Climate impacts on Canadian subarctic peatland C accumulation and storage potentially mediated by moisture, peatland development, and fire

Abstract:

Warming climate may cause permafrost regions to become stronger sources of C due to increased decomposition or fire. However, the dynamics of soil C storage in subarctic peatlands—one of the largest sinks of high-latitude soil C—potentially complicate this hypothesis for two reasons: (1) Warming may increase C storage, presumably as productivity in temperature-limited regions is enhanced by longer growing seasons relative to losses from decomposition. (2) Regional climatic effects on C accumulation may be mediated by local processes, such as peatland development (e.g., bog vs. fen, permafrost vs. no permafrost) and associated differences in moisture and attendant response of fires to moisture. We examined 13 peat cores spanning the Holocene across a continental gradient in moisture/seasonality, from the Canadian continental interior (permafrost plateau bogs in Manitoba) to eastern coastline (unfrozen bogs along the southern boundary of permafrost in Labrador). We used 90 AMS ¹⁴C dates, percent carbon, and bulk density measurements to estimate sedimentation and C accumulation rates. Macroscopic charcoal was used to determine local fire severity. Macrofossil analysis was conducted to determine historical changes in the plant community and peatland type. The effects of past climatic changes depend on peatland development and moisture. Sites in Labrador were classified as wet, poor fens for much of their history, and fires were practically nonexistent. C accumulation rates were greatest during the Holocene Thermal Maximum and lower during more recent Neoglacial cooling. In contrast, C accumulation in Manitoba sites appeared to be greatest during initial wetter fen phases and slower during subsequent, drier bog phases. Fire was more common and severe in Manitoba during the bog phases, and the combination of drier bogs and fire slowed C accumulation rates during the HTM, thereby making C storage less sensitive to past climate. These results suggest that in regions that are potentially less moisture limited, C accumulation rates are high and may be sensitive to regional climatic changes. However, in drier regions, or where peatlands transitioned early from fen to bog, the combination of lower productivity and increased fire may constrain C accumulation response to regional climate.