| Summary: | Changing sub‐Arctic plant communities can be an important feedback to climate change, via shifts in quantity and quality of litter production. Litter inputs to soil have appreciable influence on soil organic matter and microbial dynamics and consequently may provide a feedback to climate change in the sub‐Arctic. As permafrost peatlands thaw in response to climate change, the community composition of vegetation has been observed to shift from smaller and woodier shrubs to larger, more biodegradable sedges. We tested the hypothesis that carbon (C) stored in plant biomass increases across a permafrost thaw gradient by sampling both above‐ and below‐ground biomass in a permafrost‐underlain palsa, partially thawed bog, and fully thawed fen, all at Stordalen Mire in northern Sweden. Surprisingly, we found that total above‐ and below‐ground biomass together do not significantly change from the intact to the fully‐thawed habitats, despite previous research showing that net ecosystem productivity (NEP) appears to be higher in the fully thawed inundated fen. The lack of observed biomass increase despite the increase in NEP observed in other studies could be explained if the higher productivity sedges in fen sites have higher turnover, and transfer that productivity to SOM through high root exudation and/or litter deposition. We also observed a shift in plant community composition associated with loss of plant biodiversity across the gradient. These results suggest that plant community succession alters the quantity, type, and diversity of plant litter inputs to the soil. Such changes in litter quantity and type may be important drivers of decomposition rates and therefore the status of the ecosystem as a source versus sink for atmospheric C.
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