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  • Väliranta, Minna; Salojärvi, Niina; Vuorsalo, Annina; Juutinen, Sari; Korhola, Atte; Luoto, Miska; Tuittila, Eeva-Stiina (2017)
    Minerotrophic fens and ombrotrophic bogs differ in their nutrient status, hydrology, vegetation and carbon dynamics, and their geographical distribution is linked to various climate parameters. Currently, bogs dominate the northern temperate and southern boreal zones but climate warming may cause a northwards shift in the distribution of the bog zone. To more profoundly understand the sensitivity of peatlands to changes in climate, we first used the plant macrofossil method to identify plant communities that are characteristic of past fen-bog transitions. These transitions were radiocarbon dated, to be linked to Holocene climate phases. Subsequently, palaeoecological data were combined with an extensive vegetation survey dataset collected along the current fen-bog ecotone in Finland where we studied how the distribution of the key plant species identified from peat records is currently related to the most important environmental variables. The fossil plant records revealed clear successional phases: an initial Carex-dominated fen phase, an Eriophorum vaginatum-dominated oligotrophic fen phase followed by an early bog phase with wet bog Sphagna. This was occasionally followed by a dry ombrotrophic bog phase. Timing of initiation and phase transitions, and duration of succession phases varied between three sites studied. However, the final ombrotrophication occurred during 2000-3000 cal. BP corresponding to the neoglacial cooling phase. Dry mid-Holocene seems to have facilitated initiation of Eriophorum fens. The peatlands surveyed in the fen-bog ecotone were classified into succession phases based on the key species distribution. In 33% of the studied peatlands, Sphagnum had taken over and we interpret they are going through a final transition from fen to bog. In addition to autogenic processes and direct climate impact, our results showed that ecosystem shifts are also driven by allogenic disturbances, such as fires, suggesting that climate change can indirectly assist the ombrotrophication process in the southern border of the fen-bog ecotone.
  • Mathijssen, Paul J. H.; Kahkola, Noora; Tuovinen, Juha-Pekka; Lohila, Annalea; Minkkinen, Kari; Laurila, Tuomas; Väliranta, Minna (2017)
    Data on past peatland growth patterns, vegetation development, and carbon (C) dynamics during the various Holocene climate phases may help us to understand possible future climate-peatland feedback mechanisms. In this study, we analyzed and radiocarbon dated several peat cores from Kalevansuo, a drained bog in southern Finland. We investigated peatland succession and C dynamics throughout the Holocene. These data were used to reconstruct the long-term atmospheric radiative forcing, i.e., climate impact of the peatland since initiation. Kalevansuo peat records revealed a general development from fen to bog, typical for the southern boreal zone, but the timing of ombrotrophication varied in different parts of the peatland. Peat accumulation patterns and lateral expansion through paludification were influenced by fires and climate conditions. Long-term C accumulation rates were overall lower than the average values found from literature. We suggest the low accumulation rates are due to repeated burning of the peat surface. Drainage for forestry resulted in a nearly complete replacement of typical bog mosses by forest species within 40 years after drainage. The radiative forcing reconstruction suggested positive values ( warming) for the first similar to 7000 years following initiation. The change from positive to negative forcing was triggered by an expansion of bog vegetation cover and later by drainage. The strong relationship between peatland area and peat type with radiative forcing suggests a possible feedback for future changing climate, as high-latitude peatlands may experience prominent regime shifts, such as fen to bog transitions.
  • Laine, A. M.; Selänpää, T.; Oksanen, J.; Sevakivi, M.; Tuittila, E-S (2018)
    During succession, plant species composition undergoes changes that may have implications for ecosystem functions. Here we aimed to study changes in plant species diversity, functional diversity and functional traits associated with mire development. We sampled vegetation from 22 mires on the eastern shore of the Gulf of Bothnia (west coast of Finland) that together represent seven different time steps along a mire chronosequence resulting from post-glacial rebound. This chronosequence spans a time period of almost 2500 years. Information about 15 traits of vascular plants and 17 traits of mosses was collected, mainly from two different databases. In addition to species richness and Shannon diversity index, we measured functional diversity and community weighted means of functional traits. We found that plant species diversity increased from the early succession stages towards the fen-bog transition. The latter stage also has the most diverse surface structure, consisting of pools and hummocks. Functional diversity increased linearly with species richness, suggesting a lack of functional redundancy during mire succession. On the other hand, Rao's quadratic entropy, another index of functional diversity, remained rather constant throughout the succession. The changes in functional traits indicate a trade-off between acquisitive and conservative strategies. The functional redundancy, i.e. the lack of overlap between similarly functioning species, may indicate that the resistance to environmental disturbances such as drainage or climate change does not change during mire succession. However, the trait trade-off towards conservative strategy, together with the developing microtopography of hummocks and hollows with strongly differing vegetation composition, could increase resistance during mire succession.
  • Piilo, Sanna; Zhang, Hui; Garneau, Michelle; Gallego-Sala, Angela V.; Amesbury, Matthew; Väliranta, Minna (2019)
    Peatland ecosystems are important carbon sinks, but also release carbon back to the atmosphere as carbon dioxide and methane. Peatlands therefore play an essential role in the global carbon cycle. However, the response of high-latitude peatlands to ongoing climate change is still not fully understood. In this study, we used plant macrofossils and peat property analyses as proxies to document changes in vegetation and peat and carbon accumulation after the Little Ice Age. Results from 12 peat monoliths collected in high-boreal and low-subarctic regions in northwestern Quebec, Canada, suggest high carbon accumulation rates for the recent past (post AD 1970s). Successional changes in plant assemblages were asynchronous within the cores in the southernmost region, but more consistent in the northern region. Average apparent recent carbon accumulation rates varied between 50.7 and 149.1 g C m(-2) yr(-1) with the northernmost study region showing higher values. The variation in vegetation records and peat properties found within samples taken from the same sites and amongst cores taken from different regions highlights the need to investigate multiple records from each peatland, but also from different peatlands within one region.