Forest Ecology and Management 149 &2001) 295±304 Saproxylic insect diversity of beech: limbs are richer than trunks Karin Schiegg*,1 Chair of Nature and Landscape Protection, c/o Swiss Federal Institute for Forest, Snow and Landscape Research WSL), Zuercherstrasse 111, CH-8903 Birmensdorf, Switzerland Received 1 April 2000; accepted 14 July 2000 Abstract The ecological value of dead wood &coarse woody debris) is broadly acknowledged, but in most commercial forests in Central Europe dead wood amounts are still low. Dead trees are removed because they are thought to obstruct forest management and to be potential sources of outbreaks of pest species. Consequently, the saproxylic fauna is impoverished in many forests of Central Europe. In places where one cannot agree on leaving entire dead trees, dead limbs could be promoted instead. However, limbs are thought not to be an appropriate habitat for saproxylic insects. I tested the hypothesis that trunks of beech Fagus sylvatica &L.) host more saproxylic Diptera and Coleoptera than fallen dead limbs of beech by comparing observed and estimated species richness and diversity. In both insect groups, limbs hosted more species and had a higher diversity than trunks. More threatened saproxylic coleopteran species were reared from limbs than from trunks. Species overlap between trunks and limbs was 55.3% for Diptera and 82.6% for Coleoptera. Hence, a considerable proportion of saproxylic insect species would bene®t if at least dead limbs were allowed to accumulate in commercial forests. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Coleoptera; Dead wood; Dimension; Diptera; Diversity; Saproxylic 1. Introduction biodiversity and especially the number of saproxylic insect species. Throughout Europe, this group today The importance of dead wood &coarse woody deb- includes a large proportion of threatened species ris) for biodiversity in forest ecosystems has repeat- &Speight, 1989; Mikkola, 1991; Mawdsley and Stork, edly been stated &Elton, 1966; Maser and Trappe, 1995; Geiser, 1998). 1984; Kirby and Drake, 1993; Kaila et al., 1994; In the recent years, an increasing number of forest Jonsell et al., 1998; Bowman et al., 2000). However, reserves has been established, where dead wood is during the past centuries, old and dead trees allowed to accumulate, but amounts in commercial have consequently been removed from most Central forests are still low &e.g. <5 m3 haÀ1 on average in the European forests. This greatly reduced overall lowlands of Germany and Switzerland; Albrecht, 1991; Brassel and BraÈndli, 1999). Even fallen bran- ches and limbs are mostly removed or burnt, at least in Switzerland &Forster et al., 1998). Actions undertaken * Present address: Department of Biology, Virginia Polytechnic to promote dead wood should therefore focus on Institute and State University, Blacksburg VA, 24061-0406, USA. commercial forests. It is often argued, though, that Tel.: 41-1-739-25-66; fax: 41-1-737-40-80. E-mail addresses: [email protected], [email protected] &K. Schiegg). dead trees in commercial forests are dangerous 1 Tel.: 1-540-231-67-20; fax: 1-540-231-93-07. because people might be harmed by limbs breaking 0378-1127/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 378-1127&00)00563-6 296 K. Schiegg / Forest Ecology and Management 149 2001) 295±304 off or by falling trunks. Additionally, dead trees are thought to obstruct forest management and to be potential sources of pest outbreaks. Where one cannot agree on leaving entire dead trees, promoting fallen branches and limbs would be an alternative way to enhance dead wood amounts. This is particularly important, as it has been shown that large distances between dead wood pieces are fatal for certain Fig. 1. A trunk eclector during installation. saproxylic insect species &Schiegg, 2000a). However, dead wood of larger dimensions is usually considered to be more appropriate for saproxylic insects as micro- climate is more stable than in limbs or branches 2.2. Data collection &Boddy, 1983; KolstroÈm and LumatjaÈrvi, 2000). As yet, no study has explicitly focused on the signi®cance Insects were collected by a modi®ed version of of dead wood dimensions for this group, though some eclectors &emergence traps, Funke, 1971; Schmitt, authors have included dead branches or limbs in their 1992; Hilt and Ammer, 1994; Irmler et al., 1996, studies &Larkin and Elbourn, 1964; Schmitt, 1992; Kleinevoss et al., 1996; ékland, 1996; HoÈvemeyer, Rauh, 1993; Hilt and Ammer, 1994; Kleinevoss et al., 1999), which can also be used on fallen dead wood 1996; Haase et al., 1998). I tested the hypothesis that lying on the forest ¯oor &Schiegg et al., 1999). Due to trunks host more saproxylic insect species than limbs the