The importance of the propagule–sediment–tide “power balance” for revegetation at the coastal frontier

Abstract

Revegetation of pioneer plants is a critical phase in community establishment for mudflats in seriously degraded coastal wetlands. We tested a hypothesis of the importance of a “power balance” among propagule resilience and sedimentary and tidal disturbances for vegetation reestablishment. Our experiment used three types of propagules (seeds, seedlings, and corms) of native Scirpus species in the fringing flats with similar tidal flows and varying sedimentary intensities in the Yangtze Estuary. Regardless of the initial planting densities, the seed germination rate was extremely low in the field situation. Although the incubated seedlings were planted directly on the bare flat, the wave movement easily flushed the seedlings, even at the site with moderate sedimentary accretion. Failure of the revegetation practice using the seed and seedling materials indicated that the combined “growing and anchoring power” of young seedlings and “stabilizing power” of the sediment were insufficient to withstand the “dislodging power” of the tidal energy. In contrast, the planting approach with underground propagules (corms) proved to be feasible for vegetation establishment at the sites with moderate and low‐level sedimentary intensities. The successful practice improved the tipping point of plant survival and tussock formation could be surpassed when the combined growing and anchoring power of seedlings that developed from corms with the stabilizing power of the sediment was greater than the dislodging power of the wave energy. However, at the site with high‐level sedimentary intensity, the excessive sediment converted to the burying stress power as seedlings developed from the corms, revealing a burial threshold for seedling survival. The risk of seedling establishment was high when the burying stress power of the sediment far outweighed the combination of the growing power of the seedlings and the sediment removal power of the tidal current and surpassed the tipping point of vegetation die‐off. Additionally, we checked the practice cost of the different approaches to ensure a highly cost‐effective revegetation planning based on site suitability. This study highlights that understanding of the propagule–sediment–tide power balance offers a tool for improvement of the revegetation and management of site‐specific sedimentary and hydrological environments for many degraded coastal ecosystems.

Need to Seed? Ecological, Genetic, and Evolutionary Keys to Seed-Based Wetland Restoration

“…Temperature, light, and moisture are primary drivers of germination and establishment; salinity, pH, alkalinity, and substrate type and texture can also be important (Ignacio Galinato and van der Valk, 1986;Shipley et al, 1989;Shipley and Parent, 1991;Coops and van der Velde, 1995;Mahoney and Rood, 1998;Borkenhagen and Cooper, 2019;Prach et al, 2019;Rosbakh et al, 2020). In wetlands that are geomorphically dynamic (i.e., riparian and floodplain wetlands, salt marshes, estuarine wetlands, mangroves), accretion and erosion dynamics with tides (daily to multi-year cycles), currents, and flooding can also limit seed germination and seedling survival (Giroux and Bédard, 1995;Mahoney and Rood, 1998;Balke et al, 2013;Ge et al, 2019).…”

A global meta-analysis on the drivers of salt marsh planting success and implications for ecosystem services

“…Planted transplants were particularly sensitive to the species used and multiple stresses including high erosion or sediment deposition, nutrient stresses and animal activity (e.g., grazing) (Supplementary Table 2 ). Planting projects or field experiments to improve survival and growth used a range of techniques, including planting in clumps or at high density 46 , 47 , transplanting cores or plugs 28 , transplanting within structures mimicking emergent traits 48 , co-transplanting with mussels 49 , species-rich planting 50 , planting with fertilization 51 , among others (Fig. 2f ; Supplementary Table 3 ).…”

“…It is clear that planting in salt marshes is often challenging, with many projects exhibiting low establishment success and high mortality 28 , 29 . Thus, there is an urgent need to understand what elements contribute to overall planting success.…”

Using Propagules to Restore Coastal Marine Ecosystems

“…Germination success was improved by preparing the substrate through weed removal (Broome et al, 1988), burning and mechanical tillage (US Environmental Protection Agency, 1979) and by investing in activities such as fencing to exclude animals. Although there have been several recent studies comparing the survival rates of corm shoots and rhizomes with larger transplantation units such as tussocks (Ge et al, 2019;Zhang et al, 2020), these do not fit our definition of propagule. Seed dispersal distances for some tidal marsh species can be limited (e.g., only a few meters for Spartina), and lateral expansion of clonal fragments is restricted to existing marsh, so manual seeding might help to restore isolated areas where naturally dispersed seeds and clonal growth would not otherwise reach.…”