Biodiversity and Conservation 6, 1±18 (1997) Dead trees left in clear-cuts bene®t saproxylic Coleoptera adapted to natural disturbances in boreal forest LAURI KAILAÃ Finnish Museum of Natural History, Zoological Museum, PO Box 17, FIN-00014 University of Helsinki, Finland PETRI MARTIKAINEN and PEKKA PUNTTILA Department of Ecology and Systematics, Division of Population Biology, PO Box 17, FIN-00014 University of Helsinki, Finland Received 27 April 1995; revised and accepted 25 October 1995 Forest management alters the pattern of forest dynamics from that in natural conditions in the boreal region. In order to examine how certain forestry measures matching natural dynamics aect forest insects, we compared assemblages of saproxylic Coleoptera on dead, standing birch trunks left behind in eight clear-cut areas with corresponding assemblages in seven mature forests in southern and eastern Finland. We used trunk-window traps for sampling. Distinct beetle assemblages were asso- ciated with the dierent habitats. Median numbers of species or specimens caught did not dier between closed forests and clear-cuts, but individual beetle species occurred unevenly among the habitats. Several beetle species associated with open forest habitat, e.g. burned forests or storm- damage areas, including species regarded as threatened in Finland, were found almost exclusively, in clear-cuts. Correspondingly, a number of beetle species occurring frequently in closed forests were not found in clear-cuts. We conclude that dead trunks left in the clear-cut areas may host not only generalist saproxylic species but also many beetle species specialized to warm, sun-exposed en- vironments, and such species may not be able to survive in closed forests. Management measures matching suppressed natural disturbances are found useful in preserving diversity in managed forests. Keywords: saproxylic Coleoptera; clear-cut; diversity; conservation; management; decaying birch. Introduction Invertebrate animals associated with the wood-decomposition system form one of the largest groups contributing to the species richness in boreal forests. Such organisms are here called saproxylic, following the de®nition by Speight (1989): `Species of invertebrates that are dependent, during some part of their life cycle, upon the dead or dying wood of moribund or dead trees (standing or fallen), or upon wood-inhabiting fungi, or upon the presence of other saproxylics'. Among the saproxylic organisms, beetles (Coleoptera) form a particularly speciose group: e.g. in Fennoscandia there are about 1300 saproxylic beetle species (unpublished database compiled by J. Stokland). Intensive forest management has reduced the amount of decaying wood in the boreal forests and, as a consequence, large numbers of saproxylic species have declined (e.g. HelioÈvaara and VaÈisaÈnen, 1984; EhnstroÈm and WaldeÂn, 1986; Rassi et al., 1992, VaÈisaÈnen ÃTo whom correspondence should be addressed. 0960-3115 Ó 1997 Chapman & Hall 2 Kaila et al. et al., 1993). In Finland, for instance, 196 saproxylic invertebrates are regarded as threatened (Rassi et al., 1992), and they have become an important target in the con- servation of forest ecosystems. Forest management has not aected all species equally, and certain saproxylic species have even bene®tted from forest management, e.g. several bark beetles breeding in freshly dead trees (Nuorteva, 1968). Most of the declined saproxylic species are associated with the later stages of wood decomposition (EhnstroÈm and Wal- deÂn, 1986). Although the biology of most saproxylic beetles is qualitatively well-known (e.g. Saalas, 1917, 1923; Palm, 1951, 1959), quantitative information on the microhabitat and mac- rohabitat requirements of individual species is still scarce. Our previous work has focused on microhabitat level: eects of tree species, polypore species and environmental variables on beetle assemblages in decaying trunks (Kaila et al., 1994, and unpublished data). It is obvious that more quantitative information is needed about the fauna in dierent kinds of forest habitat to supplement our knowledge on the macrohabitat requirement of dierent species and ecological groups (but see e.g. BistroÈm and VaÈisaÈnen, 1988, VaÈisaÈnen et al., 1993; Kaila et al., 1994; Siitonen, 1994; Thunes, 1994). So far, attention has been focused mainly on old-growth forests. However, natural disturbances, such as forest ®res, storm damages, and ¯oods caused by beaver, create open, sun-exposed areas with a considerable amount of dying and dead wood. In primeval taiga such patches emerge locally quite frequently (SireÂn, 1955; Zackrisson, 1977, Kuu- luvainen, 1994; SyrjaÈnen et al., 1994), and regionally they may always have been present with moderately small interpatch distances, easily covered by the dispersal capacity of saproxylic beetles. These areas house a number of specialized saproxylic species (e.g. Ahnlund and Linde,1992; Esseen et al., 1992; Berg et al., 1994). Such