Pioneer salt marsh species Scirpus mariqueter disperses quicker in summer with seed contribution from current and last year

Salt marshes are widely considered to be the most valuable ecosystems for their important ecosystem services. Considering that pioneer species often spread rapidly over large areas in a short time under suitable conditions, there is a pressing need to understand their rapid dispersal (seed and seedling) mechanism and post-germination growth and fate. Based on remotely sensed images, field surveys and manipulative experiments, the rapid dispersal mechanism of S. mariqueter, one of the key native pioneer species at the Yangtze Estuary, was investigated. The results showed that (1) the average seaward expansion rate (the area of new dispersal in current year to that of the last year) of S. mariqueter was about 61% (maximum 181.7%) from 2013 to 2019 in Chongming Dongtan, and the expansion area in summer (August) was twice as large as that in spring (May). (2) Seeds from current year facilitate the dispersal of S. mariqueter with the contribution rate up to 27%, in addition to the seeds from last year. (3) The special germination regime of leaves sprouting prior to roots largely enhanced the buoyancy of the newly germinated seedlings, allowing them to have a potential long-distance dispersal. (4) The total event processes, including seed soaking and germination (7 days), root taking (2 days), forking (6 days), were completed within 15 days, which is highly consistent with the spring tidal cycle. These results illustrated important mechanism for quick expansion of pioneer salt marsh species at a large scale. It suggests that the successful seedling establishment of salt marsh species are closely linked to windows of opportunity (WoO), which provide scientific support for the conservation and restoration of coastal wetland ecosystems.