Holocene vegetation dynamics and hydrological variability in forested peatlands of the Clay Belt, eastern Canada, reconstructed using a palaeoecological approach

Abstract

Forested peatlands are widespread in boreal regions of Canada, and these ecosystems, which are major terrestrial carbon sinks, are undergoing significant transformations linked to climate change, fires and human activities. This study targets millennial‐scale vegetation dynamics and related hydrological variability in forested peatlands of the Clay Belt south of James Bay, eastern Canada, using palaeoecological data. Changes in peatland vegetation communities were reconstructed using plant macrofossil analyses, and variations in water‐table depths were inferred using testate amoeba analyses. High‐resolution analyses of macroscopic charcoal >0.5 mm were used to reconstruct local fire history. Our data showed two successional pathways towards the development of present‐day forested peatlands influenced by autogenic processes such as vertical peat growth and related drying, and allogenic factors such as the occurrence of local fires. The oldest documented peatland initiated in a wet rich fen around 8000 cal. a BP shortly after land emergence and transformed into a drier forested bog rapidly after peat inception that persisted over millennia. In the second site, peat started to accumulate from ~5200 cal. a BP over a mesic coniferous forest that shifted into a wet forested peatland following a fire that partially consumed the organic layer ~4600 cal. a BP. The charcoal records show that fires rarely occurred in these peatlands, but they have favoured the process of forest paludification and influenced successional trajectories over millennia. The macrofossil data suggest that Picea mariana (black spruce) persisted on the peatlands throughout their development, although there were periods of more open canopy due to local fires in some cases. This study brings new understanding on the natural variability of boreal forested peatlands which may help predict their response to future changes in climate, fire regimes and anthropogenic disturbances.

Peat deposits store more carbon than trees in forested peatlands of the boreal biome

“…3 ) that suggest high decay rates compared to non-forested Sphagnum peatlands (mean OM density: 0.105 g cm −3 ) 35 —which likely explains the low rates of C sequestration over millennial timescales in these forested peatlands. This could be attributed to the persistently low water tables over millennia which favour higher aerobic decomposition rates in the upper peat layers—as observed by Magnan et al 37 from testate amoeba analysis in similar woody-dominated horizons in another forested peatland 15 km apart from the present study sites. The low LORCA values could also be attributed to important organic layer combustion during fire events, as forested peatlands have relatively high woody biomass as opposed to non-forested peatlands.…”

Spatiotemporal evolution of paludification associated with autogenic and allogenic factors in the black spruce–moss boreal forest of Québec, Canada

“…4 and 5). Tree mortality caused by fire or other disturbances facilitates light penetration on the ground surface and raises water tables as a result of lower evapotranspiration (Fenton and Bergeron, 2011; Schaffhauser et al, 2016), thus creating favorable conditions for Sphagnum growth and peat accumulation (Fenton et al, 2005, 2007; Fenton and Bergeron, 2006; Simard et al, 2007; Magnan et al, 2018).…”

“…Paludification is influenced by a combination of autogenic (topography and mineral substrate) and allogenic (fire and climate) factors. The invasion of sphagna and the accumulation of a thick organic layer lead to a lowering in soil temperatures (Van Cleve et al, 1983), a decrease in soil nutrient availability (Prescott et al, 2000), a rise in water table (Taylor et al, 1987; Fenton and Bergeron, 2006; Magnan et al, 2018), and a decline in forest productivity (Simard et al, 2007). At the landscape scale, the paludification process and subsequent widespread peatland formation can also cause a reduction of fire recurrence (Cyr et al, 2005) and modify forest vegetation composition, particularly a decrease in the abundance of balsam fir ( Abies balsamea ; Ali et al, 2008; Messaoud et al, 2014) and jack pine ( Pinus banksiana ; Payette et al, 2012).…”

Drainage ditches enhance forest succession in a raised bog but do not affect the spatial pattern of tree encroachment

“…However, two types of communities comprising spatial-ecological system of raised bogs: open mires and bog woodland are among the most endangered habitats in Europe [ 4 ]. The processes of shrinking of lakes and associated peat bog development have been going on gradually throughout the Holocene [ 5 ], but in present times it has been intensified by global changes, especially climate warming [ 6 ] and the eutrophication effects caused by increased availability of nitrogen [ 7 ]. There is an urgent need to understand the processes shaping their structure and species composition to maintain peat bogs in the landscapes strongly affected by global changes and by various forms of human management, like peat extraction, decreased and unstable water resources, pollution from agriculture [ 8 , 9 ].…”