Supplementary file 1_Ecosystem service trade-offs of upstream afforestation: an integrated hydrological-economic assessment in the Sorraia basin, Portugal.docx

Southern Europe is particularly vulnerable due to seasonal water scarcity, erosion-prone soils and intensive land use. Upstream afforestation can improve ecosystem functioning, although outcomes depend on species choice and management, and resulting environmental and economic trade-offs are poorly understood.</p>Methods<p>Using the Sorraia basin (Portugal) as a case study, this study presents a hydrological–economic assessment of upstream afforestation scenarios: expanding eucalyptus plantations (EUCINT) versus restoring native cork-oak forests (RESTOAK) under current and mid-century climate conditions, focusing on streamflow, sediment and phosphorus loads (modelled using SWAT) and related ecosystem services, including agricultural production, reservoir sediment management, water purification and recreation, alongside direct economic returns associated with land-use change.</p>Results<p>The results show that afforestation has limited effects on streamflow (+1% under RESTOAK; up to −9% under EUCINT) compared to projected climate change impacts, but substantially influences sediment and nutrient dynamics.

Sediment loads into reservoirs increase by 85–185% under EUCINT but decrease by 36–75% under RESTOAK, while phosphorus loads decline by 26–65% and 36–75%, respectively. Economic outcomes are driven primarily by increased sediment management costs, water quality benefits and land-use returns, whereas effects on irrigated agriculture and water-based recreation are comparatively limited. RESTOAK generates substantial net welfare gains compared to Business as Usual (BAU), whereas EUCINT results in net welfare losses despite improvements in water quality.</p>Discussion<p>By integrating process-based hydrological modelling with economic valuation, this study demonstrates that afforestation outcomes depend on local biophysical conditions, species choice, management practices and the land-use systems being replaced.