Felled or standing retained wood—it makes a difference for saproxylic beetles

Forest Ecology and Management 175 (2003) 425±435

Felled or standing retained woodÐit makes a difference for saproxylic beetles Mats Jonsella,*, Jan Weslienb a

Department of Entomology, Swedish University of Agricultural Sciences, Box 7044, SE-75007 Uppsala, Sweden b SkogForsk, SE-75183 Uppsala, Sweden Received 26 June 2001; received in revised form 29 January 2002; accepted 15 April 2002

Abstract High stumps are often retained at clear cuttings to increase the abundance of habitat patches for saproxylic (wood living) insects. However, these high stumps constitutes a very uniform dead wood habitat which probably supports only a part of the saproxylic fauna. Therefore, we compared the saproxylic fauna of high spruce stumps with the fauna of long and short felled boles of spruce. We also investigated the associations between insect species and polypore fungi growing in the wood. All wood units were created at the same occasion on a clear cut in SW Sweden. The dominating species of bark beetles and longhorn beetles were surveyed in the ®rst year after the cutting. Four years later, the fauna was sampled again by sifting bark samples and all species found were determined. In total we recorded six species early in the succession and 43 four years later. Two species were red-listed. Three out of ®ve statistically tested early successional species had signi®cant associations with some of the wood types, while the corresponding ®gures later in the succession were six of 15. Three of the 15 species in the late succession were also signi®cantly associated with the presence of fruiting bodies of the polypore fungus Fomitopsis pinicola. We concluded that retaining felled wood in addition to high stumps may provide an important means of diversifying the dead wood substrates, which may in turn increase the number of saproxylic species on a site. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Biodiversity; Coleoptera; Dead wood; High stumps; Fungi

1. Introduction Concern that biodiversity in forests may be reduced by intensive management has resulted in certain measures being taken in Scandinavian forestry to decrease the negative impact on various organisms. One perceived cause of loss in biodiversity is the low amount of dead wood in managed forests (Esseen et al., 1997; È stlund, 1998; Fridman and Walheim, Linder and O *

Corresponding author. Tel.: ‡46-18-672876; fax: ‡46-18-672890. E-mail address: [email protected] (M. Jonsell).

2000), which threatens the existence of many species in Scandinavia (Berg et al., 1994). Therefore, recommended measures during forest operations include the retention and intentional creation of dead wood. This is, for example, included in the certi®cation standards for Swedish forestry (FSC-council, 2000; PEFC, 2000). Many saproxylic (wood living) insects, including red-listed species, have been caught in window traps placed on retained trees on clear cuts (Ahnlund, 1996; Kaila et al., 1997; Sverdrup-Thygeson (2002); Martikainen, 2001; Jonsell and Eriksson, 2001). Such traps measure the activity of insects around the trunk, which to some degree re¯ects the fauna that reproduce in the

0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 1 4 3 - 3

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wood, but to a very large extent also encompasses many species that are transient visitors. Herbivores and ground dwellers are very conspicuous examples of such visitors, but among the saproxylic species visitors are harder to recognise. In this study, we investigated bark samples, which gives a more direct measure of the species that really dwell, and probably reproduce, within the wood. Only two such studies have been done following felling operations (Hansson, 1998; NitteÂrus, 1998), both of which investigated the fauna of arti®cially created high stumps. The results of these studies strongly suggest that stumps of this kind provide useful sites for reproduction of some wooddwelling insects, and thus bene®t the saproxylic fauna. Sun-exposed wood may even be essential for the survival of some species (Ahnlund and Lindhe, 1992; Kouki et al., 2001), and most red-listed saproxylic insects in Sweden can breed or prefer to breed on substrates that are fully exposed to the sun (Jonsell et al., 1998). Most of the dead trees that have been retained so far in Sweden are high stumps of spruce (Picea abies). One reason for this is that spruce contains butt-rot more often than other tree species, and if trees with the disease are chosen the economic loss is negligible. However, high stumps of spruce represent only one speci®c kind of dead wood and many insect species require other tree species or other types of wood (Palm, 1959; Jonsell et al., 1998). One easy method for increasing the diversity of wood substrates would be to create lying trunks in addition to high stumps (EhnstroÈm, 2001). It has been indicated that standing and lying trunks have somewhat different faunas (Palm, 1951, 1959) and about 60% of the insect species living in association with wood decaying fungi have differing probabilities of occurrence depending on whether the wood is standing or lying (Jonsell et al., 2001). Sverdrup-Thygeson (2002) also showed that there were differences in species number and composition in window-trap catches between aspen snags and their respective tops. However, the study presented here is the ®rst that has experimentally tested whether adding lying wood to the stumps increases the diversity of saproxylic insects. One factor that probably exerts a very important in¯uence on the insect fauna in a decaying trunk is the fungal species that rot the wood. This dependence is very straightforward for specialist insects that live

