Forest cover, landscape patterns, and water quality: a meta-analysis

Context Forest disturbance and management can significantly affect water quality. The understanding of the quantitative relationships between forest change/forest landscape patterns and water quality is critical for designing forest management practices to ensure a sustainable clean water supply. Objectives We quantified the relationships between (1) forest cover and (2) forest landscape patterns and water quality parameters. We also assessed contributing factors (i.e., climate zones, seasonality, watershed properties, forest types, and watershed context) to the variations in the above-mentioned relationships. Methods A meta-analysis based on published case studies around the globe was conducted to address the stated objectives. Results (1) Forest cover significantly affects the selected water quality parameters with higher forest cover rates overall improving water quality. (2) Surprisingly, the forest-water quality relationship is more controlled by watershed properties (i.e., slope, watershed size) and forest characteristics than by climate, except dissolved oxygen. The mixtures of coniferous and broadleaved forests improve the positive effects of forest cover on TN and TP compared to monocultures, while natural forests have significantly positive effects on phosphorus parameters compared to planted forests. (3) Forest landscape fragmentation causes negative effects on all selected forms of nitrogen (i.e., TN, nitrate, and ammonia). (4) The regulating effects of forests are influenced by the land use contexts in a watershed where agricultural lands significantly degrade water quality. Conclusions Forests can have positive effects on water quality. Deforestation and forest landscape fragmentation lead to declines in water quality. However, there are large variations in water quality responses depending on the selected water quality parameters, watershed properties, forest types, climate, and land use contexts.