Abstract Disturbance may shift microbial communities from one state to another. However, species differ in their ecological characteristics and their abilities to withstand disturbance. No single species or individuals of a species exist alone, but they are parts of complex interaction networks including species above- and belowground. In boreal and subarctic forests, almost all plants and a high number of fungi form mycorrhizas at the plant roots. In mycorrhiza, the fungal partner harvests nutrients for the host plant and, in return, gains carbon from the plant. In general, these common associations benefit both partners, but as heterotrophs, fungi are dependent on carbon photosynthesized by plants, whereas plants can survive alone as autotrophs. In addition to mycorrhizal fungi, also other fungi, such as endophytes, saprotrophs and pathogens, live in and on plant roots. This thesis concerns the impacts of disturbance on fungi living in plant roots and in soil near the roots. I hypothesized that i) root-associated fungal (RAF) and soil fungal communities and colonization types change after disturbance, that ii) the observed shifts relate to disturbance intensity and that iii) they co-occur with changes in soil conditions and vegetation. Changes in RAF were studied as changes in root fungal colonization, or in fungal community composition. The latter were detected with next-generation sequencing methods. The responses of RAF to disturbance seemed to be context dependent and related to sources of fungal communities (e.g. soil, RAF networks), environmental conditions (e.g. soil pH and nutrients) and host performance. It seems that abundances of those RAF species, which are present in the roots first (priority effect), may be increased by disturbance. Research produced new information related to ecological roles of the genera Phialocephala and Meliniomyces. Altogether, the results indicate connections between both abiotic and biotic environments and RAF, and host species viability and RAF. / Tiivistelmä Häiriöt voivat siirtää eliöyhteisön tilasta toiseen. Lajien ominaisuudet ja häiriönsietokyvyt eroavat toisistaan. Mikään laji tai yksilö ei elä yksin, vaan lajit ovat osa maan ylä- ja alapuolelle ulottuvia monimutkaisia vuorovaikutusverkostoja. Boreaalisissa ja subarktisissa metsissä lähes kaikki kasvit ja useat sienet muodostavat sienijuuren eli mykorritsan. Mykorritsassa sieniosakas hankkii isäntäkasville ravinteita ja saa vastavuoroisesti kasvilta hiiltä. Tavallisesti nämä vuorovaikutussuhteet hyödyttävät molempia sienijuuren osakkaita, mutta toisenvaraisina (heterotrofeina) sienet ovat riippuvaisia kasvien yhteyttämästä hiilestä, kun taas tuottajina (autotrofeina) kasvit voivat elää itsenäisesti. Mykorritsasienten lisäksi kasvien juurissa elää yleisesti myös muita sieniä kuten endofyyttejä, saprotrofeja ja patogeeneja. Tämä väitöskirja käsittelee häiriön vaikutuksia sieniin, jotka elävät kasvien juurissa ja juuria ympäröivässä maassa. Hypoteesieni mukaan i) juurissa ja maassa elävien sienten yhteisöt ja kolonisaatiotyypit muuttuvat häiriön jälkeen, ii) muutokset liittyvät häiriön voimakkuuteen ja iii) muutokset tapahtuvat samanaikaisesti maan olosuhteiden ja kasvillisuuden muutoksien kanssa. Juurissa elävien sienten esiintymisen muutokset tutkittiin sienten kolonisaation tai yhteisörakenteen muutoksina. Sieniyhteisöt selvitettiin NGS-menetelmien avulla. Juurissa elävien sienten vasteet häiriöön vaikuttavat olevan tilannesidonnaisia ja liittyvän sienilajien lähteisiin (esim. maa, juurisieniverkostot), ympäristömuuttujiin (esim. maan pH, ravinteet) ja isäntäkasvin menestymiseen. Häiriö voi vahvistaa juurissa ensimmäisenä läsnä olevien sienilajien menestymistä (prioriteettivaikutus). Uutta tietoa tuotettiin Phialocephala ja Meliniomyces –sienisukujen ekologiasta, jota tunnetaan huonosti. Kaiken kaikkiaan, tulokset osoittavat yhteydet sekä elottoman ja elollisen ympäristön ja juurten sieniyhteisön että isäntäkasvin elinkyvyn ja juurten sieniyhteisön välillä.

In the work described in this thesis I studied how increases in nitrogen (N) inputs may affect plant community structure in boreal forest understorey vegetation. These phenomena were investigated in N fertilization experiments and along a national N deposition gradient. After five years of N additions, large changes in understorey vegetation composition were observed in the fertilization study. In plots that received 50 kg N ha'1 year"1 (N2), the abundance of the dominant species, Vaccinium myrtillus, decreased on average by 32 %. No decrease was observed in control plots during the same period. In contrast, the grass Deschampsia flexuosa responded positively to increased N input, being on average more than five times as abundant in the N2 treatments as in controls. Also an increase was seen in the incidence of disease caused by the parasitic fungus Valdensia heterodoxa on leaves of V. myrtillus following N additions. The parasite was on average nearly twice as abundant in N2 plots than in control plots. This could be explained by increased N concentrations in host plant tissue. Disease incidence also increased following experimental additions of glutamine to leaf surfaces of V. myrtillus, suggesting a causal connection between plant N concentration and performance of the fungus. The parasite also played a key role in the observed changes in understorey species composition. D. flexuosa was more abundant in patches in which V. myrtillus was severely affected by V heterodoxa. This suggests that V heterodoxa mediates the increased abundance of D. flexuosa following increased N additions. The fungus mediates changes in the composition of understorey vegetation mainly by increasing light availability via premature leaf loss of V. myrtillus. The incidence of disease due to the parasite was on average higher in large than in smaller N-treated plots, indicating that the response to N fertilization is spatially scale dependent. This shows that using small plot sizes in experiments that simulate changed environmental conditions may be problematic, as important interactions may be underestimated. Comparison of the occurrence of understorey species between regions with different rates of N deposition revealed that the occurrence of the two dwarf shrubs V. myrtillus and V. vitis- idaea was lower in regions with high N deposition compared to regions with low deposition. The opposite pattern was found for V heterodoxa. This is consistent with expectations from N fertilization experiments. For D. flexuosa no differences in occurrence were found between the different regions investigated. The effects on vegetation and mycorrhizal fungi observed following N additions were also found to be long lasting. Nine years after termination of the fertilization, no signs of recovery were detected, and nearly 50 years after termination characteristic signs of N fertilization were found among bryophytes and mycorrhizal fungi. This suggests that the time needed for re-establishment of the original biota following N-induced changes may be substantial. / <p>Diss. (sammanfattning) Umeå : Umeå universitet, 2002, härtill 5 uppsatser</p> / digitalisering@umu