Modelling the effects of forest use change on brownification of Finnish rivers under pressures of acidification and climate change

Browning of surface waters due to increased terrestrial loading of dissolved organic matter is observed across the Northern Hemisphere. Brownification directly influences freshwater productivity and ecosystem services like water purification. Brownification often is explained by changes in large-scale anthropogenic pressures and ecosystem functioning, including acidification and climate change. Land use or cover changes and forestry measures have recently been observed to be one reason for the increase in brownification. Climate change influences brownification by increasing temperatures and thus stimulating the decay of dissolved organic carbon in soils, and by changing the timing and intensity of precipitation and snowmelt. A decrease in sulphur deposition is assumed to increase soil organic matter solubility. In Finland, productive forests cover about 66% of the land area. This study aimed to examine the effect of forest use changes on water browning in Finland under pressure of acidification and climate change. EU land use policies (Biodiversity Strategy, LULUCF Policy) influence land use but also forestry practices. Finland is committed to the EU's goal of protecting 30% of land and sea areas, and 10% of them strictly. The LULUCF regulation agrees how carbon sinks and greenhouse gas emissions from the land use sector are considered in the EU's climate goals until 2030. Finland aims to keep forests as carbon sinks. When studying the environmental effects of land use/cover changes due to these policies, environmental influence on biodiversity, and ecosystem services (sustainability of forestry, and water quality) should be simultaneously considered. We modelled organic carbon loading from river basins under changes in global pressures (climate and deposition) by mathematical models. We combined the watershed scale model (Simply-C) with scenarios of climate change, atmospheric deposition, and forest use change (1985-2060). We used daily data from five global climate models (CMIP5) under representative concentration pathway (RCP) scenarios RCP4.5 and RCP8.5. For atmospheric sulphur deposition, we used the chemical transport model results that are based on the EMEP MSC-W model (v4.4) and the MATCH model results. We explored two forest use scenarios that focus on potential changes taking place in the forested areas in Finland: 1) forest management, and 2) forest protection. The forest management scenario was based on simulations of clear-cut following Finnish national recommendations with the PREBAS forest growth and carbon balance model. Forest protection scenarios were based on spatial data of forests with high conservation value, optimized by Zonation programme. Modelling results indicated that global influence (atmospheric deposition, climate change) seemed to weaken in southern Finland after 2016. That gave more space for the effect of local forest use change due to different EU land use policies. Forest use change was more influential in river basins dominated by organic soils than in mineral soils. In northern Finland brownification seemed to continue, mainly driven by climate change.