Miljøhensyn i skog. Relativ betydning av naturreservater, nøkkelbiotoper, livsløpstrær og kantsoner

Abstract

Rapporten presenterer resultater fra et samarbeidsprosjekt mellom NINA og NORSKOG. Vi har sammenliknet samme type død ved i de tre forvaltningskategoriene naturreservat, nøkkel-biotop og slutthogsthensyn (kantsoner og gjensatte grupper av livsløpstrær), og sett på artsan-tall og -sammensetning av både biller i nydød osp og sopp på granlæger. Undersøkelsen er foretatt i fire ulike landskaper i Sørøst-Norge, nær Oslo, i sør- til mellomboreal sone. Gran er dominerende treslag, og alle stokkene har vært valgt ut fra blåbær- eller småbregneskog. For insektsundersøkelsen, som ble utført i tre av landskapene, ble det felt ferske ospetrær, kappet i lengder på 1 meter. De ble plassert ut stående og parvis med 1,5 meters mellomrom, ett par per prøveflate. Alle tre arealavsetningskategorier ble studert i tre landskap, med 24 stokkpar i hvert av dem, 72 stokkpar totalt. En vindusfelle ble festet til hver stokk. Med to stok-ker var det mulig å sammenlikne billemangfoldet som ble tiltrukket av stokkene med det som faktisk ble produsert i stokkene. For å få til dette ble en av stokkene tildekket med et nett an-nethvert år, etter at begge var åpne for kolonisering i år 1. Vindusfellen hang inne i nettet og fanget insekter som klekket fra stammen. Av vedlevende billearter som inngår i analysen her, ble det registrert 345 arter med til sammen 13208 individer, av 573 arter fanget totalt. Mange arter ble funnet i lave antall; 60 vedlevende arter (17 % av alle vedlevende arter) kun med ett individ. Den vanligste arten, kortvingen Hap-loglossa villosula, forekom med 775 individer (6 % av alle vedlevende individer). 154 av de vedlevende artene er tilknyttet osp i større eller mindre grad, mens 11 har osp som eneste eller foretrukne treslag. Til sammen 21 arter er truet eller nær truet. Både vanlige og rødlistede biller som er tilpasset å leve i nydød osp, syntes langt på vei å kun-ne utnytte slikt substrat uavhengig av om det var lokalisert i et naturreservat, en nøkkelbiotop eller i et slutthogsthensyn. Biller som er spesialisert til å leve i nydød osp, ble fanget i lik arts-rikdom i våre eksperimentstokker i alle tre kategoriene, mens de øvrige ospeassosierte billear-tene hadde høyest artsrikdom i tilknytning til stokker som stod i nøkkelbiotopene. Slutthogst-hensyn-stokkene tiltrakk seg det høyeste artsmangfold av vedlevende biller totalt, inkludert mange arter som ikke kan leve i våre eksperimentstokker. Dette indikerer at forekomst av til-strekkelige mengder egnet dødved-substrat er viktigere enn hvor i landskapet det forekommer, for denne gruppen av biller og i våre studielandskap. Vedboende sopp ble registrert på 390 granstokker, fem for hvert par av ospestokker. For to av landskapene ble alle de tre kategoriene undersøkt, mens for de to øvrige landskapene ble stu-dert bare naturreservat og nøkkelbiotop. Til sammen ble det registrert 205 arter av vedboende sopp, fordelt på 30 poresopper, 169 barksopper og 8 corticoide heterobasidiomyceter. Hele 69 arter ble bare funnet én gang, mens fem arter hadde over hundre stokkforekomster. Totalt ble det observert 23 rødlistearter. Disse utgjorde 161 (5,9 %) av totalt 2745 stokkforekomster. I tillegg ble det undersøkt et antall granlæger i granplantasjer med ungskog, samt foretatt noen mer kvalitative studier i mer påvirket gammelskog. Svært få av disse stokkene hadde rødliste-artsforekomster. Norge, Vestskogen, Østmarka, Nordmarka, Hadelandsåsen, vedboende sopp, biller, skogbehandling, gammelskog, fragmen-tering, rødliste, vedlevende, naturreservat, kantsone, slutt-hogsthensyn, evighetstrær, nøkkelbiotop, MiS (miljøregistrering i skog), naturtypelokalitet, Norway, wood-inhabiting fungi, beetles, forest management, old forest, old-growth, fragmentation, red list, saproxylic, nature re-serve, margin zone, retention patch, woodland key habitat, for-est environmental inventories, nature type locality

