Effects of habitat quality, quantity, and spatiotemporal isolation on saproxylic organisms in boreal forests

Dead wood ecology emerged at the end of the last century, when it was discovered that the reduction and fragmentation of old-growth forests led to loss of species. Managed forests were not able to maintain a species richness similar to that of natural forests. Many species in old-growth forests depend on rotting wood. The loss of dead wood is one of the causes that led to the endangerment of forest species. A theoretical framework applicable to populations inhabiting fragmented landscapes, the metapopulation theory, has been utilized in the ecological research of dead wood dependent species. According to metapopulation theory extinction of local populations of species is related to the size of the habitat patches and colonization of empty habitat patches depend on the distance to and size of inhabited patches. In an equilibrium state, when colonization and extinction rates are equal, only part of the habitat patches suitable for the species are inhabited. As the fragmentation of the landscape continues, the proportion of inhabited patches decreases. When the amount of suitable habitat falls below a certain threshold value, the metapopulation of the species goes extinct, because the colonization of new empty habitat patches is not enough to compensate for the loss of local populations. Dead wood is a temporary habitat that, when it decays, becomes unsuitable for the species, causing deterministic local extinction. The survival of the species requires that the species is able to colonize new dead trees at least at the same rate as the old dead wood pieces become unsuitable for the species. There are many species of beetles and fungi that depend on rotting wood at some point in their life cycle. Each species of saproxylic, i.e., dead wood dependent species, is specialized to varying degrees for certain dead wood types and stages of wood decay. The diversity of the beetle species is high in boreal forest conifers in the early stages of decay. This dissertation focuses on the species of beetles and fungi living on dead Norway spruces and Scots pines based on data collected in Pirkanmaa and Kainuu regions in Finland, and in Russian Karelia. In my research, I study how the quality of dead wood, its quantity and landscape-level prevalence affect the presence of pre-selected species. In Chapter I, the subject of the study is a rare beetle, Agathidium pulchellum, which had previously only been found in a few places in old-growth forests. In our research, we show that the beetle reproduces and its larvae feed on the spores of a specific slime mould species, Trichia decipiens. The abundance of T. decipiens depended on the abundance of spruce and aspen trees in the study plots, also occurring in rare numbers in commercial forests. Agathidium pulchellum only occurred in natural forests where the host slime mould grew in sufficient density. In Chapters II, III and IV, I study specialist species that are very specific in terms of their habitat requirements. These species need certain types of host trees for their reproduction. As a control, I use general species that reproduce on many kinds of decaying trees. In Chapter II, I study whether certain specialist saproxylic beetles and wood-inhabiting fungi are missing from spruce swamp forest patches that have been isolated for a long time. With the help of a model based on metapopulation theory, I calculated an isolation index to describe the degree of isolation of the spruce swamp forest patches and the time duration of isolation. As controls, I used two common beetle species and two common wood-inhabiting fungi species. The results obtained were contradictory. The incidence of five out of the eight specialist species decreased with isolation. Contrary to our hypothesis, some very common generalist species also appeared to be affected by isolation. The effects of isolation and fragmentation are indeed difficult to demonstrate, as the most sensitive species may have already disappeared from many habitat patches. The results emphasize the importance of using generalist species as qualitative control species. Recently dead standing pines occur not only in natural forests but also in commercial forests. The species of beetles that live on them is the subject of research in chapters III and IV. In Chapter III, I find out whether certain specialist species are more sensitive to changes in stand-level