Consistent centennial‐scale change in European sub‐Arctic peatland vegetation toward Sphagnum dominance—Implications for carbon sink capacity

Abstract

Climate warming is leading to permafrost thaw in northern peatlands, and current predictions suggest that thawing will drive greater surface wetness and an increase in methane emissions. Hydrology largely drives peatland vegetation composition, which is a key element in peatland functioning and thus in carbon dynamics. These processes are expected to change. Peatland carbon accumulation is determined by the balance between plant production and peat decomposition. But both processes are expected to accelerate in northern peatlands due to warming, leading to uncertainty in future peatland carbon budgets. Here, we compile a dataset of vegetation changes and apparent carbon accumulation data reconstructed from 33 peat cores collected from 16 sub‐arctic peatlands in Fennoscandia and European Russia. The data cover the past two millennia that has undergone prominent changes in climate and a notable increase in annual temperatures toward present times. We show a pattern where European sub‐Arctic peatland microhabitats have undergone a habitat change where currently drier habitats dominated by Sphagnum mosses replaced wetter sedge‐dominated vegetation and these new habitats have remained relatively stable over the recent decades. Our results suggest an alternative future pathway where sub‐arctic peatlands may at least partly sustain dry vegetation and enhance the carbon sink capacity of northern peatlands.

The Growth and Carbon Sink of Tundra Peat Patches in Arctic Alaska

“…Due to the observed synchronicity of FBT‐like changes in peat patches within the time window of the recent warming period with no exception, we suggest that these shifts likely occurred in response to century‐long, climate‐induced, dramatic shifts in site conditions under anthropogenic warming with a comparable mechanism previously proposed for recent FBT in northern peatlands (Granlund et al., 2022; Loisel & Yu, 2013; Magnan et al., 2022; Piilo et al., 2023). Interestingly, the fact that peat patches in lower positions on the hillslope shifted to Sphagnum peat during the decadal‐scale cooling period of 1950–1975 CE seems to contradict the warming‐induced transition (Figure 6a).…”

“…This model highlights that the onset of Sphagnum peat could be triggered by climate-related allogenic forcing that involves multiple reinforcing feedback loops to sustain or accelerate the autogenic forcing (Loisel & Bunsen, 2020;Loisel & Yu, 2013). Due to the observed synchronicity of FBT-like changes in peat patches within the time window of the recent warming period with no exception, we suggest that these shifts likely occurred in response to century-long, climate-induced, dramatic shifts in site conditions under anthropogenic warming with a comparable mechanism proposed for recent FBT in northern peatlands (Granlund et al, 2022;Loisel & Yu, 2013;Magnan et al, 2022;Piilo et al, the fact that peat patches in lower positions on the hillslope shifted Sphagnum peat during the decadal-scale cooling period of 1950-1975 CE seems to contradict the warminginduced transition (Figure 6a). However, this contradiction can be reconciled if this decadal-scale cooling was too weak in magnitude and impact compared to the prior centennial-scale warming or did not reverse the continuous warming-induced changes in site conditions that have already reached a critical threshold ready for a regime shift (Belyea, 2009;Loisel & Bunsen, 2020).…”

“…The spatiotemporal changes of northern peatlands, including their inceptions, accumulations, and expansions, play a major role in the global C cycle and have affected atmospheric greenhouse-gas concentrations through time (MacDonald et al, 2006;Stocker et al, 2017;Yu et al, 2010Yu et al, , 2011. Although peatland development and carbon accumulation are controlled by a diversity of internal and external drivers as revealed in many site-specific studies (Klein et al, 2013;Payne et al, 2016;Piilo et al, 2020), global data syntheses indicated that longer and warmer growing seasons are crucial to promote peatland inception and growth, while the factor of increased effective moisture is secondary and regionally important (Charman et al, 2013;MacDonald et al, 2006;Morris et al, 2018;Treat et al, 2019;Yu et al, 2009Yu et al, , 2010. As the Arctic becomes warmer (and wetter) and the trend continues (McCrystall et al, 2021;Rantanen et al, 2022), the tundra landscape will, in the future, expand its climate space to become more like current boreal and sub-Arctic regions that have supported the development of extensive peatlands during the Holocene (Yu et al, 2009) (Figure 1a).…”

Recent hummock establishment in the margin of a subarctic fen, Finnish Lapland

“…2022; Piilo et al . 2023). In addition, testate amoeba‐based water table reconstructions suggest a trend of deepening water tables in non‐permafrost peatlands throughout Europe during the past c .…”