on fruiting bodies of fungi, and many publications describe relationships between insects and fungal hosts, e.g. Fossli and Andersen (1998) and Jonsell (1999). There may also be close relationships among species living inside the wood, although the fungus is present only as inconspicuous mycelia (Wallace, 1954; Lawrence, 1989). As well as the insects being dependent on the fungi, the fungal species may also be dependent on the conditions of the wood (Renvall, 1995). Our ®rst aim was to investigate if there was any difference in saproxylic beetle fauna between lying boles and standing high stumps. This comparison was done separately for each species, which also gives faunistic data relevant to conservation work. In the same way we also tested whether there were differences between long lying boles from the upper part of the tree and short boles from the basal part of the same tree. In a further step we correlated the occurrences of insect species with fungal species present on the wood units. We also listed beetles found in too low numbers to be statistically treated, to record rare species that might use retained wood on clear cuts. 2. Material and methods The study was done on a clear cut in the SW part of Sweden, at Mangskog in the county of VaÈrmland (598500 N/128200 E, 175 m a.s.l.). The forests in the area are conifer-dominated and the most common conifer tree is spruce (P. abies). The species pool present in the area could not be expected to be especially rich or poor as suggested by the presence of key biotopes or reserves. In a radius of 10 km around the experiment site there are no forest reserves and just a few key biotopes with natural forest (http:// www.svo.se), which is normal for Swedish forests. The forest on the site was cut in October 1995, when the trees were about 90 years old. Wood from living trees was then retained, for monitoring in this survey. The experimental setup was partly determined by the sampling procedure adopted in another study at the same site (Jacobsson et al., 2000). Three sample trees per plot (six plots, each of 314 m2) representing trees with the mean diameter  one standard deviation were chosen. Wood samples (discs for nutrient analysis) were taken at breast height (1.3 m above ground level), at 30 and 50% of tree height. The lying boles in the

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427

Table 1 Characteristics of the wood units from which beetles were sampled (®gures given are means with minima and maxima in parentheses) Wood type

n

Length/height (cm)

Diameter (cm)

Remaining bark (%)

Short felled boles Long felled boles High stumps

18 18 18

120 (113±124) 585 (425±748) 231 (220±240)

29.3 (19.5±42.0) 23.8 (16.5±35.0) 26.4 (16.5±40.0)

53.9 (10±90) 46.7 (10±80) 39.4 (10±70)

present study are the remaining parts of these trunks. In summary, on each of six plots we had three high stumps (chosen to resemble the diameter at breast height of the three sample trees), three short lying boles consisting of the basal part of the tree and three long lying boles consisting of the trunk above the basal part. The long boles consisted of the tree part immediately above breast height (with two exceptions: two such long boles were taken away by mistake during the logging operation). Instead, one additional long bole from another sample tree was left (the part between and 50% of tree height). We measured length of the boles, and the height and diameter of the stumps (Table 1) as well as the number of fruiting bodies of wood decaying fungi on all wood units. We also estimated the proportion of the bark that remained on the trunks. The diameters were measured at breast height on the high stumps and at the middle of the lying boles. All 54 wood units were fully exposed to the sun as they were situated out on the open clear cut. The trees from which the wood was retained stood on plots that had been treated earlier with different nitrogen fertilising regimes (see Norstedt et al., 1999; Jacobsson et al., 2000). As no effect of the fertilising regime on the faunal composition in the deadwood could be detected (Jonsell and Weslien, unpubl.) the material from all plots were pooled. On 30 June 1996, i.e. the ®rst summer after the clear cut, we investigated the density of some early successional species. Only the most dominant consumers of the inner bark were recorded, i.e. bark beetles and longhorn beetles. On all snags and boles we estimated the percentage of the bark area covered with galleries of different bark beetle species. If the galleries were restricted to one side of the tree bole their position was noted with respect to whether they were on the upper or lower sides of lying boles, and their orientation in the high stumps. The stumps and logs were carefully searched for signs of attack by bark- and wood-boring insects, such as entrance and exit holes, boring dust,

