We compared the CO 2 exchange and its controls in the plant communities of a strongly patterned aapa mire, the Kaamanen fen in northern Finland. Based on a systematic vegetation inventory and an ordination analysis, four plant community types were chosen for the study: Ericales-Pleurozium string tops, Betula-Sphagnum string margins, Trichophorum tussock flarks and Carex-Scorpidium wet flarks. We measured plant community CO 2 exchange with a closed chamber technique during the growing season of 2007 and early summer of 2008. Nonlinear regression models were used for simulating the CO 2 exchange over the measurement period for different mire components and for the whole mire. The CO 2 exchange dynamics distinguished two functional components in the mire: an ombrotrophic component (Ericales-Pleurozium string tops) and a minerotrophic component (other plant community types). Minerotrophic plant communities responded similarly to environmental controls, the most important of these being variation in leaf area and aerobic peat volume (water level). The ombrotrophic component dynamics were more obscure; frost and possibly peat moisture played a role. The minerotrophic communities functioned as effective CO 2 sinks in the simulation, while the net CO 2 exchange of the ombrotrophic community was close to zero. The smaller NEE of the ombrotrophic community was due to less efficient photosynthesis per unit leaf area in combination with high ecosystem respiration resulting from a large aerobic peat volume. Our study shows that a fen/bog functional dichotomy can also exist within one mire. Wet minerotrophic communities within northern mires can act as effective CO 2 sinks.

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In restored peatlands, recovery of carbon assimilation by peat-forming plants is a prerequisite for the recovery of ecosystem functioning. Restoration by rewetting may affect moss photosynthesis and respiration directly and/or through species successional turnover. To quantify the importance of the direct effects and the effects mediated by species change in boreal spruce swamp forests, we used a dual approach: (i) we measured successional changes in moss communities at 36 sites (nine undrained, nine drained, 18 rewetted) and (ii) photosynthetic properties of the dominant Sphagnum and feather mosses at nine of these sites (three undrained, three drained, three rewetted). Drainage and rewetting affected moss carbon assimilation mainly through species successional turnover. The species differed along a light-adaptation gradient, which separated shade-adapted feather mosses from Sphagnum mosses and Sphagnum girgensohnii from other Sphagna, and a productivity and moisture gradient, which separated Sphagnum riparium and Sphagnum girgensohnii from the less productive S. angustifolium, S. magellanicum and S. russowii. Undrained and drained sites harbored conservative, low-production species: hummock-Sphagna and feather mosses, respectively. Ditch creation and rewetting produced niches for species with opportunistic strategies and high carbon assimilation. The direct effects also caused higher photosynthetic productivity in ditches and in rewetted sites than in undrained and drained main sites.

1. Peatland rewetting aims to restore biomass accumulation from peat-forming plants for climate change mitigation, biotope conservation and water purification purposes. Boreal spruce swamp forests in Europe have suffered heavily from drainage for forestry and are now a focus of restoration efforts. 2. We measured Sphagnum height and biomass increment by allowing Sphagnum to grow through mesh nets located in nine undrained, nine drained and 18 rewetted boreal spruce swamp forests. 3. At the moss patch level, rewetting led to a recovery of Sphagnum growth: height increment and biomass increment at the rewetted sites (5Á9 AE 0Á7 cm year À1 and 147 AE 15 g m À2 year À1 , mean AE SE) were similar to increment at the undrained sites (4Á9 AE 0Á5 cm year À1 and 128 AE 12 g m À2 year À1 ), while remnant patches of Sphagnum at the drained sites showed smaller increment (2Á8 AE 0Á8 cm year À1 and 76 AE 19 g m À2 year À1 ). Sphagnum in the ditches at the drained sites showed similar increment as the moss at the undrained and rewetted sites, while ditches at the rewetted sites had the greatest increment (8Á7 AE 0Á7 cm year À1 and 183 AE 16 g m À2 year À1 ). A higher water-table increased Sphagnum growth, and Sphagnum riparium grew more rapidly than the other species. 4. At the ecosystem level, where information on moss cover was utilized, rewetting had increased Sphagnum production to values close to the undrained sites. At the drained sites, biomass production was 8 AE 17 g (mean AE SE) m À2 year À1 , at the rewetted sites 42 AE 15 g m À2 year À1 and at the undrained sites 66 AE 12 g m À2 year À1 . Sphagnum cover was the most important variable that determined Sphagnum production in the ecosystem. 5. Synthesis and applications. Drained spruce swamp forests appear to be a suitable target for restoration as they respond readily to rather inexpensive rewetting. The result points out that even in boreal conditions, restoration results can be achieved in a time-scale of years rather than decades. At the ecosystem level, rewetting had resulted in a biomass gain of 34 g m À2 year À1 when compared to drained sites.

To be presented, with the permission of the Faculty of Agriculture and Forestry of the University of Helsinki, for public criticism in lecture room B2, B-building (Viikki Campus, Latokartanonkaari 7, Helsinki) on May 8 th 2015, at 12 o'clock noon. Drainage to increase timber production has drastically decreased the area of undrained spruce swamp forests in northern Europe. In restoration by rewetting, drainage ditches are blocked to restore the original hydrology and, ultimately, the structure, function and ecosystem services of undrained boreal spruce swamp forests. This study quantifies the restoration success of rewetting regarding plant community composition, moss community carbon assimilation potential, Sphagnum biomass production and surface peat biogeochemistry, and aims to determine the main controls of success. The study sites comprised 18 rewetted, nine undrained and nine drained spruce swamp forests in southern Finland, complemented by sites in the Šumava Mountains, Czech Republic. Drainage had taken place decades prior; the rewetted sites varied in their rewetting age from 1 to 15 years. The results show that rewetting has to raise the water table above a threshold to initiate any changes in the drained ecosystem. If the threshold is crossed, the changes that occur will be rapid. Two strands of development emerged throughout the different components of the ecosystem: development towards the undrained reference state and development towards a new direction, different from both the undrained and the drained state. Rewetting created favourable conditions for Sphagnum photosynthesis. Sphagnum mosses recovered in cover and biomass production rapidly. The new growth started the accumulation of the porous surface organic matter layer characteristic of mires, which increased microbial decomposition activity in the surface organic layer towards undrained levels. Meanwhile, rewetting applied on the compacted, physicochemically altered peat created wet, unstable hydrological conditions, which increased the cover of opportunistic plant species in the understory and caused high NH4 mobilization and CH4 production in the surface organic layer. Demanding spruce swamp forest species were lacking at the rewetted sites, but rewetting was successful in restoring the common species and directing the ecosystem towards mire-like functioning.Keywords: peatland forest, vegetation, Sphagnum, moss photosynthesis, biomass production, biogeochemistry 4 ACKNOWLEDGEMENTSThis thesis is a result of the efforts of the many people who gave their time and expertise to the research project. Thank you for accompanying me on this venture. First, I want to express my gratitude to my supervisor Eeva-Stiina Tuittila. Thank you for your insightful thinking and persistent support. To my co-authors, thank you for your contribution. It has been a privilege to work with so many intelligent, creative and generous people. Your knowledge, skills and enthusiasm, be it in peatland restoration, plant ecophysiology, statistics or soil biogeochemistry, ...