## 1 Introduction  
Salt marshes are unique coastal ecosystems that arise in sea-to-land interfaces, usually in low-energy tidal environments [1; 2] that collectively support abundant habitats and provide countless valuable ecosystem services [3]. For instance, as exceedingly productive ecosystems, salt marshes are of extreme importance for coastal web foods [3] and partake a major role in climate regulation as they act as carbon reservoirs [4; 5; 6]. Moreover, they also bear a vital role both in water quality improvement and nutrient cycling as they are able to retain and filter several components of the ecological chain, such as excess nutrients, contaminants, and microbes [3; 7; 8]. Additionally, they assure important coastal protection [3; 9] due to their ability to effectively provide wave/floodwater attenuation and shoreline stabilization [10; 11; 12]. However, over the last decades, coastal marshes' ecological processes have been severely impaired [3; 8]. Salt marsh area has been and still is being reduced at alarming rates, mostly as a repercussion of ongoing detrimental human pressures and activities [2; 3; 13] to which, more recently, major concerns over salt marshes' vulnerability to climate change have been added [14; 15]. Given their lower elevations and regular exposure to the action of tidal waves, many argue that salt marshes are particularly threatened by the current accelerated sea level rise (SLR) rates, which often reach values greater than those under which they developed [13; 14; 16; 17]. Indeed, salt marsh ecology is strongly tied to tidal dynamics. Tides influence nearly every aspect of these ecosystems, from the initial stages of colonization by biofilms and microphotothos [1; 2; 18] to the determination of suitable conditions for the establishment of pioneer vascular plants [1; 2; 19], to even habitat delimitation and maintenance [2; 20; 21; 22]. Additionally, tidal flow, along with freshwater runoff and groundwater inputs, command the supply of water, sediments, and nutrients, in addition to the regulation of salinity and oxygen gradients, ultimately impacting plant settlement, composition, and productivity [1; 8; 13; 23]. Similarly, vascular plants, once established, also undertake a vital role in salt marsh dynamics as they improve environmental conditions that further promote the growth of denser vegetation that continuously stimulates the feedback of more accretion, aeration, dampening, stabilization, and the establishment of new less tolerant species [8; 24; 25]. Soon enough, pioneer species start giving place to others, following an ecological successional course in the early stages [8; 18]. As the salt marsh matures, zonation patterns parallel to the shore start to unravel, splitting the salt marsh into low, mid, and upper regions [2; 26; 27], as plant communities assemble in patches following a gradient of zonation from low to higher marsh zones [1; 8; 28]. Broad zonation patterns have been observed and described globally [29], most of them revealing past succession events defined by physical processes and environmental gradients [2; 3; 18; 28; 30]. The clear influence of hydrology on salt marshes' habitat delimitation has led some authors to consider this mechanism as the primary factor regulating salt marshes' composition and function [14]. However, in recent decades, several studies demonstrated that community structure may also reflect the influence of biotic factors and that ecological interactions, such as competition and/or facilitation, bear a key role in zonation as well [3; 8; 31]. Alternatively, a new line of thought emerged, bringing consensus to a more flexible perspective that endorses different zonation constraints at different spatial levels. As reviewed by [PERSON] and [PERSON] [32], abiotic factors regulating zonation at lower borders can consist of (1) elevation; (2) frequency of flooding; (3) salinity; (4) redox potential; (5) ionic composition and soil pH; (6) soil moisture content; (7) topography; (8) latitude; and (9) climate.