Tall fescue invasion fostered by grazing exclusion and nutrient addition in a temperate grassland

Forage species that escape from rangelands can invade natural grasslands, altering their composition and functioning. Native plants and wild herbivores may limit exotic species, whereas domestic herbivores and nutrient enrichment often favour invasion in the short term. However, the long-term interplay of top-down (herbivores) and bottom-up (nutrients) forces in controlling forage invasions remains uncertain. We evaluated how cattle grazing exclusion and nutrient addition influenced the invasion of Festuca arundinacea (tall fescue) in the Flooding Pampa (Argentina). Using three manipulative experiments spanning almost 30 years (1989–2016), we studied (i) the effect of cattle exclusion on species composition after invasion and (ii) how exclusion combined with nutrient enrichment (NPK) affected tall fescue, plant groups, and diversity. Tall fescue first appeared in 1995, when cattle exclusion still promoted native grasses. By 2016, it dominated cattle exclosures (>80% cover) but remained scarce in grazed areas (<5%). Nutrient addition with exclusion increased tall fescue size and competitiveness, reducing native cover and richness. Declines in richness reflected both a direct negative effect of nutrients and the indirect effect of tall fescue expansion facilitated by cattle absence. In sum, cattle exclusion shifted from favoring native grasses to enabling invasive forage dominance, especially under nutrient enrichment. Long-term interactions between grazing and resources strongly shape invasion outcomes. Facing increasing forage invasions limited nutrient inputs and domestic grazing might sustain native plant communities.