Growing recognition of the importance of nature-based flood defense for coastal protection has created a need to restore degraded coastal wetlands in flood-prone areas. Establishment of the foundational species that engineer these systems is however hampered by physical and biological thresholds. To bridge these thresholds, conventional restoration methods approach the reestablishment of degraded ecosystems through transplantation of organisms in dense congregations to take advantage of conspecific facilitative mechanisms provided by the transplanted species. Such efforts can however be costly and remain sensitive to failure in the early stages due to the high attrition rates of vulnerable transplanted organisms in hostile, unmodified environmental conditions. We test an alternative to this organism-centric approach by creating a Window of Opportunity (WoO) for establishment. The WoO is created by directly alleviating environmental stress by modifying the abiotic conditions so as to mimic the presence of established ecosystem engineers through artificial means. In a multi-year field experiment we used biodegradable structures that approximate the effects of dense vegetation, and thereby modify the bed morphology of a mudflat adjacent to a degraded saltmarsh to stimulate seedling establishment.
Results/Conclusions
In our initial field experiment we demonstrate that (1) the biodegradable mimicry structures caused the formation of stable sediment hummocks and that (2) this led to the spontaneous recruitment of saltmarsh pioneers in the following growing season. These recruits were composed primarily of the annual pioneering Salicornia genus, with densities of up to 140 individuals m-2 within the structures, a 60-fold increase over ambient densities. Lower abundances of five other perennial species were also found exclusively within structures. Furthermore, recruits grew to be on average 5 cm taller inside of the structures than in the ambient environment by the end of the growing season. In successive experiments we demonstrate how the recruitment success is the aggregate result of (1) enhanced seed retention, (2) accelerated growth rates, and (3) suppressed mortality rates from bed erosion within the sediment hummocks. We argue that the effectiveness of the structures in this scenario is due to the close alignment of their impact on abiotics with the facilitative mechanisms present within expanding saltmarsh patches, which offer ideal conditions for saltmarsh recruits.