resin ¯ow, and loss of bark. Species were mainly identi®ed on the basis of gallery characteristics, but small pieces of bark were also removed to search for living or dead insects, which could be used in identi®cation. Such adult samples revealed that the representatives of the genus Orthotomicus included both O. suturalis (Gyll.) and O. laricis (F). We also noted the presence or absence of the wood boring ambrosia beetle Trypodendron lineatum (Olivier). Insect species occurring later in the successional decay of the wood were investigated after four summers, on 1 and 2 September 1999. On these dates we took sieve samples consisting of 0.25 m2 bark from each wood unit. The samples were taken at breast height on the high stumps and on the side of the boles if possible. In some cases the bark had fallen to such an extent that the sampling area had to be shifted nearer the ground where bark was still present. At the same occasions we also counted the number of rearing holes of the early successional species Monochamus sutor and the number of fungal fruiting bodies. The sieved bark samples were brought back to the lab in textile bags where they were placed in Tullgren-funnels for at least 24 h. All saproxylic beetles (Coleoptera) and Aradus species (Hemiptera) were identi®ed to the species level, except for a few dif®cult cases that were determined to the genus level (see Table 3). Red-list categories are after GaÈrdenfors (2000). 2.1. Statistics For each species we measured the density as individuals per m2 bark. Differences in density between the three wood types were tested for statistical signi®cance using the Kruskal±Wallis test (non-parametric ANOVA) with corrected H for tied ranks (Zar, 1984). The test was applied to species occurring in at least ®ve samples. In cases where these tests gave a signi®cant outcome we used the non-parametric Tukey test for multiple comparisons to locate differences between

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wood types (Zar, 1984). For T. lineatum, where we only had presence/absence data, Fisher's exact test was used (Zar, 1984). Differences in species numbers between wood categories were tested with an ANOVA. Associations between insects and fungal species were calculated by using presence/absence data in 2  2 contingency tables, which were analysed statistically with Fisher's exact test using SAS version 6.12. All other statistical calculations were done with Statview for Mac version 5.0.1. 3. Results 3.1. Variables measured on the wood Before analysing the association of the measured variables with beetle species we investigated whether there were any systematic differences between the three wood types. The percentage of remaining bark varied widely for all wood types, ranging from 10 to 90% (Table 1). Six of the 54 trunks had only 20% or less of the bark remaining. There were, however, no systematic differences between the three wood types (Mann±Whitney test, H ˆ 4:04, p ˆ 0:13). The diameter differed signi®cantly between categories, as the short boles were thicker than the long boles (Tukey test, p < 0:05) and the stumps had intermediate diameters (Table 1). 3.2. Total species list In total 49 saproxylic species were recorded in the study. We found six early successional species: ®ve bark beetles (Hylurgops palliatus (Gyllenhal), Orthotomicus spp. (laricis (Fabricius) and suturalis (Gyllenhal)), Pityogenes chalcographus (Linneaus), Ips typographus (Linneaus) and T. lineatum (Olivier)) and one longhorn beetle (M. sutor (Linneaus)). The

sieve sampling carried out later in the succession yielded in total 499 individuals including 43 saproxylic species (Table 2): one of which (Aradus corticalis) was a bug (Hemiptera), and the rest beetles (Coleoptera). Two species are on the Swedish red-list: Euthia linearis (NT) and Ipidia binotata (VU). In total, we found a higher number of species in the two types of boles than in the high stumps (Table 2). The average number of species per wood unit had the same pattern, but there was no signi®cant difference between the categories (ANOVA, F ˆ 2:29, d:f: ˆ 51, p ˆ 0:11). The number of species unique to any category was also higher for both types of bole than for the high stumps. 3.3. Early successional species Among early successional species M. sutor was much more common on long boles than on any other substrate (Fig. 1d, Kruskal±Wallis test: H ˆ 18:5, p < 0:0001). Among the other early species H. palliatus was most common on high stumps (Fig. 1a) and was, without exception, found on the north±northeast facing side of the stumps. P. chalcographus was most common on long boles (Fig. 1b). In both these cases there were signi®cant differences between the wood types (for H. palliatus: H ˆ 6:1, p ˆ 0:048; P. chalcographus: H ˆ 7:0, p ˆ 0:031). Orthotomicus spp. and T. lineatum did not show any signi®cant preferences for the wood types (Fig. 1c and e, H ˆ 2:7, p ˆ 0:27; Fisher test, p ˆ 0:27, respectively). I. typographus occurred only in two samplesÐone from a long bole and one from a high stump. 3.4. Later successional species Of the 43 species found later in the succession, 15 were found in at least ®ve wood units and were

