Afforestation and subsequent restoration of raised and blanket bogs: impacts on water table depth and pore-water quality

There has been increasing interest in peatlands over the last two decades after their recognition as a primary worldwide carbon store and potential to mitigate climate change. Afforestation is a significant global source of peatland degradation, and concentrated efforts to restore affected peatlands are ongoing. Our study monitored the pore-water quality, with respect to the water table depth (WTD), over 18 months from a raised bog and a blanket bog study site where the first forest-bog restoration work started in 2002. We collected pore-water samples from 4 small, hydrologically disconnected catchments which included intact forestry, a near-natural bog and two restored catchments of differing ages and restoration techniques, at each of the study sites.

 

The WTD was significantly different between the afforested catchments (deepest) and near-natural bog (shallowest), increasing with time since restoration for the restored catchments at both study sites. In periods of low rainfall, the WTD receded faster at the raised bog site, which may be because of the increased water demand from more mature tree stands. There were significant spatial and temporal variations in pore-water chemistry. However, dissolved organic carbon (DOC) and soluble reactive phosphate (PO₄-P) concentrations were significantly higher than the near-natural bog (difference in means of 30.37 mg L^-1 and 412 µg L^-1, respectively), 5-6 years after restoration. DOC and PO₄-P concentrations reduced with time since restoration, and, at the blanket bog site, there was no significant difference between the near-natural bog and a catchment that had been restored 17 years earlier. Principal component analysis (PCA) showed that DOC, PO₄-P and nitrite (NO₂-N) concentrations are controlled by similar processes and primarily a component of the restoration work; dissolved ammonium (NH₄-N), water table depth and electrical conductivity were more closely associated with afforestation. Higher NH₄-N concentrations within the forest pore-water are likely because of increased mineralisation rates within the peat after a lowering of the WTD through drainage and the water demands from the trees; increased electrical conductivity is likely connected to atmospheric scavenging from forest canopies.

 

The lowland raised bog site had significantly higher mean DOC concentrations which we hypothesise could be a result of increased plant production or the hydrological differences between lowland raised bog and blanket bogs. The humic fractions of the DOC, measured by the E4:E6 ratio of absorption, were significantly higher in the restored sites of the raised bog and negatively correlated with the depth to the water below the surface. We found significant differences between the afforested microforms (furrows, original surface and ploughed ridges) for many of the pore-water variables measured, and we believe newly developed ground smoothing techniques could help restore the natural balance. Dominant vegetation cover was also a significant factor, and other methods such as plug planting of bog species could be beneficial.