Mountainous grasslands in South America represent highly diverse ecosystems that offer a broad spectrum of benefits to people. These include the regulation and purification of water, prevention of soil erosion, provision of livestock feed, and preservation of cultural heritage. Previous research has underscored the significant role of grazing in shaping the diversity and productivity of grassland ecosystems, particularly in highly productive, eutrophic systems. In such systems, grazing has been demonstrated to restore grassland plant diversity by reducing primary productivity. However, it remains unclear whether these findings are applicable to South American mountainous grasslands, where plants are adapted to different environmental conditions. To address this gap, we conducted a meta-analysis of experiments that excluded livestock grazing. The goal was to quantify the impact of grazing exclusion on plant diversity and productivity across mountainous grasslands in South America. Our findings, akin to studies conducted in temperate grasslands, revealed that herbivore exclusion led to an increase in aboveground biomass. Nevertheless, it also resulted in a reduction in species richness and Shannon diversity. Interestingly, the effects of grazing exclusion became more pronounced with longer exclusion durations, yet remained robust to various climatic conditions, including mean annual precipitation and mean annual temperature, as well as the evolutionary history of grazing. In contrast to results observed in temperate grasslands, the reduction in species richness due to herbivore exclusion was not correlated with increased aboveground biomass. This suggests that the processes regulating (sub)tropical grassland plant diversity may differ from those in temperate grasslands. Further research is imperative to better comprehend the specific factors influencing plant diversity and productivity in South American montane grasslands and to discern the ecological implications of herbivore exclusion in these unique ecosystems..

1. The reduction of plant diversity following eutrophication threatens many ecosystems worldwide. Yet, the mechanisms by which species are lost following nutrient enrichment are still not completely understood, nor are the details of when such mechanisms act during the growing season, which hampers understanding and the development of mitigation strategies. 2. Using a common garden competition experiment, we found that early-season differences in growth rates among five perennial grass species measured in monoculture predicted short-term competitive dominance in pairwise combinations and that this effect was stronger under a fertilisation treatment. 3. We also examined the role of early-season growth rate in determining the outcome of competition along an experimental nutrient gradient in an alpine meadow. Early differences in growth rate between species predicted short-term competitive dominance under both ambient and fertilized conditions and competitive exclusion under fertilized conditions. 4. The results of these two studies suggests that plant species growing faster during the early stage of the growing season gain a competitive advantage over species that initially grow more slowly, and that this advantage is magnified under fertilisation. This finding is consistent with the theory of asymmetric competition for light in which fast-growing species can intercept incident light and hence outcompete and exclude slower-growing (and hence shorter) species. We predict that the current chronic nutrient inputs into many terrestrial ecosystems worldwide will reduce plant diversity and maintain low biodiversity state by continuously favouring fast-growing species. Biodiversity management strategies should focus on controlling nutrient inputs and reducing the growth of fast-growing species early in the season.