Table 2 Numbers of beetle species and individuals caught in sieve samples four summers after felling

Total No. of individuals Total No. of species Average No. species No. of unique species

Short felled bole

Long felled bole

High stump

Total

149 30 4.6 8

209 31 5.2 7

141 20 3.2 1

499 45 4.3 ±

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429

Fig. 1. Frequencies of primary consumers estimated as: percentage of bark area covered by beetle galleries (a)±(c), number of beetle rearing holes/m2 (d) and beetle presence/absence (e). Bars with different letters mark signi®cant differences (Tukey test, p < 0:05).

therefore analysed statistically regarding their habitat associations (Table 3). Six of these had a distribution signi®cantly different between wood types. For the remaining nine species we could not detect any difference in abundance between wood types. Among species that were most abundant on high stumps Hadreule elongatula had a strongly signi®cant pattern (Table 3), being found on only one bole, while occurring on 10 high stumps. Cis comptus had a distribution between wood types that was signi®cantly different from equal, with most ®ndings on high stumps. I. binotata was more common on high stumps than on short boles (although the difference was not quite statistically signi®cant p < 0:05) with long boles as an intermediate type (Table 3). Lygistopterus sanguineus tended also to be most common on high stumps. Several other species were more common on boles than on high stumps. A. corticalis was signi®cantly more common on long boles than on high stumps, with short boles as an intermediate class (Table 3). Three other species, Pteryx suturalis, Tyrus mucronatus, and

Gyrophaena boleti had signi®cant differences between the wood types according to the Kruskal± Wallis test and no individuals of these species were found on the high stumps. 3.5. Relationships with fungi Fruiting bodies of two fungal species were observed. Of these, Gloeophyllum sepiarium, was signi®cantly more common on long boles than on any other wood type (Table 4; Kruskal±Wallis test: H ˆ 32:9, p < 0:0001; Tukey test at signi®cance level p ˆ 0:01), while the presence of Fomitopsis pinicola had no signi®cant relationship to wood type (H ˆ 1:9, p ˆ 0:38). As an alternative we also tested the presence/absence of these species against the three wood types in contingency tables which yielded the same result: p < 0:0001 for G. sepiarium and p ˆ 0:26 for F. pinicola. Analysis of the presence/absence of the two fungal species showed them to be negatively associated with each other (contingency table, Fisher test, p ˆ 0:049).

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Table 3 Number of individuals per m2 bark and (in parentheses) the number of wood units in which insects were present in sieve samples taken four summers after clear cuta. Species are presented in systematic order Species Tachyta nana (Gyllenhal 1810) Pteryx suturalis (Heer 1841) Eutheia linearis (Mulsant 1861) Stenichnus bicolor (Denny 1825) Gabrius splendidulus (Gravenhorst 1802) Nudobius lentus (Gravenhorst 1806) Bibloporus bicolor (Denny 1825) Euplectus spp. Tyrus mucronatus (Panzer 1803) Acrulia inflata (Gyllenhal 1813) Phloeocharis subtilissima (Mann. 1830) Scaphisoma sp. Sepedophilus sp. Dadobia immersa (Erichson 1837) Dinaraea angustula (Gyllenhal 1810) Dinaraea aequata (Erichson 1837) Gryophaena boleti (Linnaeus 1758) Bolitochara pulchra (Gravenhorst 1806) Leptusa pulchella (Mannerheim 1830) Leptusa fumida (Erichson 1839) Lygistopterus sanguineus, larvae (L. 1758) Melanotus castanipes (Paykull 1800) Ptinus subpilosus (Sturm 1837) Epuraea variegata (Herbst 1793) Ipida binotata (Reitter 1875) Arpidiphorus orbiculatus (Gyll. 1808) Rhizophagus dispar (Paykull 1800) Atomaria elongata (Erichson 1846) Cerylon histeroides (Fabricius 1792) Corticaria longicollis (Zetterstedt 1838) Cis glabratus (Mellie 1848) Cis comptus (Gyllenhal 1827) Cis punctulatus (Gyllenhal 1827) Ennearthron cornutum (Gyllenhal 1827) Hadreule elongatula (Gyllenhal 1827) Bitoma crenata (Fabricius 1775) Mordella sp. Curtimorda maculosa (Naezen 1794) Rhagium inquisitor (larvae) (L. 1758) Hylobius abietis (L. 1758) Hylastes cunicularius (Erichson 1836) Crypturgus spp. Aradus corticalis a

