Abstract
Forested riparian corridors are a key management solution for halting the global trend of declining ecological status of freshwater ecosystems. There is an increasing body of evidence related to the efficacy of these corridors at the local scale, but knowledge is inadequate concerning the effectiveness of riparian forests in terms of protecting streams from harmful impacts across larger scales. In this study, nationwide assessment results comprising more than 900 river water bodies in Finland were used to examine the importance of adjacent land use to river ecological status estimates. Random forest models and partial dependence functions were used to quantify the independent effect of adjacent land use on river ecological status after accounting for the effects of other factors. The proportion of adjacent forested land along a river had the strongest independent positive effect on ecological status for small to medium size rivers that were in agricultural landscapes. Ecological quality increased by almost one status class when the adjacent forest cover increased from 10 to 60%. In contrast, for large rivers, adjacent forested land did not show an independent positive effect on ecological status. This study has major implications for managing river basins to achieve the EU Water Framework Directive (WFD) goal of obtaining good ecological status of rivers. The results from the nationwide assessment demonstrate that forested riparian zones can have an independent positive effect on the ecological status of rivers, indicating the importance of riparian forests in mitigating the impacts of catchment-level stressors. Therefore, forested buffer zones should be more strongly considered as part of river basin management.
Highlights
The ecological status of freshwater ecosystems has degraded worldwide, requiring urgent management efforts and policy actions to counter the prevailing trend (Grizzetti et al 2017; Reid et al 2019)
Forest percentage was highest for small streams with low amounts of agricultural land use in the catchment, and forest percentage was lowest in medium size streams with high amounts of agricultural land use in the catchment
In small to medium rivers with > 10% agricultural land use, ecological status increased with an increasing proportion of forest buffer zones, indicating that increasing forest cover in buffer zones increased the ecological status of the river
Summary
The ecological status of freshwater ecosystems has degraded worldwide, requiring urgent management efforts and policy actions to counter the prevailing trend (Grizzetti et al 2017; Reid et al 2019). An important mitigation action to counter the negative effects of agricultural land use on the ecological status of freshwater is to maintain or restore forested buffer zones along water courses (e.g., Singh et al 2021; Cole et al 2020; Pavlidis and Tsihrintzis 2018; Stutter et al 2012). Evaluations of the importance of forested buffers on ecological status in large river assessments are lacking, which is a significant knowledge gap regarding the efficient implementation of environmental policies such as the European Union Water Framework Directive (WFD, European Commission 2000). Forested riparian zones could help mitigate climate warming effects on freshwater biotas (Sponseller et al 2001; Thomas et al 2016; Turunen et al 2021) and contribute to the retention of excess nutrients, contaminants and organic material originating from human land use in a catchment (e.g., Kiffney et al 2003)
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