The report presents the results of a project in cooperation between the Norwegian Institute for Nature Research (NINA) and The Norwegian Forest Owners´ Association (NORSKOG). We have compared the same type of dead wood in the three conservation categories of nature reserve, woodland key habitat and retention patch (riparian margin zones and retention trees) and studied species number and species composition of both beetles in newly dead aspen logs and fungi on spruce logs. The study has taken place in four different landscapes in South East Norway, close to Oslo in the south and middle boreal zones. Spruce is the dominating tree species, and all of the logs have been selected from bilberry or small fern forest. For the study of early-successional beetles associated with aspen, fresh aspen trees were cut into units of one meter in length. The wood units were transported out to the study sites and placed upright, spaced about one meter apart, one pair for each study plot. All the three categories of area-based conservation measures were studied in three landscapes, each of them with 24 pairs of logs, 72 all together. A flight interception trap was mounted on each wood unit. Using two logs for each plot it was possible to compare the diversity of beetles that were attracted by the logs with what was in fact produced in the logs. To manage this, one of the logs was covered by a net every second year after that both of them were open for colonization in the first year. The flight interception trap was hanging inside the net and caught insects which hatched from the log. Of wood living beetles included in this analysis, 345 species were recorded with a total of 13208 individuals, of 573 species sampled totally. Many species were sampled in low numbers; 60 wood-inhabiting species (17 % of all the wood-inhabiting species totally) were recorded with only one individual. The commonest species, the rove beetle Haploglossa villosula, occurred with 775 individuals (6 % of all wood-inhabiting individuals). 154 of the wood-inhabiting species are associated with aspen to some degree, whereas 11 have aspen as the only or preferred tree species. Altogether, 21 species are threatened or near threathened. Both common and red-listed beetles which are adapted to live in newly dead aspen seemed to a large degree to be able to use such a substrate independently of whether it was localized in a nature reserve, a woodland key habitat or in a retention patch. Beetles specialized to live in newly dead aspen were sampled in about the same number in our experimental logs in all of the three categories, whereas the other aspen-associated beetle species had their highest species number for logs standing in the woodland key habitats. The retention patch logs attracted the highest diversity of wood-inhabiting beetles totally, including many species which cannot live in our experimental logs. This indicates that the occurrence of sufficient amounts of suitable substrate of dead wood is more important than where in the landscape it occurs for this group of beetles in our studied landscapes. Wood-inhabiting fungi were recorded on 390 spruce logs, five for each pair of aspen logs. For two of the landscapes all of the three categories were studied, whereas for the two other landscapes only nature reserve and woodland key habitat were studied. Altogether, 205 species of wood-inhabiting fungi were recorded; 30 polypores, 169 corticiaceous fungi and 8 corticioid heterobasidiomycetes. As many as 69 species were only found once, wheras five species were found on more than 100 logs. Totally, 23 red-listed species were found. They constituted 162 (5.9 %) of a total of 2745 log occurrences. A number of spruce logs were studied from spruce plantations in young forests and some qualitative studies were performed in old forests more influenced by forestry. Very few of these logs had occurrences of red-listed species. In contrast to beetles wood-inhabiting fungi on spruce logs exhibited a distinct contrast in total species number and number of red-listed species between nature reserves and woodland key habitats on the one side and retention patches on the other side. The latter category is most often in the form of margin zones along mires. However, also for fungi, there is some variation: In cases when the margin zones were broad and consisted of forest with the character of nature forest, also this type seemed to function as a habitat for numerous species, included some red listed species. We also found a modest difference between the different conservation categories. However, both beetles and fungi showed greater difference between the different landscapes. The utility effect of setting aside an area, either as nature reserve, woodland key habitat, or retention patch, is high and complimentary for all of the three categories, evaluated from the criteria we have measured in the project. Given that the economic cost of setting aside trees is the same for all categories, the economic loss also becomes the same. But this prerequisite is not always true, and in reality, separate trees which are set aside, probably cost more per m3 than the same amount of trees in forest reserves. Anyway, small scale considerations may to a larger degree become localized where they have the least cost, so that this effect can be reduced. The results also indicate that there is some degree of freedom of choice as to how area should be set aside, even though the ecological results also stress the significance of larger areas rich in dead wood in closed forest for wood-inhabiting fungi. The study further stresses that the woodland key biotopes have an important function to conserve red-list species. This does not only refer to signal species which in many cases have been recorded beforehand, but also to other red-listed species which cannot or only with difficulty can be identified macroscopically, but which were recorded in this study. Possible dispersal limitations of fungi and possible roles of woodland key habitats and retention patches in a dispersal network are discussed. Our results also show the significance of the width of the margin zone. Therefore, it will be favourable both with respect to biology and economy that the margin zones have sufficient width, so that they can house sufficient amounts of the right elements of dead wood in a short and long term perspective, and that they despite of small size may retain some kind of old forest environment on a small scale. From the results and current knowledge about the biology of wood-inhabiting fungi and beetles, it is reason to believe that the three categories of nature reserves, woodland key habitats and margin zone complement each other in a positive way, especially with respect to the need both for large and robust conservation areas and the occurrence of a network, which ensure the possibility for rapid dispersal and establishment. There is therefore every reason to continue current policies with an emphasis on all three categories of conservation measures.