Priority effects, not fire alone, determine the success of invasive alien plant species

1. Fire is a key disturbance known to facilitate plant invasions, but the mechanisms driving competitive outcomes, especially how they are shaped by the pre-existing stage of invasion, remain unclear.2. We experimentally assessed how fire and pre-disturbance dominance affect performance and competitive interactions between a widespread invasive alien plant species (IAPS, Urochloa arrecta ) and a resident native plant species (NPS, Hemarthria altissima ). We simulated fire across a gradient of IAPS dominance, hypothesizing that fire’s impact would depend on the initial invasion stage.3. The IAPS’s advantage was driven by a superior regenerative strategy, not by consistently higher biomass production. Post-fire, the IAPS’s sprout production was critically dependent on its own dominance, more than doubling when at high abundance. Crucially, this high IAPS dominance suppressed the number of NPS sprouts and prolonged their sprouting time. In contrast, at low abundance, the IAPS’s regenerative capacity was credibly reduced.4. Synthesis . Our findings reveal that fire facilitates plant invasion not simply by creating opportunity, but by amplifying the regenerative and suppressive traits of an already-dominant invader. This demonstrates that priority effects and propagule pressure are key mediators of post-disturbance success. Considering the increasing records of fire disturbances and plant invasion processes worldwide, these findings contribute to a more profound understanding of the rationale behind the IAPS dominance in fire-disturbed environments.