Windows of opportunity for salt marsh establishment: the importance for salt marsh restoration in the Yangtze Estuary

Abstract

Restoration has been promoted as an important strategy to reverse the decline of salt marsh ecosystems. Physical and biological processes limiting the colonization of bare tidal flats by pioneer salt marsh species are commonly recognized. Recently, the window of opportunity (WoO) concept has been proposed as a framework to provide an explanation for the initial establishment of biogeomorphic ecosystems on tidal flats under high physical stress. Understanding the thresholds for early seedling establishment and colonization is critical for the successful restoration and management of this threatened ecosystem. In this study, we investigated the WoOs for the establishment and colonization of a Scirpus mariqueter salt marsh at a large‐scale restoration site in the Yangtze Estuary. A set of field monitoring and measurements were conducted to identify the potential physical and biological thresholds that could provide a mechanistic insight on the establishment and colonization of this pioneer marsh. The results showed that the successful colonization and expansion of the S. mariqueter marsh on the tidal flat required passing both physical and biological thresholds to open WoOs for establishment and colonization. The concurrence of WoOs, that is, propagule availability in the early growing season at a suitable tidal flat elevation with a benign sedimentary regime together with the removal of competition from invasive species, is presumed to be essential for the success of S. mariqueter marsh establishment and colonization. In applying such ecological insights, the seed‐sowing field experiment proved that by surpassing these thresholds, an S. mariqueter salt marsh can be successfully restored in a cost‐effective manner. We suggest that these WoO‐related establishment and colonization thresholds as well the cost‐effectiveness of using these WoOs should be considered in future large‐scale restorations of salt marshes.

The onset of secondary seed dispersal is controlled by germination‐features: A neglected process in sudden saltmarsh establishment

“…This hypothesis is supported by field observations and remote sensing analysis in the Yangtze Estuary, China (Fig. 1), where there was rapid colonization of large bare areas by single cohorts of seedlings when local seed banks were largely absent and primary seed-fall had already occurred (Yuan et al 2020 for Scirpus mariqueter; Zhao et al 2020a for Spartina alterniflora). This indicates that massive secondary seed dispersal and seedling establishment occur together over a short but specific period.…”

“…1), despite earlier measurements showing that local seed banks were largely absent (Yuan et al 2020;Zhao et al 2020a). This indicates the massive dispersal of seeds and/or seedlings during spring, which is consistent with findings from a field survey revealing a strong reduction in seed reserves within nearby saltmarshes during spring (Yuan et al 2020). Seeds trapped within the saltmarsh after local primary dispersal will most likely be waterlogged and lose buoyancy from late autumn to early spring.…”

“…Considering the CDNR in the Yangtze Estuary, many cohorts of S. mariqueter and S. alterniflora seedlings suddenly appeared at the mudflat during spring (Fig. 1), despite earlier measurements showing that local seed banks were largely absent (Yuan et al 2020;Zhao et al 2020a). This indicates the massive dispersal of seeds and/or seedlings during spring, which is consistent with findings from a field survey revealing a strong reduction in seed reserves within nearby saltmarshes during spring (Yuan et al 2020).…”

“…S. mariqueter is the most widely spread native pioneer species in the Yangtze Estuary (Fig. 1a) and is regarded as a favorable habitat for local biodiversity and hence an important species for local ecological conservation and restoration in the Yangtze Estuary (Hu et al 2015 a ; Yuan et al 2020).…”

“…The vegetation cover at both sites in 2017 and 2018 was extracted from a Pleiades satellite remote sensing image for July 24, 2017, and an aerial image taken by drones on July 19, 2018, respectively. Images were processed and vegetation was classified as described by Yuan et al (2020). ( g ) Schematic distribution of the locations for hydrodynamic (ALEC was used to measure current velocity and direction; RBR was used to record water depth) and bed elevation changes measurement at the Scirpus ‐site.…”