Red-list categoryb

NT

VU

Short felled bole

Long felled bole

High stump

0 4.4 (5)a 0 0 0.9 (3) 0 0.4 (1) 0.4 (2) 3.6 (8)a 0.4 (2) 0.2 (1) 1.1 (3) 0.2 (1) 2 (5) 0.4 (1) 1.3 (2) 0.9 (1)a 0.2 (1) 2 (6) 0 0.2 (1) 0.2 (1) 0.4 (2) 0.2 (1) 0 0.2 (1) 2 (6) 0.2 (1) 1.3 (2) 0.4 (2) 0.7 (2) 0a 0 0 0a 0.9 (2) 0 0 0.4 (2) 0.2 (1) 0 1.3 (3) 5.8 (8)a,b

0.4 0a 0.7 0.2 0 0 2.2 0.2 3.3 0 0.4 1.8 0 2.2 0 0.2 4.7 0 4.9 0.4 0 0.2 0 0.4 0.7 0 0.7 0.7 0.2 1.3 0.4 0.9 1.1 0.2 0.4 0.2 0.2 0.2 0 0 0.4 3.3 12.9

0 0a 0 0 0 0.2 (1) 0 0.2 (1) 0a 0 0.4 (2) 0 0 0.7 (2) 0 0.4 (2) 0a 0 0.7 (2) 0.7 (2) 1.1 (4) 0 0.4 (1) 0.2 (1) 4 (5) 0 0.4 (2) 0 0 2 (6) 0 3.8 (5)a 0.4 (2) 0 12 (10)b 0.2 (1) 0 0 0.2 (1) 0 0 1.1 (2) 2 (5)a

(2) (3) (1) (3) (1) (6)a (1) (4) (6) (1) (7)a (5) (2) (1) (2) (3) (3) (2) (1) (5) (2) (2)a (2) (1) (1)a (1) (1) (1) (2) (5) (13)a

p 0.0045

0.0096 0.13 0.29 0.79 0.0026 0.3 0.06

0.057 0.21 0.24 0.046 0.0001

0.4 0.01

Statistics denote Kruskal±Wallis test. Letters indicate differences between wood categories according to Tukey's test of multiple comparisons. b Red-list categories are after (GaÈrdenfors, 2000). NT: near threatened; VU: vulnerable.

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431

Table 4 Percentage of units of each wood type in which two fungal species, F. pinicola and G. sepiarium, occurreda. Numbers with different letter mark signi®cant differences (Tukey test, p < 0:05) Species

Short felled bole

Long felled bole

High stump

H

p

F. pinicola G. sepiarium

50 50a

61 89b

78 0a

1.94 32.9

0.38 0.0045

a H-values and probability levels refer to ®gures derived using the Kruskal±Wallis test, ranking wood units according to the number of fruiting bodies found.

