How much is too much forest? Catchment sensitivity and silvicultural controls on water yield in temperate grassland afforestation

The expansion of Eucalyptus and Pinus monocultures across the Pampa biome has solidified a simplistic paradigm: "more trees equals less water." However, this critical review argues that the traditional focus on forest cover percentage is insufficient for predicting catchment hydrological health. Despite observed runoff reductions of 20–60%, there is unexplained data dispersion, pointing to an ecohydrological "black box": arising from the interaction between forest structure and management (e.g., LAI, density, spatial configuration), intrinsic catchment sensitivity, and subsurface water access. This article integrates catchment hydrology and landscape ecology to challenge current land-suitability frameworks, which often overlook ecosystem resilience thresholds and human water supply. At the regional scale, afforestation is highly uneven. Basin-scale analysis shows that 93% of catchments contain less than 10% planted forest cover, whereas only 35 catchments exceed 50% and a very small subset (n = 6) surpasses 70%, often overlapping with recharge-sensitive zones. As a result, hydrological risk is spatially concentrated rather than regionally pervasive, emerging where intensive plantation systems coincide with sensitive hydrological settings. By integrating catchment hydrology with silvicultural controls on vegetation structure and water use, this review reframes afforestation impacts as a continuous productivity-water tradeoff rather than a fixed threshold response. It proposes a mechanistic framework in which hydrological outcomes emerge from the coupling between site-inherent properties and management-dependent traits, and outlines a research agenda focused on process-based, spatially explicit forest planning beyond static land-suitability models.