Pre-disturbance dominance, not fire alone, determines the success of an invasive alien plant species

Background Fire is a key ecological disturbance that can facilitate plant invasions, but the mechanisms driving post-disturbance competitive outcomes, especially the role of pre-existing species abundance, remain poorly understood. We conducted a greenhouse experiment using wetland plant species to test how fire and pre-disturbance dominance influence performance and competitive interactions between a widespread invasive alien grass (Urochloa arrecta)and a resident native grass (Hemarthria altissima). Fire was simulated across a gradient of invasive dominance, including native monoculture (0%), native dominance (25%), invasive dominance (75%), and invasive monoculture(100%), allowing us to assess whether post-fire competitive outcomes depend on initial community composition. Results The competitive advantage of invasive alien plant species (IAPS) was driven by a superior regenerative strategy rather than by consistently greater biomass production. After fire, IAPS sprout production was critically dependent onits initial dominance, more than doubling under high-abundance conditions. Crucially, high IAPS dominance suppressed the number of native plant species (NPS) sprouts and delayed their emergence. In contrast, at low pre-disturbance abundance, the IAPS exhibited reduced regenerative performance. Conclusions Our findings show that post-fire invasion success is density-dependent: only when already dominant does the invasive species gain a regenerative advantage by suppressing native sprout production and delaying the native emergence. These results demonstrate that pre-disturbance dominance and vegetative propagule pressure, particularly from the bud bank and clonal vegetative structures, are key mediators of post-disturbance success. Given the increasing global frequency of both fire disturbances and plant invasions, our study provides a mechanistic explanation for why fire promotes invasion under some dominance contexts but not others.