Some insect species showed a strong association to one or the other of the two fungal species (Table 5). I. binotata, H. elongatula and A. corticalis had statistically signi®cant positive associations with F. pinicola, while M. sutor and L. sanguineus showed associations to G. sepiarium. However among wood types, M. sutor and G. sepiarium had the same association pattern (cf. Fig. 1d and Table 4). When the association between the two species was tested only on short boles, the pattern was far from signi®cant (contingency table, Fisher test, p ˆ 0:99). Thus, the association between Table 5 Associations between fungal species and saproxylic insects, shown by p-values for Fisher tests of contingency tablesa Species

4. Discussion

Association with F. pinicola

G. sepiarium

Primary consumers H. palliatus P. chalcographus Orthotomicus spp. T. lineatum M. sutor

0.10 0.005 ( ) 0.079 (‡) 0.073 (‡) 0.55

0.51 0.054 (‡) 0.78 0.39 0.0091 (‡)

Later successional species P. suturalis T. mucronatus Scaphisoma sp. D. immersa D. aequata G. boleti L. pulchella L. sanguineus (larvae) I. binotata R. dispar C. longicollis C. comptus H. elongatula Crypturgus spp. A. corticalis

0.99 0.75 0.99 0.52 0.64 0.13 0.52 0.36 0.02 (‡) 0.99 0.99 0.99 0.004 (‡) 0.47 0.0097 (‡)

0.99 0.99 0.99 0.99 0.36 0.076 (‡) 0.34 0.045 (‡) 0.71 0.99 0.54 0.43 0.0091 ( ) 0.99 0.172

a

M. sutor and G. sepiarium for the whole material seems to be a result of the two species having similar associations to wood type. P. chalcographus was negatively associated with F. pinicola (Table 5) and H. elongatula was negatively associated with G. sepiarium. For H. elongatula and G. separium the negative association might be explained by the two species having opposite association patterns with wood types, and/or a negative association between them per se (Tables 3 and 4). However, there is only one observation of the species in wood types other than high stumps, so this could not be tested statistically.

Positive or negative associations are noted when p < 0:10.

4.1. Felled or standing trees We found a number of insect species that were clearly more or less restricted to either high stumps or boles. Fewer differences could be detected between the two bole types. However, many species were found quite infrequently. Of the total 43 species found, we could only test the pattern of 15 statistically, suggesting that this study is more likely to have underestimated the numbers of species associated with either standing or lying trees. The differences between the wood categories are probably largely an effect of water content. Although it has not been studied in very many cases, moisture content has proven to be a signi®cant variable, to which the abundance of some saproxylic species is correlated (Thunes and Willassen, 1997; Midtgaard et al., 1998; Jonsell et al., 2001). As well as determining water availability, moisture content also affects the temperature of the substrate, and it is not only saproxylic insects that are affected by these variables.

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Wood-decaying fungi also have different optima for these factors and they, in turn, have probably very important in¯uences on the insect fauna (see below). We have not measured the moisture content of the wood. However, we are quite sure that the coarse short boles were moister than the long boles since they had thicker bark, and they were in full contact with the ground throughout their entire length. The high stumps were most exposed to wind and sun, but had thick bark and only one cut end, so it is dif®cult to compare them with the lying stems. The moisture in the bark and wood will also to some extent depend on the weather situation. This may have some effect especially on the early successional species as most of these species are present during one season only. May and June 1996 (before the ®rst survey of the stumps) were rather cold (Anonymous, 1997), and therefore the bark and wood may have desiccated slower than during a normal year. The later successional species will probably perceive minor effects from the weather as they spend several years, with consecutive generations, in the same wood piece. Thus, they are affected by several seasons with different weather conditions summing up to some average. Nevertheless, the summer and especially the autumn of 1999 were rather warm, although not very dry (Anonymous, 2000). However, there is no data known to us that suggest that seasonal weather have any important effect on the substrate selection of wood-living insects.

strongly associated with fruiting bodies of F. pinicola (Korinek, 1935). The cisid H. elongatula has also been observed many times in association with F. pinicola (M. Jonsell, pers. obs.), but mainly on thick mycelium strings inside the trunks and almost never in fruiting bodies (Jonsell et al., 2001). The third species, I. binotata, has not previously been very clearly associated with any speci®c fungal species although it has been found on F. pinicola before (Ahnlund and Lindhe, 1992, Jonsell, unpubl.). For G. sepiarium it was more dif®cult to de®ne the association between the fungus and the insects as it had a strongly skewed distribution between wood types. For most comparisons within single types of wood there were too few observations to make an analysis. The longhorn beetle M. sutor and the fungus G. sepiarium were both much more common on the long boles than on the other wood types. Thus, they were associated with each other. M. sutor is an early successional species that severely affects the material it attacks as the large galleries make the bark fall off which, in turn, causes the wood to become drier. As G. sepiarium is mostly found on wood exposed to the sun (Kotiranta and NiemelaÈ, 1981), the activity of M. sutor probably helps make the logs suitable for it. Another, more speculative possibility is that M. sutor may disperse the fungus by carrying spores or pieces of mycelium.

