Field sampling of source populations
Populations of P. lanceolata included in this study were part of the coordinated project PlantPopNet (Buckley et al. 2019). In the growing seasons of 2015 and 2016, we sampled 46 populations across the species’ range (29 native and 17 non-native populations; Fig. 2, Table S1, S2 in Supporting Information), spanning a wide range of climatic, management and plant community conditions, and a wide range of genoypes (Smith et al. 2020). For each population, we monitored all individual plants within 0.25 m2 plots along 10 m transects until we reached a minimum of 100 plants (Buckley et al . 2019). We recorded for each plant the number of rosettes and the flowering status (flowering vs. non-flowering). For each rosette, we recorded the number of leaves, the size of the longest leaf, the number of flowering stems if any and the length of the most developed inflorescence. We used these measurements to estimate biomass and total inflorescence length at the whole plant level (see further details on Appendix S1). In a subset of populations and outside the monitoring plots, we collected leaves for the estimation of specific leaf area (SLA; 25 populations) and seeds for the greenhouse experiment (15 populations; Table S1, Appendix S1).