AMF limitation at the invasion front: differential benefits to edge versus core populations shape the expansion of an invasive plant

Abstract Arbuscular mycorrhizal fungi (AMF) are key mutualists in plant invasions, but their availability may constrain range expansion if plant spread outpaces fungal dispersal. In this study, we tested whether core and edge populations of an invasive plant differ in their AMF associations and how this affects expansion-related traits. Using a combination of field surveys and a greenhouse experiment, we investigated the effects of AMF on the growth, competitive ability, and reproduction of the invasive plant Galinsoga quadriradiata in its core and edge populations. Field data confirmed that core populations had significantly higher root AMF colonization than edge populations. In the greenhouse, edge populations exhibited greater mycorrhizal dependency; AMF inoculation significantly shortened their duration of vegetative growth phase (DVGP) and enhanced both growth and reproductive output in the edge populations but generally had no such effects on the core populations. However, the presence of competitors, particularly intraspecific ones, weakened these plant-AMF interactions. The results of piecewise structural equation models revealed distinct plant-AMF interaction pathways for the core versus edge populations of G. quadriradiata. Our findings demonstrate that plant-AMF interactions differentiate during range expansion, with edge populations suffering from an AMF deficit that can limit their growth and delay reproduction. AMF not only promotes invader performance but also adjusts its temporal niche by accelerating life history. Consequently, AMF availability acts as a critical ecological filter at the expansion front, serving as both a potential promoter and a limiter of invasive spread, with population-level variation in symbiosis shaping invasion dynamics.