Will fluctuations in salt marsh - mangrove dominance alter vulnerability of a subtropical wetland to sea-level rise?

To avoid submergence during sea-level rise, coastal wetlands build soil surfaces vertically through accumulation of inorganic sediment and organic matter. At climatic boundaries where mangroves are expanding and replacing salt marsh, wetland capacity to respond to sea-level rise may change. To compare how well mangroves and salt marshes accommodate sea-level rise, we conducted a manipulative field experiment in a subtropical plant community in the rapidly subsiding Mississippi River Delta. Experimental plots were established in spatially equivalent positions along creek banks in monospecific stands of Spartina alterniflora (smooth cordgrass) or Avicennia germinans (black mangrove) and in mixed stands containing both species. To examine the effect of disturbance on elevation dynamics, vegetation in half of the plots was subjected to freezing (mangrove) or wrack burial (salt marsh), which caused shoot mortality. Vertical soil development was monitored for six years with the surface elevation table-marker horizon system.

为避免海平面上升引发的淹没，滨海湿地可通过累积无机沉积物与有机质，实现土壤表面的垂直抬升。在红树林扩张并取代盐沼（salt marsh）的气候过渡带中，湿地应对海平面上升的能力或发生改变。为对比红树林与盐沼适配海平面上升的能力差异，我们在快速沉降的密西西比河三角洲的亚热带植物群落中开展了控制性野外实验。实验样地设置在空间位置等效的潮沟岸带，分别设立于互花米草（Spartina alterniflora，光滑绳草）、白骨壤（Avicennia germinans，黑红树林）的单优群落，以及包含两个物种的混生群落中。为探究干扰对土壤高程动态的影响，我们对半数样地的植被分别施加冷冻处理（针对红树林）与枯落物掩埋处理（针对盐沼），二者均可造成地上茎秆死亡。我们借助地表高程表-标记层（surface elevation table-marker horizon, SET-MH）系统，对土壤垂直发育过程进行了为期6年的监测。