tent-like construction of the eclectors, pieces of by comparing observed and estimated species richness dead wood could be enclosed to rear saproxylic insects and diversity of saproxylic Diptera and Coleoptera &Fig. 1). Four eclectors were installed in each of the 14 breeding in trunks and limbs of beech Fagus sylvatica plots. I de®ned parts of stems of fallen dead beech &L.). Beech is potentially the dominating tree species trees &diameter at the smaller end >20cm; L 1:5m) in the forests of the Swiss lowlands &BraÈndli, 1998). It as `trunks'. Accordingly, large beech limbs without is important to know the number and identity of the side branches &diameter at both ends 5±10cm; species developing in limbs relative to those breeding L 1:0À1:5 m) lying on the forest ¯oor were de®ned in trunks in order to evaluate the signi®cance of limbs as `limbs'. Two traps per plot contained each a part as a habitat for saproxylic insects. of a trunk &trunk eclectors) and another two traps per plot were ®lled with limbs &limb eclectors), giving 28 traps of each type, 56 in total. The limbs were col- 2. Methods lected from the forest ¯oor within 10m around the traps. I measured the length and diameter of each piece 2.1. Study area of dead wood and calculated the surface area and volume enclosed in each trap. As stage of decay The study was carried out in the forest reserve of greatly in¯uences the community composition of Sihlwald &478150;88330) on a NE-facing slope shaped saproxylic insects &Derksen, 1941), only dead beech by several small valleys 10km south of Zurich, Swit- wood at a medium stage of decay was considered. zerland. The entire forest covers 10km 2 and is domi- Medium stage of decay was de®ned according to nated by beech and spruce Picea abies &Karst.), Albrecht &1991): the wood is partly softened and followed by ash Fraxinus excelsior &L.) and ®r Abies shows clear signs of colonization by saproxylic insects alba &Mill.). Dead wood averages 6.3 m3 haÀ1 of and fungi. The bark is partly lose but less than 10% of mainly beech and spruce &own unpublished data). I its surface is covered by moss. The collecting vials selected 14 study plots between 500 and 800 m asl were attached and ®lled with a 2% formaldehyde similar to each other in terms of aspect, stand structure solution at 24 April 1996, when collecting started. and age as well as tree species composition. Plot They were emptied monthly until 25 November 1996. centres were at least 600 m apart to ensure indepen- All specimens of Diptera and Coleoptera were identi- dence of the collections. ®ed by various specialists to species level except K. Schiegg / Forest Ecology and Management 149 2001) 295±304 297 for Psychodidae, Chironomidae, Cecidomyiidae, and for Coleoptera &R2 0:17, n 28, P 0:04) when Phoridae. volume was used as independent variable. Neither for Diptera &R2 0:04, n 28, P 0:31), nor for 2.3. Species classification Coleoptera &R2 0:12, n 28, P 0:09) such a relationship could be established when using surface Only saproxylic species were included in the ana- area as the independent variable. I therefore compared lyses classi®ed according to McAlpine et al. &1981), the assemblages on the basis of equal volumes. As Freude et al. &1964±1983), Koch &1989±1992), Smith dead wood volume in the trunk eclectors &meanÆ &1989), and on information given by the specialists S:D::0:168 Æ 0:061 m3) was on average four times responsible for the species identi®cation. I categorised greater than in the limb eclectors &0:041 Æ 0:007 m3), a beetle species as a potential forest pest, when it was I pooled the data of four randomly selected limb recorded by Schwenke &1982) as having caused con- eclectors and chose one trunk eclector with a volume siderable damage to forest stands. Based on the clas- as close as possible to the summed volume of the four si®cations given by Koch &1989±1992), I separated the limb eclectors. I repeated this procedure seven times beetle species into predatory &i.e. feeding mainly on until all 28 limb eclectors and seven trunk eclectors other living arthropods, see also Hammond, 1997) and were considered, which resulted in a total dead wood non-predatory species, as well as into stenotopic, volume of 1.15 m3 for each trap type as basis for eurytopic and ubiquistic species. Stenotopic species comparisons of species numbers. occur only in particular habitat types, eurytopic spe- cies in several habitats similar to each other, and 2.5. Diversity measurements ubiquistic species are found in various kinds of dif- ferent habitats. A beetle species was regarded as Diversity was measured with the Shannon index threatened, when it was recorded in the Red List of &Lloyd et al., 1968) and Fisher's a &Fisher et al., 1943) Germany &Geiser, 1998).