Resources addition experiment
To further test whether large-GS species would be favored over small-GS species by increasing available resources, we obtained data from a long-term field resources manipulation experiment that was conducted in the typical steppe of Duolun, Xilingol in northern China (116. 3°E, 42.0°N). The site has a MAP of 360 mm and MAT of 2.1°C and is close to the eastern side of the transect described above. The study design is described by (Xu et al. 2012); because we aimed to assess the effects of resource limitation generally and not of the specific resources (for which limitation may differ by site), we selected only two treatments from this experimental setup: no (control) vs. combined multiple resource addition. The control plots only received ambient precipitation while the resource addition plot received additional water and nutrients on top of the ambient precipitation from 2007 to 2016 as described by (Xu et al. 2012). Water was added weekly (15 mm via sprinkler) during the growing season, which added up to 180 mm yearly. Nutrients were added in May and July as CO(NH2)2 and Ca(H2PO4)2, leading to 10 g N m-2 yr-1 and 10 g P m-2 yr-1. Each treatment was applied at the plot level (8 m × 8 m) and had 7 replicates. Aboveground biomass was harvested at the end of growing seasons every year from a 0.3 m × 0.3 m quadrat randomly chosen from within every plot. Biomass was sorted by species and was oven-dried at 65 °C for 48h to constant weight. From May to July 2017, genome size was measured for each species following the same procedure as in the transect study. Community weighted mean (CWM) genome size was calculated based on biomass for each plot. A linear mixed model was applied on CWM genome size, with resources treatment and experimental year as fixed effect while plot as a random effect. All analyses were conducted in R software 3.6.2 (R Core Team 2019).