Restoration of afforested peatland: Effects on pore- and surface-water quality in relation to differing harvesting methods

Abstract

The restoration of drained and afforested peatlands is carried out by removing trees and blocking forestry drains to reinstate key ecosystem services such as peatland habitat provision to support specialised biodiversity assemblages and carbon storage. Although restoration is a slow process, in the short-term, management interventions result in physical and biogeochemical disturbances, impacting upon pore- and surface-water quality. These impacts may vary depending on the restoration techniques used. Here, we compare the effects of two restoration techniques that both include blocking of main forestry drains but that vary in the amount of tree material removed from afforested sites: stem only harvesting (“standard harvesting”, STD) and whole tree harvesting (“enhanced harvesting”, ENH). We measured their short-term (0–1 year) effects on pore- and surface-water quality in a replicated BACI study. We assessed pore- and surface-water quality using principal response curves, comparing restoration treatments with afforested and open bog controls.Our results showed the largest effects in surface- and shallow pore-water after restoration, where DOC, PO43−, NH4+ and K showed the greatest concentration increases, in both ENH and STD plots, relative to controls. Lesser increases were measured for Al, Ca, Fe and Mn concentrations. Meanwhile, in deep pore-water, water quality changed comparatively little during the study. We also found a post-restoration (mean) rise in water table of 10 cm in both ENH and STD treatments.In general, ENH plots showed larger concentration increases in surface-water, likely related to the additional disturbance caused during brash removal (i.e. more intensive management). However, concentrations had begun to decline for many parameters by the end of the study, which was attributed to the removal of a major source of nutrients (brash) by this technique. In contrast, STD plots showed greater concentration increases in shallow pore-water, given (on average) a 5 cm shallower water table and with all brash remaining on site. Although longer-term monitoring would be required to test this, our results suggest that there remains merit in removing brash (compared to stem-only harvest) when considering longer-term recovery to bog.