Reduced phenological differences under nitrogen enrichment facilitate invasion by a late‐growing plant

Summary Although invasion success is often attributed to the early phenology of the invader, many late‐growing invaders also thrive in resource‐enriched environments. However, the mechanism behind this paradox remains poorly understood. Here, we tested how nitrogen (N) enrichment influences competition between the late‐growing invader Spartina alterniflora and the early‐growing native Phragmites australis in a coastal salt marsh. Using field experiments and modern coexistence theory, we tracked changes in growth timing due to N enrichment and measured their effects on niche differences (ND) and fitness differences (FD). We found that N enrichment advanced the growth phenology of S. alterniflora but not that of P. australis , thereby reducing their temporal separation. This N‐induced phenological synchrony weakened stabilizing ND and amplified the invader's fitness advantage, shifting the competitive outcome from coexistence under ambient N conditions to the strong suppression of the native plant under N enrichment. Our findings reveal a critical, yet often‐overlooked mechanism: N enrichment helps late‐growing invaders not only by increasing their competitiveness but also by eroding the temporal ND that buffer native communities. This highlights that such resource‐driven alterations in phenological strategies are a potent determinant of invasion success under global change.