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Peatlands are the most space-efficient terrestrial carbon sink on earth, storing more carbon than all other vegetation types in the world combined. The amount of carbon input into peatlands is determined by the primary production and decomposition of plants. The fragile relationship between these two processes is massively disturbed by intensive land use and the associated drainage of large peatland areas, releasing as much carbon dioxide annually as global air travel. Aiming for the substantial reduction of greenhouse gas emissions, rewetting measures have been initiated worldwide to protect and sustainably manage peatlands by restoring the waterlogged conditions required for peat formation. However, the increase in droughts across Europe adds another threat for peatlands by lowering water tables and affecting plant productivity, litter decomposition and phenology, which can reduce their potential for carbon storage.
Fens are minerotrophic peatlands that make up over a third of the peatland area in Europe. The growth and turnover of root biomass is particularly important for the formation and degradation of peat in fens; thus, a special focus should lie on root dynamics research. However, despite their pivotal role for peat formation, we still lack knowledge about root responses to environmental changes caused by rewetting or drought in fens. This thesis aims to advance our knowledge about root processes as well as their abiotic drivers in drained and rewetted fen peatlands of NE Germany, and how they may be affected by an extreme drought. For this purpose, destructive (i.e. in-growth cores, litter bags, soil coring) along with non-destructive measurements (i.e. minirhizotrons, NDVI) were used in situ in forested (alder forests) and graminoid-dominated (sedges and grasses) plant communities representative of the prevailing fen peatlands of Central Europe.
In this thesis, I investigate the environmental drivers of root growth (Chapters I-III), the annual production and decomposition (Chapter II), phenology and temporal dynamics of root growth (Chapters I and III), and the response of root biomass distribution and their functional traits to environmental changes linked to rewetting (Chapter IV). To understand the fundamental differences in productivity of plant communities on mineral and organic soils, above-and belowground phenology and their environmental drivers were compared among different temperate ecosystems (i.e. a beech forest, a forested peatland and two graminoid-dominated fen peatlands) in Central Europe (Chapter I). The study provides evidence that generalizations of aboveground to belowground production are not likely to reflect seasonal dynamics in temperate fen peatlands. Furthermore, the study shows that fine root production can be up to 10 times higher for peatland plant communities than for a beech forest on mineral soil, highlighting the importance of roots for contributing substantially to the formation of organic soils. By comparing annual productivity and decomposition between drained and rewetted fens, it is shown that rewetted fens maintained their productivity under the drought conditions experienced in Central Europe in the year 2018, leading to a higher carbon storage potential despite similar decomposition rates (Chapter II). A deeper understanding on the drivers of this high productivity in the rewetted sites is provided by the analysis of temporal dynamics of root growth and their potential abiotic drivers (Chapter III). Here, the important role of root phenology in the maintenance of productivity of rewetted fens under drought conditions is revealed, since higher root productivity in response to rewetting was driven by an extension of the growing season rather than through a higher growth rate (Chapter III). This thesis shows that rewetting can be beneficial for plant production under drought conditions, which is central to the maintenance of the carbon sink function of peatlands (Chapters II and III). Rewetting maintained high water tables, favouring a plant community adapted to water saturation and also to fluctuating environmental conditions, and thus a community able to cope with periodic water table drawdowns that might increase in the future. Contrarily, drainage caused water tables to constantly drop below rooting depth of plants that might be adapted to drier conditions, but not drought. To gain a deeper understanding of the changes that roots undergo with rewetting and their potential effects on soil carbon storage, a fourth study focuses on the changes in biomass distribution and functional traits of roots along the soil profile (Chapter IV). Together with root age determination the study indicates higher rates of carbon turnover in shallow soil layers and higher belowground carbon investments with rewetting compared to drainage in a forested peatland.
This thesis demonstrates that generalizations of phenological events from plant communities of mineral to organic soils, even though they face the same macroclimatic conditions, are misleading, as they are not subject of the same environmental controls (Chapter I). Rewetting of forest and graminoid-dominated fen peatlands supports their function as carbon sink by enhancing renewed carbon sequestration in form of root biomass (Chapters II-IV). Knowledge about root phenology is crucial to understand plant productivity of peatlands, one of the main drivers of organic matter accumulation (Chapter III). Even though roots are pivotal for mediating the input of carbon into the soil, their dynamics remain one of the least understood aspects of plant function. This thesis contributes to fill this knowledge gap by shedding light on root processes that contribute to the formation of peat and the complexity of the underlying abiotic drivers in rewetted and drained fens in face of a warmer and drier climate.