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We use paleoecological techniques to investigate how Canadian High Arctic wetlands responded to a mid-twentieth century increase in growing degree days. We observe an increase in wetness, moss diversity, and carbon accumulation in a polygon mire trough, likely related to ice wedge thaw. Contrastingly, the raised center of the polygon mire showed no clear response. Wet and dry indicator testate amoebae increased concomitantly in a valley fen, possibly relating to greater inundation from snowmelt followed by increasing evapotranspiration. This occurred alongside the appearance of generalist hummock mosses. A coastal fen underwent a shift from sedge to shrub dominance. The valley and coastal fens transitioned from minerogenic to organic-rich wetlands prior to the growing degree days increase. A subsequent shift to moss dominance in the coastal fen may relate to intensive grazing from Arctic geese. Our findings highlight the complex response of Arctic wetlands to warming and have implications for understanding their future carbon sink potential.

Plain Language Summary The response of Arctic wetland ecosystems and carbon stores to climate change is uncertain. We investigate the response of wetland ecosystems in the Canadian High Arctic to twentieth century climate warming. We use proxies for changes in vegetation (plant macrofossils) and wetness (testate amoebae) preserved in the wetland soil in combination with radiocarbon dating to reconstruct the past ecology of these wetlands. This approach allows us to explore beyond the timeframe of monitoring studies. Our results suggest that wetland type is an important determinant of the response of ecological, hydrological, and soil carbon accumulation to climate warming. Our findings highlight the clear but complex response of Arctic wetlands to twentieth century warming. This has important implications for understanding the future carbon sink potential of these ecosystems.