While the volume of DWR had a negative effect on the abundance of bark beetles when burning was included (probably due to increased fire intensity), it had no clear effects without burning. Although larger amounts of damaged spruces generally attract more

colonizing bark beetles into restored or windfall areas (Eriksson

et al., 2005), the number of colonized logs was negatively correlated with the volume of DWR on the harvested plots of this restoration

experiment (Eriksson et al., 2006). We found that the number ofP.

chalcographus collected along the boles of unburnt spruces decreased strongly with increasing DWR level. Thus, the number of bark beetles dispersing to the restored areas may have been restricted by the sizes of the source populations. Another possible explanation for the lack of effect of DWR level is that the harvested plots were characterized by the wealth of logging residue such as branches and cut stumps that is known to be utilized by several

saproxylic species (Sippola et al., 2002), including also bark beetles

such asP. chalcographus(Hedgren, 2004). The volume of logging

residue was likely to be equal among all the harvesting with DWR

treatments making the proportional differences in the amount of resources smaller between the treatments.

4.2. The effect of fire and harvesting on the abundance of bark beetles along host tree boles

Samples collected in traps attached to host trees represent a collection of beetles emerging from and colonizing the trees and also reflect beetle activity around the trees. We found that the number of bark beetles in these traps was particularly high (compared to the flight-intercept traps, the number of beetles per trapping day was tenfold) and that the majority of beetles collected were newly emerged from the trees (i.e., callow adults identified by not fully developed coloration). This suggests that most of the beetles collected were truly associated to the particular trees. In addition, the treatment effects were not consistent with those observed by flight-intercept traps (i.e., burning increased the abundance of bark beetles in flight-intercept traps but less beetles were collected along burnt than unburnt host trees). Therefore, our interpretation is that samples collected in traps attached to host trees reflect the quality of the trees as breeding substrate for bark beetles and can thus be used as indirect measure of resource quality. However, results must be interpreted with some caution because the reproductive success of beetles was not directly measured.

We found that the abundance of bark beetles was lower in the traps attached to burnt spruces than those attached to unburnt

spruces, and that in particular the abundance ofI. typographuswas

negatively affected by fire. In addition, the abundance of P.

chalcographusin the traps attached to burnt spruces was lowest at

highest volumes of DWR, at which the fire was most intense (Lilja

et al., 2005), while the abundance ofI. typographusin the traps was low at all harvesting with DWR levels. Thus, although burnt areas provide a wealth of dead wood resources for bark beetles and

burning makes trees more susceptible to bark beetle attacks (Fettig

et al., 2007), fire may decrease the nutritional quality of phloem material and burnt trees may desiccate rapidly making the

resource less suitable for bark beetles (Wikars, 2002; Johansson

et al., 2006). This effect may be particularly strong in Norway

spruce because of its thin bark. The numbers of three common phloem-feeding bark beetle species have been found to be

decreased in fire-damaged stands in pine forests in Florida (Hanula

et al., 2002), and in particular heavily charred trees are known to

host very low insect densities (Saint-Germain et al., 2004),

although they may host high species diversity (Wikars, 2002).

The abundance of bark beetles along tree boles was also affected by harvesting, and the effect differed between burnt and unburnt trees. Among traps attached to unburnt spruces, harvest-ing increased the number of bark beetles, which was probably due to that the logged habitat itself attracted more bark beetles because of sun-exposed conditions and volatiles released from logging residues. Sun-exposed disturbance areas are favoured by

several dead wood dependent species (Kouki et al., 2001). For

example,I. typographus(Peltonen, 1999; Go¨thlin et al., 2000) andP.

chalcographus(Hedgren, 2004; Johansson et al., 2006), are known to prefer open areas and forest edges over forest interior. In contrast, among traps attached to burnt spruces, bark beetle numbers were negatively affected by harvesting, which is likely to reflect differences in the quality of dead wood resource. While the fires were intense on harvested plots, the unharvested plots did not burn particularly well and the proportion of trees directly killed by

fire was low (seeLilja et al., 2005). Thus, the weakened trees on

burnt unharvested plots may have provided a better resource to bark beetles compared to the heavily charred trees on burnt harvested plots.

