Assessing the Value of Testate Amoebae and their Functional Traits in Detecting Climate Change-Induced Peatland Drying

Climate change-induced drying is projected for northern peatlands, but evidence on its extent is inconsistent. Testate amoebae (TA) are a water table depth proxy that also responds to vegetation succession and abiotic changes. However, it is not well-known how TA communities and functional traits differ between unaffected and drying areas in different peatland types. Thus, we compared TA communities and functional traits in undrained control areas and areas (initially similar in vegetation and WT) subjected to moderate water level drawdown (WLD) for ∼20 years. The experiment covers a rich fen, a poor fen, and a bog. Arboreal vegetation has been established in the fen WLD areas. Taxonomic composition and functional traits differed between the WLD and control, most notably in the rich fen and the least in the bog, mirroring the vegetation. Eleven taxa favored WLD or pristine conditions; six taxa had a site-specific preference. Small tests and apertures, heterotrophy, and siliceous tests were more common in the WLD than control areas, but in the bog, these differences were not significant. Overall, besides drying, the establishment of arboreal vegetation and changes in abiotic conditions affected TA community and trait compositions in the fens, showing that they deliver information not only about water table depth but also sensitivity/resistance of peatlands. When aiming for quantitative water table reconstructions, account should be taken that the secondary changes amplify the signal of WLD in fens, whereas the resistance of vegetation and testate amoeba communities may hinder it in bogs, possibly causing bias when the reconstruction runs through a fen-bog-transition.