Long-term invasion dynamics of Spartina increase vegetation diversity and geomorphological resistance of salt marshes against sea level rise

Abstract

The cordgrass Spartina anglica C.E. Hubbard (Poaceae) is an invasive transformer in many salt marsh ecosystems worldwide. Relatively little is known about the capacity of Spartina to accelerate salt marsh succession and to protect salt marshes against sea level rise. We analyzed long-term changes in vegetation and elevation in mainland salt marshes of the European Wadden Sea in Schleswig-Holstein, Germany, to estimate the impact of non-native Spartina on the geomorphological resistance of salt marshes to sea level rise and on changes in species diversity. From 1989 to 2019, the Spartina-zone shifted and expanded upwards to elevations of the high marsh zone and Spartina increased in frequency in several salt marsh vegetation communities. At sites where Spartina dominated the vegetation already three decades ago, elevation and species diversity increased with a higher rate compared to sites lacking Spartina. The median change rates reached for elevation MHT +8.6 versus +1.5 mm per year, for species richness +3 versus $$\pm$$ ± 0 species per three decades, and for evenness +0.04 versus −0.08 per three decades, regarding plots with versus without former Spartina dominance, respectively. Invasion of salt marshes by Spartina and its continued, long-term presence were associated with increased elevation and species diversity in the face of sea level rise.

Subsurface aeration of tidal wetland soils: Root-system structure and aerenchyma connectivity in Spartina (Poaceae)

“…Elevation mean high tide (MHT) was referenced against the German Ordnance Datum and regional mean high tide gauge as described in Granse et al (2020).…”

“…The hybrids S.  townsendii and S. anglica inhabit a wide variety of habitats and marsh zones (e.g., Gray et al, 1991;Hacker et al, 2001;Granse et al, 2020) in-and outside the parental range (e.g., Maricle et al, 2006;Wong et al, 2018;Proença et al, 2019, Granse et al, accepted). S. anglica and parental S. alterniflora have gained attention as invasive species which spread in coastal habitats around the globe (e.g., Ranwell, 1967;Strong and Ayres, 2013;Bortolus et al, 2015;Shang et al, 2019).…”

The Overlooked Hybrid: Geographic Distribution and Niche Differentiation Between Spartina Cytotypes (Poaceae) in Wadden Sea Salt Marshes

“…From 2017 to 2019, Spartina was additionally sampled along 19 transects in mainland salt marshes at the North Sea coast of Schleswig-Holstein, Germany, hereafter referred to as focus area. In the focus area, the transects were arranged as described in Granse et al (2021). Accounting for differences in tidal range, the focus area was split into six regions (Fig.…”

“…To analyze intercytotypic differences in phenotypes, Spartina cover was recorded on 1029 plots of 1 × 1 m 2 in the focus area. The positions of the plots were the same as the plots described in Granse et al (2021) which are based upon 362 resampled plots from a 30-year-old initial vegetation study (Suchrow and Jensen 2010) and 667 additional plots located in newly developed salt marsh areas. The plots were arranged in clusters at 50-400 m intervals along each transect (see Online Resource, Fig.…”

“…As a basis for analyzing the ecological niche of both Spartina cytotypes along the elevational salt marsh gradient, elevation of all 1122 plots (including 93 uncovered plots) in the focus area was determined by means of a Laser Leveling device (LL500 combined with HL700 receiver by Trimble, CA, USA) and by averaging five elevation measurements per plot as described in Granse et al (2021). The elevation of plots was calculated in relation to nearby benchmarks of the vertical control survey-net of the German height reference system or to permanently installed sedimentation erosion bars.…”