4.2. Associations with fungi

The attack patterns displayed by the six early successional species are consistent with known habitat preferences and agree to a large extent with earlier studies (Schroeder et al., 1999 and references therein). H. palliatus is a shade-loving species and was present on all high stumps on the north or east facing sides. In an earlier study (Schroeder et al., 1999), I. typographus was found to attack only stumps that had been cut in the winter or spring, whereas no stumps cut in autumn were attacked. In accordance with these observations, we found very low frequencies of I. typographus as all stumps were cut in October. P. chalcographus is known to reproduce in many types of spruce substrate, from quite coarse to very thin stems. In this study it was most common in the long boles that lacked thick, rough bark. In the study by Schroeder et al. (1999), M. sutor was found in rather

In the boles and stumps we investigated there is probably a wide ¯ora of wood-decaying fungi. Most fungal species are, however, very cryptic and exist merely as mycelia in the wood, or can be seen as small, short-lived fruiting bodies. We detected only species with conspicuous long-lived fruiting bodies and found two such fungal species. However, the fruiting bodies represent only the sexual stage of the fungal life cycle and our survey thus gave a minimum estimate of where the two fungi were present. We found three insect species that were positively associated with F. pinicola: the two beetles I. binotata and H. elongatula and the bug A. corticalis. A. corticalis has earlier been associated with ``stumps of spruce'' (Coulianos, 1989) and is more speci®cally

4.3. Species composition

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high frequencies on the stumps while in our study it was largely con®ned to the long boles. The difference in these results might be due to the fact that the beetles had the opportunity to choose in our study. M. sutor is known to prefer lying, sun-exposed stems and the results of Schroeder et al. in this respect were somewhat surprising. The early successional stage was monitored only during the ®rst summer. Earlier studies have shown that several species, e.g. H. palliatus, Orthotomicus spp., and T. lineatum attack also in the second summer (Butovitsch, 1971; Schroeder and Eidmann, 1993; Schroeder et al., 1999). Therefore we may have underestimated the abundance of these species and probably also missed some species that are common in the second summer, e.g. bark beetles of the genus Dryocoetes. The number of saproxylic species we found in the later successional stages, 43, is similar to the numbers seen in previous investigations. Hansson (1998) found 57 saproxylic species but used a collection method that caught beetles that emerged from the stump, and which was in operation throughout the entire ®eld season. In a survey of aspen and birch stumps NitteÂrus (1998) found, in total, 77 beetle species, most of which were saproxylic. As two tree species and several successional stages were included in the cited study, one would expect a large number of species to have been detected than in our investigation. Two of the species found in this study are red-listed. Of these, the biology of E. linearis is rather poorly known, but it has mostly been found in association with deciduous trees and/or with ants (Palm, 1959). The three specimens found here suggest that spruce wood may also be useful for this species. The other red-listed species, I. binotata, is considered rare in Sweden and its abundance in this study is somewhat surprising. It seems to be favoured by severe disturbance such as ®re or clear cutting, as long as the required substrates are retained (Ahnlund and Lindhe, 1992). It is not usually present in managed forests, with stands like the site studied here, so it may have colonised from an unknown source in the surrounding landscape. In landscapes with a rich saproxylic fauna many species could be expected to colonise retained wood on clear cuts (Ahnlund and Lindhe, 1992; Martikainen, 2001; Jonsell and Eriksson, 2001). However, if red-listed species are found in retained wood

