Reviewing peatland forestry: Implications and mitigation measures for freshwater ecosystem browning

Abstract

Freshwaters of the boreal and temperate regions have experienced increased browning during the last decades. Browning, or brownification, is mostly driven by increased organic carbon (OC) and iron concentrations. It can cause detrimental changes in aquatic ecosystems through effects on chemistry, physics and ecology. Additionally, browning can impact aquatic greenhouse gas emissions, increase the costs of drinking water treatment and weaken the recreational value of water bodies. All these impacts call for means to mitigate the excess export of humic matter to aquatic ecosystems. Browning has often been associated with decreased atmospheric sulphur deposition and climate change-induced alterations in temperature, vegetation and the hydrological regime of catchments. Lately, it has been attributed to changes in land use, especially drainage of peatlands for forestry purposes and afforestation. In this review, we evaluate the impacts of peatland forestry operations on OC leaching and assess possible mitigation measures. We reveal that traditional, even-aged forest management with clear felling, site preparation and ditch cleaning creates hot spots for OC leaching and hot moments for carbon-rich runoff due to fluctuations in hydrology, peat decomposition and peat exposure. Simultaneously, most of the current water protection measures are either ineffective or ambiguous in retaining OC, especially in a dissolved form. Hence, management practices that prevent future peat decomposition and reduce hydrological connectivity are essential to reduce negative water quality impacts. We conclude that a systemic change would be needed to prevent formation of loading and excess OC leaching. More research is needed, but it seems that a transition towards more natural and diverse peatland forest management with abundantly applied continuous cover forestry, mixed forest cover and improved catchment retention by peatland restoration could result in fewer undesirable water quality impacts. Protecting biodiversity and counteracting negative climate impacts requires the integration of updated management practices into future forest policies and guidelines, as the green shift towards a bioeconomy will create an increasing demand for the sustainable use of peatland forests.