Life-history plasticity of intertidal salt marsh in response to sea level rise: Salinity and inundation modulate size-dependent flowering of Spartina alterniflora

• Salinity reduces size and accelerates flowering, while inundation does the opposite. • Inundation duration primarily drives life-history trait plasticity of salt marshes. • Plastic life-history strategies promote wetland resilience to global change. The life-history theory posits that the optimum threshold size for flowering determines the transition in resource allocation, resulting from trade-offs between reproduction, survival, and vegetative growth. The stresses such as drought and nutrient limit can induce flowering at smaller sizes in terrestrial ecosystems, but how the factors in coastal wetland influence the threshold size for flowering remains unclear. Here, we employed the widely distributed salt marsh plant, Spartina alterniflora , and established an in-situ mesocosm array along the estuarine gradient to manipulate sediment porewater salinity and inundation duration, the key factors jointly shaping species distribution of salt marsh community. We found that higher salinity reduced plant size and advanced flowering, whereas prolonged inundation increased plant size and delayed flowering. Consequently, the threshold size for flowering decreased with increasing salinity but increased with longer inundation duration. Furthermore, these abiotic factors affected the threshold size for flowering both directly and indirectly, with inundation duration emerging as the primary determinant. Given that salt marsh ecosystem productivity and functioning are threatened by the increasing salinity and inundation duration associated with sea-level rise, our findings suggest that shifts in plant life history strategies promote sexual reproduction for regeneration and may contribute to the ecosystem resilience under global change.