Warming neither accelerates degradation of alpine grasslands nor promotes restoration of degraded alpine grasslands on the Tibetan Plateau

It is still debated whether warming induces degradation of alpine grasslands and whether warming accelerates the restoration of degraded alpine grasslands. Here we conducted a warming experiment using open top chambers to explore warming effects on plant composition and diversity, aboveground net primary productivity (ANPP) and net ecosystem carbon exchange (NEE) in alpine grassland at three degradation levels (i.e., no-degradation, moderate degradation and desertification) on the Tibetan Plateau from 2014 to 2016. There were no interactive effects between warming and degradation level on the variables measured in our study, except for plant diversity. Warming enhanced plant species diversity at no-degradation sites but had no significant influence on species diversity at sites of moderate degradation and desertification. Generally, warming alone had no significant effects on ANPP or seasonal average NEE during the experimental period, but its effect on them varied with year. In addition, compared with non-degraded sites, heavy degradation decreased ANPP by 35.8%, 44.2% and 63.8%, and reduced NEE by 71.8%, 63.4% and 70.3% in 2014, 2015 and 2016, respectively. The effects of moderate degradation on ANPP and NEE varied with year, enhancing ANPP by 37.3% and 37.7% in 2015 and 2016, but moderate degradation had no remarkable influence on seasonal average NEE compared with non-degraded sites. These results suggest that warming neither accelerates degradation of alpine grasslands nor promotes restoration of degraded alpine grasslands on the Tibetan Plateau.