Coastal wetlands are some of the most vulnerable ecosystems on the frontlines of global change (Craft et al. 2009; Schuerch et al. 2018). As hurricanes and drought become more common, and human infrastructure expands (i.e. canal creation, groundwater extraction for irrigation), the influx of seawater further inland into freshwater dominated systems (i.e. saltwater intrusion) has become a more severe environmental stressor (Hoeppner et al. 2008; Manda et al 2014; Zhou et al. 2017; Bhattachan et al. 2018; Herbert et al. 2018). Soil salinization is an increasing threat throughout the world to biologically diverse wetlands that are home to a high number of endemic species which are vulnerable, threatened, or endangered (Herbert et al. 2018; Noss et al. 2015). One of the most dramatic manifestations of salinization is the rapid mortality of trees in freshwater wetlands, leading to the expansion of “ghost forests” (Kirwan and Gedan 2019; Tully et al. 2019; Ury et al. 2021; White et al. 2021). The potential for biodiversity, carbon storage, and plant community change has prompted recent research on the main drivers of changes in freshwater wetlands and brackish marshes (Pezeshki et al. 1990; Pennings et al. 2005; Desantis et al. 2007; Schile et al. 2017; Greene et al. 2019; Taillie et al. 2019; Smart et al. 2020).
Plant species in these coastal landscapes exhibit wide variation in their sensitivity to the magnitude and frequency of salinization (Poulter et al. 2009). Elevated salt exposure causes osmotic and ionic toxicity stress in plants (Kozlowski, 1997), and alters their ability to use available nutrients in the soil (Eallonardo et al. 2013). Increased salt exposure lowers hydraulic conductance, which leads to reducing whole-plant performance (e.g. growth rates), impeding photosynthetic function, causing leaf loss (Chaves et al. 2009; Parida and Das 2005ab), and limiting regeneration by inhibiting seed germination (Krauss 1998, Koslowski 1997). Physiological traits govern plants’ ability to tolerate, or adapt to, increased stress (Munns and Tester 2008), and adaptations for tolerating salinity are typically associated with trade-offs against growth or reproduction, so the relative abundance of salt-adapted plants should be associated with the historical regime of salt exposure in an area.
Mechanisms responsible for the sensitivity of individual plant taxa to salinity are well studied (Parida and Das, 2005; Parihar et al. 2015); however, comprehensive assessments from field studies that use a quantitative approach for understanding the consequences of species-level sensitivity to salinity on community patterns are sparse in the literature. Often, studies are limited to a small number of species, and the metric of sensitivity is largely based on syntheses, or controlled greenhouse experiments that can be limited in scope (Parida and Das, 2005; Negrão et al. 2017). In addition, the majority of prior research has focused on forest overstory trees, or agricultural crop responses to salinity. Although trees and agricultural crops are ecologically and commercially important, the herbaceous dominated communities that make up the ground layer of coastal forested wetlands, and the majority of biomass in fresh and saltwater marshes, support a high proportion of coastal plain plant diversity (Bratton 1976). In addition to being more diverse, ground layer communities are likely to be more responsive to environmental change than overstory trees because of their shorter life span. Changes in the presence, absence, and relative abundance of ground layer plant taxa may thus serve as an indicator of the abiotic conditions that a community is experiencing, and has experienced in the recent past (Osland et al. 2019).
We know plant species respond to salt exposure quite differently; however, one key unknown is whether variability in sensitivity to salt exposure is distributed somewhat continuously across species in an assemblage, or if a substantial fraction of species in a regional pool exhibit shifts in abundance at similar points along a gradient. The latter response would suggest that the system can be described as having a critical threshold, or tipping point (Biesner et al., 2003; Scheffer et al., 2012). Identifying these thresholds can be useful for multiple reasons. First, if we can identify critical thresholds, we can detect areas on the landscape that are particularly likely to undergo transitions, and where management interventions might be necessary. Secondly, if monitoring of environmental conditions is difficult or costly, identifying sets of species that are most likely to shift in abundance as thresholds are crossed, and focusing monitoring on their presence, absence, or abundance can serve as an alternate tool (i.e., biosensor) for monitoring environmental quality and ecosystem health.
An additional challenge of assessing exposure to marine salts is that while topography and elevation determine vulnerability to flooding and salinity, the vulnerability of ecosystems to salinization is more complex. In addition to rising sea levels along the shoreline (Horton et al. 2009; Kemp et al., 2011), marine salts can penetrate inland well above the high tide line via storm surge, mixing during drought events, and groundwater intrusion. (Manda et al. 2014; Herbert et al. 2015). Shoreline ecosystems are continuously exposed to marine salts, while in inland wetlands, ecosystem salinization depends upon the retention of salts which can vary due to soil exchange, evaporative concentration, or incorporation into organic matter (Herbert et al. 2015). Furthermore, marine salts are chemically complex and the various constituent ions are likely to be differentially retained, and have different effects on vegetation (Tully et al. 2019). Plant responses are integrated responses to salt exposure over time, and previous work on plant responses to saline stress in mature trees found that while changes in tree basal area showed similar declines in response to concentrations of chloride, sulfate, sodium, and elevation, changes in composition were more weakly associated with these drivers (Ury et al. 2020). Given the greater diversity of the plant community in the ground layer and its more rapid turnover time, it seems likely that composition will be affected by the components of salinity, but to what degree, and whether responses are consistent across different components needs further investigation.
The aim of this study was to identify whether there are critical thresholds where large changes in ground layer community composition occur in non-tidal coastal wetlands. Here, we address three questions: 1) Is salinity a primary environmental driver of ground layer plant community composition in coastal wetlands,2) Are there critical thresholds of salinity exposure that are associated with shifts in composition, and 3) If we identify thresholds, are community responses consistent across multiple components of salinity?
To address these questions, we combined understory, ground layer vegetation data from multiple sources across the Albemarle-Pamlico Peninsula (APP) in eastern North Carolina. Both vegetation and soils data were used to calculate taxa-specific indicator values using a well-known indicator species analytical approach (Dufrene et al. 2013) within Threshold Indicator Taxa ANalysis (TITAN, Baker and King 2015), a statistical model for detecting thresholds of community change.