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

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 estuary 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.生活史理论（life-history theory）认为，开花最佳阈值尺寸决定了资源分配的转变，而该转变源于繁殖、存活与营养生长之间的权衡。干旱、养分限制等环境胁迫可使陆地生态系统中的植物在更小体型时开花，但滨海湿地中的各类因子如何影响开花阈值尺寸，目前仍不明晰。本研究以广泛分布的盐沼植物互花米草（Spartina alterniflora）为实验材料，沿河口梯度构建原位中宇宙（mesocosm）阵列，对沉积物孔隙水盐度与淹没时长这两个共同塑造盐沼群落物种分布的关键因子进行人工调控。研究结果显示，更高的盐度会降低植株体型并提前开花，而更长的淹没时长则会增大植株体型并延迟开花。由此，开花阈值尺寸随盐度升高而降低，但随淹没时长增加而升高。此外，这些非生物因子可通过直接与间接途径影响开花阈值尺寸，其中淹没时长是主要调控因子。鉴于海平面上升引发的盐度升高与淹没时长增加正威胁着盐沼生态系统的生产力与功能，本研究结果表明，植物生活史策略的转变会促进有性繁殖以实现种群更新，或有助于全球变化背景下生态系统恢复力（ecosystem resilience）的提升。