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4.3. The abundance of bark beetles and the risk of elevated tree mortality outside restored areas

Bark beetles are capable of dispersing several kilometers in

forests (Wermelinger, 2004). However, the majority of newly

attacked trees byI. typographushave been found to occur within

100 m of an old attack (Wichmnann and Ravn, 2001). Thus, the

spatial scale of our study to determine whether restoration treatments lead to elevated abundance of bark beetles outside restored areas appears reasonable. The abundance of bark beetles in the adjacent forests was studied during the flying season of the species of interest in the first post-treatment year (in 2003) based on the assumption that the primary colonizers had colonized the restored plots immediately after treatments (in 2002) and would disperse from the plots following successful reproduction. We found that although the abundance of bark beetles was strongly elevated within burnt and harvested plots, the numbers of bark

beetles in the adjacent forests remained relatively low. Only fewI.

typographus were collected along the trap lines. Although the

abundance of P. chalcographus was significantly elevated up to

50 m outside restored plots, abundances were nevertheless very

low compared to that inside the plots.Eriksson et al. (2006)found

that the number of dead spruces withI. typographuswas very low

at the edges of the harvested plots of this experiment during 3 years after treatments, and that there were no differences between the treatments. This suggests that bark beetles are not likely to disperse into the ‘‘healthy’’ forests around restored areas. Even if the abundance of bark beetles strongly increased after restoration treatments, it seems that the population densities did not reach the level in which bark beetles would have been able to attack live trees and overcome host defenses. The reasons for this may include reduced breeding success in fire-killed trees, originally small source populations, and the fact that all the dead wood was created simultaneously that led to lack of breeding material once the resource was fully utilized. However, if must be noted that if colonization is not limited by small size of source populations, providing large quantities of dead wood might enable the populations grow large enough to successfully attack healthy trees after the original resource has been exhausted.

4.4. Conclusions and practical implications

Controlled burning and partial harvesting with DWR increased the number of bark beetles, including species that are potential forest pests of Norway spruce forests. The abundance of bark beetles was highest when restoration treatments included both burning and harvesting. This is likely due to burnt areas providing more resources for bark beetles than unburnt areas, however, burnt Norway spruces were a less suitable resource for bark beetles than unburnt dead spruces. In addition, the abundance of bark beetles on burnt areas decreased with the volume of DWR,

probably because DWR levels also influenced fire intensity (Lilja

et al., 2005) which may have negatively affected resource quality (e.g., nutritional quality of cambium and amount of moisture). Therefore, in terms of pest management, burning might be a safer way to create large quantities of dead wood than simply girdling or felling the trees. Without burning, the volume of DWR had no clear effect on the abundance of bark beetles, which may have been due to that the number of colonizing individuals was restricted by the size of source populations or that the effect of DWR was masked by the effect of logging residue. Under the conditions of our study, restoration does not seem to cause any severe risk of elevated levels of tree mortality in the adjacent forests. However, we note that creating large quantities of dead wood within a small area at consecutive years, or low-intensity burnings that do not directly

kill the trees but make them susceptible to bark beetle attacks for several years, might enable the populations to grow to outbreak levels. The results we reported here may also not be applicable to

other geographic regions, especially to those whereI. typographus

is able to produce more than one generation per year which is very unlikely in Finland.

Acknowledgements

The restoration experiment was established by the Forest Ecology section of the University of Helsinki, and we thank especially Timo Kuuluvainen, Saara Lilja and Pasi Puttonen for co-operation. The burning and harvesting treatments were carried out by the land-owners and helped by numerous students and volunteers. Jarno Nevalainen and Satu Kuntsi carried out a substantial part of beetle identification. Jarno Nevalainen helped also in the field work and the material was sorted by Satu Kuntsi and Elina Manninen. Two referees provided valuable comments on the manuscript. The study was funded by the MOSSE research program of the Ministry of Agriculture and Forestry, Otto A. Malm’s Foundation (research grant to T.T.), Jenny and Antti Wihuri’s Foundation (research grant to T.T.), Metsa¨miesten Sa¨a¨tio¨ Founda-tion (research grant to V.L.), The Academy of Finland and The Centre of Excellence in Evolutionary Research of University of Jyva¨skyla¨.

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