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on all clear cuts irrespective of where they are situated, it rather suggests that the red-list should be revised because such a species is too wide spread to be redlisted. This was the case with another species in this study, H. elongatula, which was formerly red-listed (EhnstroÈm et al., 1993). 4.4. Management implications Surveys on biodiversity are often restricted to counting the number of species present. Here, this would have led to the conclusion that high stumps are less valuable than the boles as fewer species were found in them. However, it is important to know which species are present in various wood types. In this case, the different wood types had faunas that were complementary to each other, even if most species could use both boles and stumps. Thus, if the stumps were supplemented with felled boles more species would be able to use the clear cut. Forestry practices are often modi®ed to mimic natural disturbances, since this is expected to minimise the negative effects on threatened organisms (Haila et al., 1994; Angelstam and Pettersson, 1997; Fries et al., 1997; Bengtsson et al., 2000). Leaving cut trees as arti®cial snags or logs is probably not very close to natural circumstances. Leaving trees intact would probably be closer to natural disturbance patterns, and create a greater variety in dead wood substrates. Some of the upper parts of trees will blow off, leaving high stumps, other trees will be uprooted by wind, some will decline slowly and others will remain vital. Factors such as sun exposure, tree species, successional stage and size of boles or stumps will vary. These factors are known to be important for saproxylic insects (Palm, 1959; Kaila et al., 1997; Jonsell et al., 1998; Jonsell et al., 2001) and if all states are present it would give room for a higher diversity of organisms that depend on dead wood. Nevertheless, cut trees, both standing and lying, have the psychological advantage of being conspicuously created for biodiversity purposes. Retained intact trees are often taken out of the forest when they fall or die as they may be regarded as wasted if they are not used. Retained high stumps and cut logs could also be easily monitored, for instance in a certi®cation process. Our study indicates that several species may bene®t from such measures on a local scale. However, the

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ecological signi®cance of taking these steps needs to be proven on a larger scale. Even if we knew exactly which species reproduce in the wood, one would still need to know the reproductive success relative to the success in the rest of the landscape to really assess the bene®t of retaining it. Acknowledgements We thank Ulf SikstroÈm for help with establishing the experiment, Martin Schroeder for comments on the manuscript and John Blackwell for correcting the English. The study was ®nanced mainly by the forest companies AssiDomaÈn AB, Holmen Skog AB, KorsnaÈs AB, SCA Skog AB and Stora Enso Skog AB, as part of the wider project ``KvaÈve 2002''. References Ahnlund, H., 1996. Vedinsekter paÊ en soÈrmlaÈndsk aspstubbe (Saproxylic insects on a Swedish dead aspen). Entomol. Tidskr. 117, 137±144, (in Swedish with English summary). Ahnlund, H., Lindhe, A., 1992. Hotade vedinsekter i barrskogslandskapetÐnaÊgra synpunkter utifraÊn studier av soÈrmlaÈndska brandfaÈlt, haÈllmarker och hyggen (Endangered wood-living insects in coniferous forestsÐsome thoughts from studies of forest-®re sites, outcrops and clearcuttings in the province of SoÈrmland, Sweden). Entomol. Tidskr. 113, 13±22, (in Swedish with English summary). Angelstam, P., Pettersson, B., 1997. Principles of present Swedish forest biodiversity management. Ecol. Bull. 46, 191±203. Anonymous, 1997. VaÈder och vatten, VaÈderaÊret 1996. SMHI, NorrkoÈping (in Swedish). Anonymous, 2000. VaÈder och vatten, VaÈderaÊret 1999. SMHI, NorrkoÈping (in Swedish). Bengtsson, J., Nilsson, S.G., Franc, A., Menozzi, P., 2000. Biodiversity, disturbances, ecosystem function and management of European forests. For. Ecol. Manage. 132, 39±50. Ê ., EhnstroÈm, B., Gustafsson, L., HallingbaÈck, T., Jonsell, Berg, A M., Weslien, J., 1994. Threatened plant, animal, and fungus species in Swedish forests: distribution and habitat associations. Conserv. Biol. 8, 718±731. Butovitsch, V., 1971. Untersuchungen uÈber das Auftreten von ForstschaÈdlingenin den von SchneestuÈrmen heimgesuchten FichtenwaÈldern des KuÈstengebiets der Provinz VaÈsternorrland in der Jahren 1967±1969. Department of Zoology, Royal College of Forestry, Stockholm. Coulianos, C.C., 1989. Nya landskapsfynd av barkstink¯yn (Hem.Het., Aradidae) jaÈmte Aradus truncatus, ny foÈr Sverige (New provincial records of Swedish ¯atbugs and barkbugs (Hem.Het., Aradidae) with Aradus truncatus Fieber, 1861, new to

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