Research site selection and basic site information

The study was conducted in the Hengduan Mountains, Sichuan Province, China (Fig. 1). This area is on the south-eastern edge of the Tibetan Plateau, and the study sites are located in Kang-Ding Valley, on the northwest slopes of Mt. Gongga along a steep elevational gradient where the lower part is characterized by mixed coniferous-broadleaved forest, followed by belts of subalpine coniferous forest, sub-alpine meadows, subalpine shrub, and alpine meadows towards higher elevations and colder climates^33,34.

Location and study design of the Mt. Gongga climate change gradient and experimental studies. (a) Map of the study area with latitude, longitude, and elevation indicated as well as the location within China (insert), and (b) schematic representation of the study design of climate change experiments replicated across four sites along the 1100 m elevational gradient in Mt. Gongga, Sichuan Province, China. Each colour represents an experimental treatment; different kinds of turf transplants, Open Top Chambers (OTCs), and control plots (see legend). Arrows detail the turf transplant treatments between specific sites. At each site, all treatments are replicated in seven experimental blocks. Elevation and the mean temperature of the growing season (air temperature in the four warmest months per year, based on the 2 m climate station measurements, dataset v) are indicated for each sites.

In 2012, four study sites along the slope were chosen to reflect the major bioclimatic variation in southwest mountain grassland vegetation along an elevation gradient of 1100 meters; the Lowland site at 3000 m a.s.l, the Middle sites at 3500 m a.s.l., the Alpine site at 3850 m a.s.l., and the High Alpine site at 4130 m a.s.l. (Fig. 1, Table 1). All the sites were placed selectively in grasslands associated with mountain gray-brown soil originating from granite³⁵. The sites were selected to represent the charactieristic grasslands at different elevations and hence climatic conditions, but to otherwise be as similar as possible with respect to environmental conditions, vegetation structure, plant community composition, slope, aspect, etc. The geographical distance between adjacent sites is on average ca 2 km.

Basic site description from the Mt. Gongga climate gradient.

The four study sites with elevation, geographical coordinates, growing season mean temperature (June–August), based on data measured at 2 m above ground between 2012 and 2016 (dataset v), growing degree days > 5 °C (GDD), and freezing days with temperatures < 0 °C based on data from 2013 (dataset v), long term annual precipitation from WorldClim version 2.0 for 1970–2000⁴⁵, soil moisture (June–August) measured at 5 cm below ground between 2012 and 2016 (dataset v) and above-ground standing biomass (dataset iii) in the four study sites along the slopes of Mt. Gongga, Sichuan Province, China.

At each site, we selected an experimental area within the grassland as homogeneous and representative of the grasslands at that elevation as possible. The experimental areas were placed on sloping ground, avoiding depressions and concave areas in the landscape and other features such as big rocks or formations that may affect microclimate, light conditions, hydrology and/or snowdrift. All sites were moderately grazed prior to the experiment by yak, sheep, cattle, goats, and/or horses, and the experimental areas were fenced and locked for the duration of the study to prevent grazing and human disturbance of the experimental infrastructure. The fenced area was mowed at the end of each growing season to mimic past grazing regimes and minimize fence effects.