Patch-level processes of vegetation underlying site-level restoration patterns in a megatidal salt marsh

Abstract

Vegetation patterns during salt marsh restoration reflect underlying processes related to colonization, reproduction, and interactions of halotolerant plants. Examining both pattern and process during recovery is valuable for understanding and managing salt marsh restoration projects. We present a decade of vegetation dynamics during salt marsh restoration (2011–2020) at a study site in the Bay of Fundy with megatidal amplitudes, strong currents, cold winter temperatures, and ice. We mainly investigated reproduction (asexual and sexual) and associated spread rates of Spartina grasses, and their health-related states (stem density, canopy height, and percent flowering) which help inform the probability of processes occurring. We also estimated modes of colonization and began quantifying the effects of interspecific interactions and environmental conditions on plant state. Spartina pectinata was the only pastureland plant to survive dike-breaching and saltwater intrusion in 2010; however, it was stunted compared to reference plants. Spartina pectinata patches remained consistent initially, before decreasing in size, and disappearing by the fifth year (2015). This early dynamic may provide initial protection to a developing salt marsh before Spartina alterniflora becomes established. Spartina alterniflora first colonized the sites in year 2 (2012), likely via deposition of rhizomal material, and then spread asexually before seedlings (sexual reproduction) appeared in year 4 (2014). Vegetation cover subsequently increased greatly until near-complete in year 9 (2019). The early successional dynamics of S. pectinata and S. alterniflora occurred spatially independently of each other, and likely contributed to sediment retention, creating an improved environment for S. patens, the dominant high marsh species in our region. Spartina patens have been slowly spreading into restoration sites from high elevation areas since year 6 (2016). We expect that competition between S. alterniflora and S. patens will result in the typical distinct zonation between high and low marsh zones. A next study will use the quantified processes for spatial-explicit modeling to simulate patterns of vegetation recovery, and to evaluate different salt marsh restoration strategies for the Bay of Fundy and elsewhere. Thus, proper identification and quantification of pattern-building processes in salt marsh vegetation recovery, the focus of our present study, was an essential step.

Comparing Pixel- and Object-Based Approaches for Classifying Multispectral Drone Imagery of a Salt Marsh Restoration and Reference Site

“…Semi-diurnal tidal amplitudes in the Cumberland Basin reach more than 12 m [57]. A managed realignment salt marsh restoration project began here in 2009 under the leadership of Ducks Unlimited Canada (DUC) and partners [23,[58][59][60]. The project consists of two restoration (B, C) and two reference (A, D) sites; the study areas of focus for the present paper were the Western reference and restoration sites (A and B, Figure 1).…”

“…Sporobolus alterniflorus; [61,62]) and Spartina patens (salt marsh hay, syn. Sporobolus pumilus), which are central during restoration [23,60]. We used imagery from when the restoration was in its 11th year (2021) after dike breach, and the low-elevation bioengineer species of salt marshes in eastern North America, S. alterniflora, dominated the site.…”

“…), sea milkwort (Lysimachia maritima), seaside plantain (Plantago maritima), seaside goldenrod (Solidago sempervirens), and sea glasswort (Salicornia maritima) [59]; hereafter, we refer to these various plants by their genus names, except for the Spartina grasses. The phenological growing period of vegetation in these marshes is from early June, when above-ground biomass first emerges from below-ground roots and rhizomes, to late September, when plants begin to senesce [23]. above-ground biomass first emerges from below-ground roots and rhizomes, to late September, when plants begin to senesce [23].…”

“…The phenological growing period of vegetation in these marshes is from early June, when above-ground biomass first emerges from below-ground roots and rhizomes, to late September, when plants begin to senesce [23]. above-ground biomass first emerges from below-ground roots and rhizomes, to late September, when plants begin to senesce [23].…”

“…Major efforts are being made to restore wetlands and the beneficial services they provide [18][19][20][21]. Understanding vegetation dynamics during wetland restoration can aid in assessing and modeling the recovery trajectory, evaluating ecosystem services, and planning future restoration projects [22,23].…”