Detection of seasonal changes in vegetation and morphology on coastal salt marshes using terrestrial laser scanning

Highlights

• TLS enables the detection of centimeter-scale marsh vegetation distribution.

• Repeated TLS surveys reveal seasonal changes in vegetation and morphology.

• Sediment supply overrules vegetation in controlling the evolution of sparse-plant marsh.

Abstract

Coastal salt marshes exhibit strong seasonal variations that may induce subsequent changes in their morphology. However, such changes are insufficiently understood due to a lack of data at seasonal timescales. Knowledge of the seasonal variability is essential for coastal management and ecosystem conservation. Terrestrial laser scanning (TLS) technology allows the rapid acquisition of high-resolution and large-scale vegetation and morphology data with an error of <3 cm. In this study, we used a TLS system to retrieve high-accuracy digital canopy models (DCM) for low vegetation species (approximately 20 cm) Suaeda salsa and digital terrain models (DTM) of a marsh in the Yellow River Delta. We conducted three TLS surveys on the same field on January 5, April 21, and November 20, 2019, which spanned from the dry to wet seasons. The centimeter-scale TLS data retrieved the seasonal variations of vegetation and elevation distributions. The DCMs of the study area showed that the percentages of vegetation-occupied regions were 13.5% and 22.5% in the dry and wet seasons, respectively. The DTMs indicated a net erosion of the study area throughout the year, but at a larger erosion rate in the dry than the wet season, that is, a rate of 2.5 and 0.2 cm/month, respectively. Spatially, regions with vegetation and zones adjacent to creeks underwent slight deposition. We inferred that the larger riverine sediment supply and higher vegetation density in the wet (summer) season mitigated erosion, while the influence of rich sediment supply was crucial. This work demonstrates the applicability of TLS in detecting the distribution of low-height vegetation species, and quantifying vegetation and morphological changes of marshes at seasonal timescales owing to its high resolution.

Introduction

Coastal salt marshes are not only one of the most productive ecosystems but are also ideal for monitoring biological responses to global climate change (Kirwan and Megonigal, 2013; Schuerch et al., 2018; Wiberg et al., 2020). Salt marshes have the natural capacity to dissipate waves and storm surges, and to keep up with sea level rise by natural accumulation of biogenic and mineral sediments (Friedrichs and Perry, 2001; Fagherazzi, 2014; D'Alpaos and Marani, 2016; Leonardi et al., 2018). However, in recent decades, salt marshes have been facing challenges worldwide due to sea level rise, fluvial sediment reduction, and the human development of coastal areas (Fagherazzi et al., 2012; Nienhuis et al., 2020). Large salt marsh losses have been documented globally, including in the Yangtze Estuary (Yang et al., 2011), the southwest of the Wadden Sea (Bakker et al., 1993), the UK coast (Foster et al., 2013), the Virginia Coast Reserve (Leonardi and Fagherazzi, 2015), and the Venice Lagoon (Bendoni et al., 2016). Fan et al. (2018) and Zhang et al. (2018) demonstrated that the tidal flats in the Yellow River Delta have retreated landward and encroached into adjacent salt marshes due to a shift of the river mouth and severe reduction in fluvial sediment supply. Wang et al. (2006) and Leonardi et al. (2018) showed that salt marshes were eroded in response to wave activities and storms. Further, salt marshes have been altered by seasonal disturbances from riverine sediment and plant growth (Temmerman et al., 2004; Bouma et al., 2013; Xie et al., 2018).

High-resolution data covering large areas on the order of kilometers are required to adequately understand salt marsh vegetation and morphological changes (Davis et al., 2017). However, given that the magnitude of seasonal changes on salt marshes is on the order of centimeters to decimeters (Xie et al., 2018), studies describing the three-dimensional vegetation and morphology features across salt marshes, are rare due to monitoring difficulty. Data acquisition using traditional methods, such as point-based GPS surveys and plant quadrat surveys, is cost prohibitive and the large spacing between data points limits the accuracy of vegetation and morphological changes (Fuller et al., 2003; Wheaton et al., 2009; Cui et al., 2011; He et al., 2019). Marine radar and remote sensing products provide alternatives for investigating large scale responses of salt marshes to seasonal disturbances (Belluco et al., 2006; Zhang et al., 2016; Bird et al., 2017). However, there are problems surrounding their application in obtaining microtopographic-scale features on salt marshes, such as data resolution limitation, surface elevation estimation, and lack of vegetation height information (Liu et al., 2014; Bell et al., 2016; Farris et al., 2019). Airborne light detection and ranging (LiDAR) technology can overcome the drawbacks of optical imagery by providing direct measurements of ground and vegetation canopy elevations (Zhang et al., 2003; Wang et al., 2009; Hladik et al., 2013). However, owing to an accuracy of approximately 15 cm, LiDAR fails to distinguish centimeter-scale variations between the vegetation canopy and bare ground on estuarine salt marshes (Schmid et al., 2011; Fernandez-Nunez et al., 2017).

Compared to the aforementioned methods, the terrestrial laser scanning (TLS) has advantages including lower costs, less survey time, larger coverage and greater improvement in the resolution of vegetation and geographic data (Guarnieri et al., 2009; Fabbri et al., 2017; Xie et al., 2017). As TLS can provide centimeter-level accuracy, it is emerging as the ideal method to rapidly detect the detailed vegetation and morphological changes of salt marshes. Although TLS has been employed to investigate salt marshes on the North Adriatic Sea coast (Fabbri et al., 2017), the Venice lagoon (Guarnieri et al., 2009), and the Yangtze Estuary (Xie et al., 2017, Xie et al., 2018), there are only a few studies on the benefits of TLSs in quantifying seasonal variations in salt marsh vegetation and morphology.

In estuarine salt marshes, morphological changes are substantially influenced by riverine sediments, which vary during different seasons. As large amounts of riverine sediments are transported into estuaries, the rates of vertical sediment accretion and lateral expansion of estuarine salt marshes increase (van Proosdij et al., 2006; Yang et al., 2008). Meanwhile, interactions between plant growth and morphological changes over salt marshes are increasingly recognized to have strong impacts on the ecological regime and morphological evolution (Friedrichs and Perry, 2001; Fagherazzi, 2014; D'Alpaos and Marani, 2016). Vegetation can reduce flow strength, trap sediment, dissipate short waves, and protect the sediment surface (Le Hir et al., 2007; Li and Yang, 2009; Bouma et al., 2014). Moreover, the modified flow patterns and the changed landforms of salt-marshes and tidal creeks can impact the spatial patterns of vegetation (Temmerman et al., 2007; Murray et al., 2008; Vandenbruwaene et al., 2015). The seasonal variations of vegetation structures observed in salt marshes greatly influence marsh evolution. Therefore, it is important to document both vegetation and morphology changes derived from seasonal variations to understand estuarine salt marsh dynamics.

The salt marshes in the Yellow River Delta, which are impacted both by seasonal plant growth and riverine sediment supplies, provide ideal testing sites for using TLS technology to examine the seasonal eco-morphological changes in estuarine wetlands (Fig. 1). Therefore, the objective of this research was (1) to obtain large-scale and high-resolution vegetation distributions and morphologies of salt marshes using TLS measurements, (2) to quantify the seasonality of vegetation change and the morphological evolution in salt marshes, and (3) to compare the roles of plant growth and riverine sediment in shaping the evolution of salt marshes.