ECOHYDROLOGICAL CONTROLS ON POST-FIRE SPHAGNUM MOSS RECOVERY IN BOREAL SHIELD PEATLANDS

Abstract

Northern peatlands are a critically important global soil carbon reserve. The largest disturbance to Boreal peatlands is wildfire and the continued carbon sink status of Boreal peatlands is largely dependent on the productivity and reestablishment of Sphagnum mosses following wildfire. The main objective of this thesis was to identify the ecohydrological controls on Sphagnum moss moisture stress in post-fire peatlands.

We assessed post-fire moss accumulation and near-surface soil water tension five-years post-fire in triplicate burned and unburned Boreal Shield Sphagnum dominated peatlands. The post-fire recovery trajectory of the Boreal Shield peatlands indicates these peatlands will remain a net carbon sink. We identified peat depth and water table depth as key controls on near-surface soil tension, as near-surface soil tension exceeded 100 hPa (an established ecophysiological threshold for Sphagnum moss) when the peat profile is completely desaturated, which only occurred in the shallowest peatlands. As a result, shallow peatlands have reduced Sphagnum moss growth and peat accumulation, resulting in shallow peatlands remaining shallow.

We also identified ecohydrological controls on near-surface soil tension in an unburned, burned, and recovered peat profile by modelling the length of time until near-surface soil tension exceeded 100 hPa under a drying scenario using HYDRUS-1D. The greatest control on near-surface soil tension was initial water table depth, with near-surface soil tension exceeding 100 hPa faster with deeper initial water table depths. When assessing the disturbance state of the profiles, near-surface soil tension in burned profiles reached 100 hPa faster than unburned or recovered, and near-surface soil tension in recovered profiles reached 100 hPa faster than unburned. In recovered profiles, when recovered Sphagnum moss depth was greater than 4 cm, near-surface soil tension took significantly longer to reach 100 hPa than shallower recovered depth.

Overall, our results suggest that peatlands with peat depths less than 30 cm have reduced drought resilience than deeper peatlands, and once peatlands recover more than 4 cm of Sphagnum moss, their drought resilience increase. We suggest that future research should explore adaptive management and restoration strategies in burned peatlands in the Eastern Boreal Shield ecozone to promote the recovery of Sphagnum mosses to at least 4 cm as a means to enhance drought resilience and the continued accumulation of Sphagnum mosses (and carbon) post fire.