density of standing pines than generalist species. The study includes 8 specialist and 8 generalist species. Four of the six specialist species were dependent on host tree density, while six of the generalist species did not respond to host tree density. The two specialist species were too rare for statistical comparison. In Chapter IV, I investigate whether species traits modulate how they react to changes in the quality and availability of host trees and what significance the quality, density and landscape frame of host trees have on the composition of the species community. The research utilizes modeling based on joint distributions, in which the occurrence of several species in the simultaneous change of several habitat variables is modeled at the same time. The composition of the species community was mainly controlled by the changes in the quality of the wood, especially the number of galleries hollowed out by Tomicus-bark beetles, but the time since the death of the tree and to a lesser extent the diameter of the tree were also important. The results show that in forests where there were more standing dead pines regionally, there were more specialist species. The results of this dissertation show that the qualitative characteristics of dead wood, such as the presence of certain fungi, are significant factors that control the composition of the species community. Species with specialized habitat requirements are particularly sensitive to changes in habitat availability. These do not occur even on suitable host trees, if the local and regional density of the host trees has fallen too low. These can also disappear from natural forests in a landscape where the availability of suitable host trees falls below the extinction threshold. Maintaining the processes that produce decaying trees is important in commercial forests in order to safeguard the diversity of the forest species.Lahopuun väheneminen metsissä on yksi metsälajien uhanalaistumiseen johtaneista syistä. Monet kovakuoriaiset ja sienet ovat jossain elinkiertonsa vaiheessa riippuvaisia lahopuusta. Tämä väitöskirjatutkimus kohdistuu kuolleilla kuusilla ja männyillä elävään kovakuoriais- ja sienilajistoon. Selvitän tutkimuksissani, miten lahopuun laatu, sen määrä ja maisematasoinen esiintyvyys vaikuttaa ennalta valittujen lajien esiintymiseen. Osoitan, että harvinainen kovakuoriainen, korukeräpallokas, lisääntyy ja sen toukat ruokailevat tietyn limasienen, nuijanuorasen itiöemillä. Nuijanuorasen runsaus riippui kuusi- ja haapamaapuiden määrästä esiintyen myös talousmetsissä harvalukuisena. Korukeräpallokasta oli vain luonnontilaisissa runsaslahopuustoisissa metsissä, joissa isäntäsientä kasvoi riittävästi. Tutkimme puuttuvatko vaateliaat kovakuoriaiset ja kääväkkäät pitkään eristyksissä olleista korpilaikuista. Verrokkina käytin kahta yleistä kovakuoriaislajia ja kahta yleistä kääväkäslajia. Saadut tulokset olivat ristiriitaisia, sillä myös kahden verrokkilajin esiintyvyys laski eristyneisyyden kasvaessa. Eristyneisyyden ja pirstoutumisen vaikutuksia on vaikea osoittaa, sillä herkimmät lajit ovat voineet hävitä valtaosalta elinympäristölaikuista. Tutkimme miten sopivien isäntäpuiden, eli pystyyn kuolleiden mäntyjen, tiheyden vaihtelu vaikutti elinympäristövaatimuksiltaan toisistaan poikkeaviin kovakuoriaislajeihin. Lajit, jotka ovat erikoistuneet vain tiettyihin lahopuutyyppeihin, olivat riippuvaisia isäntäpuiden tiheydestä. Sen sijaan yleislajit, jotka lisääntyvät myös hakkuukannoissa ja -tähteissä, eivät reagoineet isäntäpuiden tiheyteen. Pystyynkuolleilla männyillä elävän kovakuoriaislajiyhteisön koostumusta ohjasivat puun laadun muutokset, etenkin nävertäjäkuoriaisten kovertamien syömäkäytävien määrä. Merkitystä oli myös puun kuolemasta kuluneella ajalla sekä vähäisemmin puun läpimitalla. Metsissä, joiden ympäristössä oli enemmän pystyynkuolleita mäntyjä alueellisesti, esiintyi runsaammin vaateliaita lajeja. Väitöskirjani tulokset osoittavat, että lahopuiden laatu ohjaa merkittävästi sillä elävän lajiyhteisön koostumusta. Erityisen herkkiä ovat elinympäristövaatimuksiltaan erikoistuneet lajit. Nämä eivät esiinny sopivillakaan isäntäpuilla, jos isäntäpuiden paikallinen ja alueellinen tiheys on laskenut liian alhaiseksi. Tällaiset lajit voivat hävitä myös luonnonmetsiköistä maisemassa, jossa soveliaiden isäntäpuiden saatavuus alittaa sukupuuton kynnysarvon. Lahopuita tuottavien prosessien ylläpitäminen on tärkeää talousmetsissä metsälajiston monimuotoisuuden turvaamiseksi