Experimental setup
The experiment was conducted in the eastern Swiss Alps (Calanda mountain, 46°88’N, 9°49’E). In October 2016, 1 m x 1 m turfs of alpine vegetation were transplanted from 2050 m to a nearby site at 2000 m (hereafter “+0°C”), and to three lower elevations with ten replicate turfs at each site. The three lower elevations created a gradient of warming temperatures: 1600 m (“+2°C”, the adiabatic rate of temperature), 1400 m (“+3°C”) and 1000 m (“+5°C”). The sites were also situated along a gradient of decreasing average soil moisture, from 34.3% at 2000 m to 22.6% at 1000 m. The changes in precipitation and temperature along the gradient were consistent with climate change predictions for the region (Appenzeller & Center for Climate Systems Modeling 2011). An additional ten turfs were transplanted to the 1400 m site, and planted with individuals of 12 lowland species. This treatment mimicked the establishment of novel low-elevation competitors in an alpine community under warmer climate.
We measured the demographic response to climate change by surveying the abundance of each alpine species in the turfs near the peak of the growing season at each site from 2017 to 2020. Weather stations recorded air and soil temperature, air relative humidity and soil moisture from May to October in 2019 and 2020 (Table 1, Figure S1). Further details of all methods are presented in the “Detailed methods” section of Supporting Information.