Study design
The study site is a moist meadow habitat within the alpine tundra at Niwot Ridge Long Term Ecological Research (LTER) site in the Front Range of the Rocky Mountains, Colorado, USA (40°03′ N, 105°35′ W). The experimental design of this study is described in detail in Smithet al. (2012) and Farrer et al. (2014). Briefly, experimental plots (1m2) were established in 2006 with a fully factorial deployment of warming (using passive, open-top plexiglass chambers), nitrogen (N) addition (using slow release fertilizer), and snow manipulation treatments (using snow fences) (See supplementary methods).
Our study focuses on warming, as this is the most consistent global change driver in alpine tundra ecosystems (Hock et al. 2019). While we assess community responses to all three global change drivers: warming, snow addition and N addition, we restrict our analyses to only the treatments that include experimental warming either alone or in combination with snow and N addition (n=40 plots). Thus, we compare the following four experimental treatments and contrast them to full control (ambient conditions): 1) warming only (W), 2) N addition and warming (NW), 3) snow addition and warming (SW) and 4) snow, N addition and warming (SNW). In addition to warming, altered snowpack is another crucial impact of global change in alpine ecosystems (Gobiet et al. 2014). While many regions, particularly at lower elevations and latitudes, are experiencing a decrease in snowpack with climate change, winter precipitation in the alpine at Niwot ridge has increased since the 1950s due to shifts in upper-air circulation patterns across the Continental Divide (Kittel et al. 2015). Finally, while atmospheric N emissions have decreased in many parts of the world, they are still increasing in highly developing areas and impacts on sensitive alpine ecosystems can be long-lasting with limited recovery of plant communities on decadal time scales (Bowman et al. 2018).