Peatlands are a major terrestrial carbon store and a persistent natural carbon sink during the Holocene, but there is considerable uncertainty over the fate of peatland carbon in a changing climate. It is generally assumed that higher temperatures will increase peat decay, causing a positive feedback to climate warming and contributing to the global positive carbon cycle feedback. Here we use a new extensive database of peat profiles across northern high latitudes to examine spatial and temporal patterns of carbon accumulation over the past millennium. Opposite to expectations, our results indicate a small negative carbon cycle feedback from past changes in the long-term accumulation rates of northern peatlands. Total carbon accumulated over the last 1000 yr is linearly related to contemporary growing season length and photosynthetically active radiation, suggesting that variability in net primary productivity is more important than decomposition in determining long-term carbon accumulation. Furthermore, northern peatland carbon sequestration rate declined over the climate transition from the Medieval Climate Anomaly (MCA) to the Little Ice Age (LIA), probably because of lower LIA temperatures combined with increased cloudiness suppressing net primary productivity. Other factors including changing moisture status, peatland distribution, fire, nitrogen deposition, permafrost thaw and methane emissions will also influence future peatland carbon cycle feedbacks, but our data suggest that the carbon sequestration rate could increase over many areas of northern peatlands in a warmer future

Peatlands of continental western Canada (Alberta, Saskatchewan, and Manitoba) cover 365 157 km2 and store 48.0 Pg of carbon representing 2.1% of the world's terrestrial carbon within 0.25% of the global landbase. Only a small amount, 0.10 Pg (0.2%) of this carbon, is currently stored in the above-ground biomass. Carbon storage in peatlands has changed significantly since deglaciation. Peatlands began to accumulate carbon around 9000 years ago in this region, after an initial deglacial lag. Carbon accumulation was climatically limited throughout much of continental western Canada by early Holocene maximum insolation. After 6000 BP, carbon accumulation increased significantly, with about half of current stores being reached by 4000 BP. Around 3000 BP carbon accumulation in continental western Canada began to slow as permafrost developed throughout the subarctic and boreal region and the current southern limit of peatlands was reached. Peatlands in continental western Canada continue to increase their total carbon storage today by 19.4 g m-2 year-1, indicating that regionally this ecosystem remains a large carbon sink.

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Peatlands are a major terrestrial carbon store and a persistent natural carbon sink during the Holocene, but there is considerable uncertainty over the fate of peatland carbon in a changing climate. It is generally assumed that higher temperatures will increase peat decay, causing a positive feedback to climate warming and contributing to the global positive carbon cycle feedback. Here we use a new extensive database of peat profiles across northern high latitudes to examine spatial and temporal patterns of carbon accumulation over the past millennium. Opposite to expectations, our results indicate a small negative carbon cycle feedback from past changes in the long-term accumulation rates of northern peatlands. Total carbon accumulated over the last 1000 yr is linearly related to contemporary growing season length and photosynthetically active radiation, suggesting that variability in net primary productivity is more important than decomposition in determining long-term carbon accumulation. Furthermore, northern peatland carbon sequestration rate declines over the climate transition from the Medieval Climate Anomaly (MCA) to the Little Ice Age (LIA), probably because of lower LIA temperatures combined with increased cloudiness suppressing net primary productivity. Other factors including changing moisture status, peatland distribution, fire, nitrogen deposition, permafrost thaw and methane emissions will also influence future peatland carbon cycle feedbacks, but our data suggest that the carbon sequestration rate could increase over many areas of northern peatlands

The spatial development and vegetation history of a large boreal peatland complex in east-central Alberta was reconstructed to examine factors that control peatland development in continental regions. Peat depth throughout the site was interpolated from over 300 depth measurements, and basal radiocarbon dates were obtained from 16 cores. Peat first initiated about 7400 calibrated 14C years BP (cal. BP), and early peat-forming communities were wet fens or marshes. Rates of expansion from these nucleation sites were dependent on both moisture availability and topography, with asynchronous expansion in different regions. Basal macrofossil assemblages suggest that paludification on slopes of large basins was the result of flooding caused by rising peatland water tables. In many areas that initiated after 3000 cal. BP, paludification involved invasion of upland forest by Sphagnum. Long-term apparent rates of peat accumulation were fastest in wet, moderate-rich fen areas where little community change has occurred over time. Macrofossil analysis of core profiles reveals a tendency for sites that initiated wet and minerotrophic to eventually be colonized by Sphagnum. However, the thickness of surficial Sphagnum layers differs between cores, and there are several examples of minimal or apparently reverse successional development.Key words: peatlands, boreal, paleoecology, vegetation succession, peat accumulation, paludification.