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

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.