Global change drivers alter multiple components of community
composition, with cascading impacts on ecosystem stability. However, few
studies have examined the complex interplay between global change
drivers, synchrony, and diversity, especially over long-term
successional dynamics. We analyzed a 22-year time series of grassland
community data from Cedar Creek, USA, to examine the joint effects of
pulse soil disturbance and press nitrogen addition on community
synchrony, diversity, and stability during transient and post-transient
periods of succession. Using multiple regression and structural equation
modeling, we found that global change drivers decreased both synchrony
and stability, thereby decoupling classic theoretical relationships,
such as the portfolio effect. While the effect of soil disturbance
weakened through time, nitrogen addition induced unexpected dynamics
with maintained long-term impacts on composition, synchrony, and
stability. Our findings underscore the need for long-term data and a
comprehensive approach when managing ecosystems under ongoing global
environmental changes.