natural disturbances are nowadays eliminated by eective ®re control and by removing dead trees from windfall gaps and ¯ooding sites. Dead trees left in clear-cut areas are at ®rst glance, essentially similar to naturally disturbed areas as microhabitats for saproxylic insects. To assess the signi®cance of such trees for saproxylic beetles we have studied the dierence between beetle-species assem- blages on dead birch trunks located inside mature forest versus trunks left to clear-cut areas after logging. We also discuss the relevance of our results for forestry practice. Material and methods Sampling method and design The sampling was conducted in two parts. First, we compared saproxylic beetle assem- blages on dead birch trunks within closed, mature forests and on open, recently clear-cut areas with some decaying birch trunks left behind, called `comparison sampling' below. Second, we sampled according to a BACI design (before-after-with-control-impact), called `BACI experiment' below (see e.g.Green, 1979). The comparison sampling was performed in two distant localities, in Heinola (61° 10¢ N 26° 08±16¢ E), situated in southern Finland, and in Juva in eastern Finland, (61° 42±45¢ N 28° 000 E). The sampling localities are situated in south-boreal vegetation zone (Ahti et al., 1968). At the time of the sampling, the habit of leaving dead standing trunks in clear-cuts was unfortunately rare, and thus, the number of clear-cut sampling sites is limited in our study. The sampling eort, and the logging years of the clear-cut patches are summarized in Table 1. Saproxylic Coleoptera in clear-cuts 3 Table 1. The number of traps in the sampling sites with logging years of the clear-cut patches. The sampling was performed during the summer of 1993 in all the sites Location Type of site Number of traps Cutting winter Heinola Forest 10 ± Heinola Forest 4 ± Heinola Open 7 1990±91 Heinola Open 2 1991±92 Heinola Open 5 1991±92 Juva Forest 3 ± Juva Forest 3 ± Juva Forest 2 ± Juva Forest 1 ± Juva Forest 2 ± Juva Open 3 1992±93 Juva Open 3 1992±93 Juva Open 1 1989±90 Juva Open 1 1991±92 Juva Open 3 1988±89 In Heinola the sampling sites were mixed forests that originated after the end of slash- and-burn cultivation in the late 19th century. Later, the forests were probably used as forest pastures, and they have been slightly managed. In spite of such management, the amount of decaying wood, especially birch, has been relatively high in the area, and we assume that the availability of this microhabitat has been continuous on a regional scale. This assumption is also supported by the occurrence of the white-backed woodpecker (Dendrocopos lecuotos) in the area. This species is dependent on dead deciduous trees, and it is now near extinction in Finland as a consequence of loss of breeding habitats (Virkkala et al., 1993). The sampling was performed from 6 May to 4 October 1993 in three clear-cut areas in Heinola, 200 m±1 km from each other. In these clear-cuts some dead and dying trees had not been removed from the managed areas. We took control samples from two plots in adjacent forest from a distance of, at most, 300 m from the corresponding managed patch. The age of the forests was 70±80 years, and the tree-species composition was birch 40 %, Scots pine 60 %. We estimated the tree-canopy coverage as ca 85 %. The tree-canopy coverage estimations were made from a sample of sighting straight upwards in 20 spots southward from each trap with 1m intervals. The estimations were always made by the same person. In Juva the sampling was performed from 22 May to 2 October 1993 in ®ve clear-cut patches, in distances of 0.5±8 km from each other, and in ®ve control forests in the vicinity of each clear-cut patch. The age of the forests was 70±80 years, and the tree-species composition varied as follows: birch 7±28 %, Scots pine 0±79 %, Norway spruce 5±93 %. In general, the forests were more conifer-dominated than in Heinola. The tree-canopy cov- erage of the Juva forests was, on average, 70 %. The BACI-experiment sampling included one of the above-mentioned Heinola clear- cuts which was sampled before clear-cutting in 1990 (sampling period from 22 May to 28 September) and again in 1993 (for sampling period, see above). To control variation between dierent years we sampled in a non-managed forest patch in 1990 and in 1993 4 Kaila et al. with the same sampling periods as in the treatment. This sampling plot was situated in Pertunmaa community ca 10 km distant form the Heinola sites. The tree-species com- position was birch 67 %, aspen 26 %, alder and Scots pine together 7 %. The tree-canopy coverage was ca 80 %. The polypore species aects the beetle assemblage caught from the trunk, and the perennial polypore Fomes fomentarius hosts a large variety of beetles (Kaila et al., 1994). In order to minimize variation caused by dierent species of fungi, we limited the study to white rotted birch trunks, predominantly decayed by this very common polypore species.