<p>It is unclear how much land use intensification ecosystems can withstand before undergoing abrupt changes in their structure and dynamics. Here we assess how the functional structure, diversity and temporal stability of 150 agricultural grasslands responded to large variations in land use intensification, namely, different intensities of fertilization, grazing and mowing. Using multi-site time series (2008–2020) of plant trait distributions, we identify two thresholds where the functional structure, diversity and stability of grasslands changed dramatically. The first threshold occurred between unfertilized and fertilized grasslands, with maximization of trait evenness indicating the persistence of plant species with diverse resource-use strategies in extensively managed grasslands. The second threshold occurred when fertilization exceeded 80 kg N ha⁻¹ yr⁻¹ or when grazing exceeded 500 livestock units days ha⁻¹ yr⁻¹, beyond which the most intensively managed grasslands were functionally poor, highly unstable and vulnerable to extreme weather events. These findings allow us to quantify the level of perturbation beyond which grasslands depart from a high biodiversity state and show that highly intensive management pushes the system to a more unstable state. The identified thresholds may provide targets for sustainable management and fertilization practices.</p>