Intraspecific Root Trait Variability Along Environmental Gradients Affects Salt Marsh Resistance to Lateral Erosion

Abstract

Recent studies in salt marshes have demonstrated the role of plant roots in sediment stabilisation, and hence the importance of marshes in providing coastal protection. However, the relative role of root traits and environmental factors in controlling sediment stability, and how intraspecific variability of root traits vary within and among marshes, remain poorly understood. In this study, we investigated which root trait(s) drive sediment stability (resistance to lateral erosion) in two marsh species with an important role in coastal protection (Spartina anglica and Atriplex portulacoides) and how the environment affects the expression of these traits. We sampled three marshes along salinity gradients in each of two estuaries in Wales (UK), establishing replicate plots in the respective dominant zones of each species. In all plots we sampled abiotic variables (sand, redox potential, pH, salinity) and root traits (root density, specific root density, root volume, root length density); in a subset of these plots (three per species in each marsh) we extracted soil-plant cores and assessed their erosion resistance in a flume. Sediment stability was enhanced by increases in root density and reductions in sand content. Abiotic variables affected root density in different ways depending on species: in S. anglica, redox was the only significant factor, with a positive, linear effect on root density; in A. portulacoides, redox had a non-linear (U-shaped) effect on root density, while sand had a negative effect. Collectively, these results show that (i) intraspecific variability in root density can influence sediment stability in salt marshes, and (ii) sediment properties not only influence sediment stability directly, but also indirectly via root density. These results shed light on spatial variability in the stability of salt marshes to lateral erosion and suggest that root density should be incorporated into coastal vegetation monitoring programs as an easy-to-measure root trait that links the environment to sediment stability and hence to the function and services provided by marshes.

Multiple trait dimensions mediate stress gradient effects on plant biomass allocation, with implications for coastal ecosystem services

“…Furthermore, in well-aerated sediments (high redox) plants would produce many coarse roots (third dimension), allowing an higher soil exploration Garnier et al, 2016) with an associated investment in ABV biomass. Previous studies demonstrated the capacity of BLW biomass to stabilize the sediment (De Battisti et al, 2019;Ford et al, 2016;Lo et al, 2017) and that of ABV biomass to attenuate wave energy (Bouma et al, 2010(Bouma et al, , 2013Möller et al, 2014;Möller & Spencer, 2002). Therefore, our study suggests that, at estuary mouths (high salinity), S. anglica plants shift towards the conservative side of the LES, investing more in BLW biomass and possibly enhancing sediment stability ( Figure 5).…”

“…This biomass decrease along salinity gradients plays a critical role for geomorphological changes in salt marshes (Bouma et al, 2009(Bouma et al, , 2010(Bouma et al, , 2013. Moreover, low redox potential in the sediment (a proxy for waterlogging) can affect root production, morphology and biomass, which plays a significant role in below-ground carbon storage and sediment stabilization (Bouma, Nielsen, Hal, & Koutstaal, 2001;De Battisti et al, 2019;Ford et al, 2016;Wang et al, 2017). Thus, changes in traits that reduce both ABV and BLW biomass production could be detrimental for several ecosystem functions and services.…”

Mimicry of emergent traits amplifies coastal restoration success

“…and expansion of otherwise vulnerable transplants were much higher. By simulating root mats using belowground establishment structures, sediments were stabilized similar to what is observed in natural matured patches 47,48 . This, in turn, enhanced both cordgrass and seagrass survival, as well as seagrass growth.…”

Salt Marsh Dynamics in a Period of Accelerated Sea Level Rise

“…Salt marshes develop near sea level in the presence of favorable conditions for plant growth and sufficient sediment supply (e.g., Allen, 2000; Bakker & De Vries, 1992; Fagherazzi et al, 2012; Ganju et al, 2015). Once established, vegetation can accumulate large amounts of sediments that promote the formation of a marsh platform, the stability of which is increased by the shear strength of plant roots (e.g., De Battisti et al, 2019). Salt marshes represent areas of high biological productivity and also provide a natural barrier against the effects of sea level rise and storms (Langley et al, 2009; Smith et al, 2015).…”