Nitrogen addition reduces alpine grassland stability via enhanced temporal synchrony and population variability

Nitrogen enrichment leads to diversity loss of plant community in grasslands, and it typically enhances productivity in short-term scale but decreases it in the long term. Short-term negative N effect on stability has generally been primarily attributed to species synchrony rather than species loss. However, the persistent species loss caused by long-term N enrichment may result in high population variability, ultimately contributing to variability in community productivity. Here, we report results of a 19 year-long experiment that manipulated livestock exclusion and N enrichment in an alpine grassland to test their effects on species richness and stability of aboveground productivity through the years. Results demonstrated that the stability of aboveground productivity declined during the first non-overlapping 5 year window which was primarily driven by increased species synchrony. In the second non-overlapping time window, decrease in stability was largely driven by compounded effects of persistent synchrony and amplified dominant species variability. In the third and fourth non-overlapping 5 year windows, functional diversity loss and subordinate species variability contributed to a continued decline in stability. Mechanistically, the losses of species and functional diversity under different N forms shaped temporal patterns of synchrony and population variability, explaining decrease in stability. These findings reveal a time-dependent destabilization cascade, i.e., initial synchrony-driven fluctuations evolved into a dual effect of synchrony and dominant species variability, ultimately disrupted by population variability. We posit that diversity help to sustains grassland functioning not only  by increasing productivity, but also by maintaining a dynamic balance between synchrony and population variability. This highlights the necessity of conserving plant diversity to secure ecosystem functioning under global change.