Tree establishment can have multiple effects on the production and biodiversity of rangelands. In mixed (C3–C4) grasslands, winter deciduous trees could favor cold-season species in the understory, improving forage availability in the most critical time of the year. Yet, they could also promote local extinctions and invasions, risking native biodiversity. We evaluate the effect of poplar planting on the structure, composition, and diversity of native grasslands in the Flooding Pampas of Argentina using a network of 9 pairs of adjacent nonafforested and afforested stands (age: 23–25 years, density: 625–1 111 plants ha⁻¹) located in different topographic positions. Phytosociological surveys, basal cover measurements, and tree volume were performed at all stands. Live plant cover was 42% lower under poplars (P < 0.05). Litter cover followed an opposite trend leaving bare soil proportions unchanged with afforestation. Afforested stands had a higher proportion of C3 species compared with nonafforested ones. Little evidence of local extinctions or invasions with afforestation was found. Poplar understories had significantly higher nonnative species cover but similar numbers and lower species diversity (Shannon–Weaver index) yet similar species richness when compared with their nonafforested counterparts. Beyond the diversification of ranch outputs, deciduous tree plantations in the Flooding Pampas can offer a good forage source in their understory that complements nonafforested natural grasslands in quality and seasonality.