Developing hydrologic thresholds for Sphagnum recolonization on an abandoned cutover bog

After 30 years of abandonment, a block-cut peatland near Cacouna, Quebec, Canada, has naturally regenerated Sphagnum mosses on <10% of its area, typical for these disturbed systems. Distinct hydrologic conditions were observed where Sphagnum has successfully recolonized, providing a basis for establishing thresholds that can be targeted by peatland restoration managers. Sites where Sphagnum mosses recolonized were characterized by high water table (mean−24.9±14.3 cm), soil moisture (ϕ) > 50%, and soil-water pressure (Ψ)≥ 100 mb. These hydrologic indicators were spatially organized according to the morphology of block- cut trenches, which typically include raised baulks, shallow ditches, and the convex skag (unused turf) deposits along the central axis of the trench. Topographically low areas like the shallow ditches (D) and lower parts of the skag (LS) adjacent to the ditches maintained ϕ and Ψ > 50% and − 100 mb, respectively, for 100% of the summer period. About 83% of all Sphagnum recolonization that occurred in the study trench did so in these areas. In more raised areas like the mid- and center portion of the skag, ϕ and Ψ eventually fell below these thresholds, and these areas generally did not support Sphagnum, except in a few localized microtopographic depressions in the lower (downslope) end of the trench. While this lower end of the trench had all the Sphagnum species that were present in the trench, even there it was only 38% of the total area. It seems that even short periods of low Ψ may restrict Sphagnum reestablishment in an otherwise favorably wet site.