Invasive plants have wide-reaching negative economic and ecological impacts, and with ongoing plant species introductions associated with globalization, invasion rates will likely continue to increase worldwide. Ornamental species comprise a majority of plant introductions and invasions, and preventing new ornamental invasions is a priority for many land managers. Climate suitability is strongly associated with alien species establishment success; however, determining potential invaders’ climate suitability under climate change is difficult. Climate change-related predictions are usually restricted to modeling approaches, and often limited in the accuracy and specificity of results. Here, we assessed the invasion potential of 37 not-yet-naturalized alien ornamental herbaceous species by measuring establishment success under experimentally-simulated projected temperature increases. Temperature increases in the field were simulated using infrared heater lamps, and we measured germination rates and seedling survival of ornamentals in grassland plots in Konstanz, Germany across two years. Multiple species traits were assessed for potential effects on establishment success under increased temperatures. We also assessed germination and seedling survival in 14 native and 13 already-naturalized alien species, to determine whether increased temperatures may alter climate suitability for previously-established species. Additionally, a disturbance treatment was applied to determine whether increased temperatures differentially affect invasibility in disturbed vs. undisturbed grasslands.
Increased temperatures overall somewhat reduced the establishment success across all species, however annual species performed better under warmer conditions than did perennial species, and greater seed weight also increased germination success under increased temperatures. Establishment success under increased temperatures did not vary with species winter hardiness designations, suggesting that this metric, frequently used to assign climate suitability, may serve as a poor predictor for alien species suitability under changing climates. Native species were more negatively affected by increased temperatures than alien ornamentals or already-naturalized aliens. Undisturbed plots were highly uninvasible, though native and perennial species were better than others at establishing in undisturbed plots. However, overall invasibility increased in undisturbed native grassland plots under increased temperatures. While invasion potential may not inherently increase with increased temperatures for all ornamental species, the reduced fitness of many native species under increased temperatures may make available many additional niches for more potential invaders. With this supporting experimental evidence of traits associated with establishment success under increased temperatures, it may be possible to make more informed predictions about likely invaders in different habitat types under new temperature regimes.