Spatial and temporal patterns of initial plant establishment in salt marsh communities

Abstract

Questions How are dispersal processes, abiotic and biotic interactions determining the initial salt marsh plant community establishment and development when connectivity is different? We aim to answer this question by analysing the spatial and temporal patterns of plant establishment along an environmental gradient at two connectivity settings. Location Back‐barrier salt marsh and tidal flats of Spiekeroog, northwest Germany. Methods We established an experiment along a saltmarsh elevation gradient with bare sediment open for spontaneous colonisation on the natural salt marsh and on the experimental salt marsh islands built on the tidal flats approximately 500 m from the natural salt marsh for low connectivity. Plant establishment was identified from georeferenced photos at least monthly. Results Experimental islands as low‐connectivity plots had limited colonisation by annual halophytes that produced large number of small seeds. Number of individuals increased with higher connectivity at patches enclosed by salt marsh. Number of individuals was highest at the mid elevations whereas peak species richness was at the upper salt marsh. Temporal patterns of seedling emergence showed increasing plant numbers until the end of April followed by a gradual decline over the season at the pioneer and lower salt marsh zones. Upper elevations on the other hand had a stable number of low individual counts over time. Spatial clustering of plant individuals indicating possible facilitation was important at the initial stages of salt marsh development at pioneer and lower salt marsh elevation, but only early in the season. Conclusions Stochastic patterns of early salt marsh colonisation indicated that success of species colonisation was determined by seed properties, seed availability and environmental conditions mediated by elevation. We found indications that at the initial stages, further colonisation was supported by already colonised plants but shifted to avoidance later in the season.

The Spiekeroog Coastal Observatory: A Scientific Infrastructure at the Land-Sea Transition Zone (Southern North Sea)

“…Using these data, it could be shown that highest species diversity and fine root biomass can be found at the intermediate inundation frequency and zonation of salt marsh vegetation reflects the availability of water sources along the inundation gradient (Redelstein et al, 2018a,b). Similarly, the initial colonization of vascular plants on salt marshes depended on the environmental conditions mediated by the elevation above mean sea level with most individuals colonizing the mid elevations, but higher elevations supporting larger species richness of initial communities (Lõhmus et al, 2020).…”

The importance of dispersal and species establishment in vegetation dynamics and resilience

“…We highlight the need to distinguish between the adult niche and the regeneration niche to understand better how the abiotic filter works (Del Vecchio et al, 2020, this issue; Lõhmus et al, 2020, this issue). The difference between the adult niche and the regeneration niche (Grubb, 1977) determines whether under certain environmental conditions, a species is capable of generative reproduction or only of surviving and possibly of vegetative reproduction.…”

A Minimalist Model of Salt-Marsh Vegetation Dynamics Driven by Species Competition and Dispersal

“…The encroachment of pioneer halophytes capable of thriving in highly hypoxic and hypersaline soils facilitates sediment accumulation, ameliorate physical stresses, and promote further marsh growth thanks to the ability of these critical foundation species (sensu Dayton, 1975) to eco-engineer their habitats (Ranwell, 1972;Jones et al, 1994;Bruno, 2000;Bruno and Kennedy, 2000;Crain and Bertness, 2006;Morris, 2007;Vandenbruwaene et al, 2011;Da Lio et al, 2013;Marani et al, 2013;Lõhmus et al, 2020;van Regteren et al, 2020). Once foundation species are established, a network of emergent vegetation organized into recurrent zonation patterns develops along the gradients of elevation and salinity that characterize tidal wetlands (Pennings and Callaway, 1992;Belluco et al, 2006;Marani et al, 2006) (Figures 1A,B).…”

“…Finally, it is often overlooked that biological dispersal and competition in tidal marshes are density-dependent processes, i.e., they are a function of local population size, as well as habitat quality and size (Bertness and Ellison, 1987;Ellison, 1987;Bouma et al, 2009;Balke et al, 2017;Lõhmus et al, 2020;Bertassello et al, 2021).…”

“…Moreover, we run exploratory numerical simulations aimed at assessing model performances in replicating results of ecological experiments involving the transplant of species outside their realized ecological niches. Even though the model can be easily extended to investigate morphological shifts from tidal flats to salt marshes driven by the colonization of pioneers species, here we do not focus on initial vegetation establishment (e.g., Balke et al, 2017;Brückner et al, 2019;Lõhmus et al, 2020) because our interest is in the medium to long-term outcomes of vegetation dynamics in mature marshes. Nonetheless, a discussion on how the initial vegetation distribution affects model results will be provided later in the text.…”