Although a variety of global changes have been shown to influence plant invasions, interactive effects of different changes have rarely been studied. We examined effects of CO2 enrichment and warming on the ability of the invasive forb Linaria dalmatica (Dalmatian toadflax) to invade semi-arid mixed-grass prairie. Twenty 2.5 m2 plots of undisturbed native prairie were treated with a factorial combination of free air CO2 enrichment (to 600 ppm) and warming (+ 1.5C in the day, + 3C at night), using ceramic heaters. CO2 treatments began in May 2006. Linaria seedlings were transplanted into all plots in June 2006, and watered periodically through August 2006 to enhance survival. Warming treatments began in May 2007.
Linaria survival, measured at the end of the 2008 growing season, more than doubled with elevated CO2 and decreased by 96% with warming. Average height of surviving Linaria plants increased with CO2 enrichment in both 2007 and 2008, by an average of 28%. Under ambient CO2, warming decreased plant height by an average of 53%, apparently due to cessation of growth early in the season. In contrast, with both CO2 enrichment and warming, L. dalmatica height continued to increase throughout the growing season, and almost matched height in CO2 enriched plots. Although the mechanisms behind these responses remain unclear, there are indications that both direct photosynthetic responses to elevated CO2, and indirect responses, mediated by soil water availability may be involved. Elevated CO2 increased both Linaria photosynthesis and soil water availability, while warming had no effect on photosynthesis, but decreased water availability. These results demonstrate that CO2 enrichment can have particularly strong effects in conjunction with warming, not only facilitating invasion, but also eliminating negative effects of warming on invasion.