Coastal salt marshes are at 50% of their historical cover and threatened by sea-level rise. Salt marsh resilience is key to their future survival, but the mechanisms of resilience are poorly understood. This thesis explored patterns and drivers of marsh erosion and expansion from patch scale (a few meters) to geographical scales across the United Kingdom, focusing on changes at the seaward edges of marshes where marsh erosion or expansion is known to take place. Resilience of salt marshes is a product of environmental context and marsh bio-physical properties that collectively govern feedback mechanisms between vegetation, sediments and hydrological forcing. The relative importance of these factors to marsh change is likely to vary depending on the spatial scale of the study, although this principle has not been addressed to date. This thesis (1) investigated the intrinsic marsh bio-physical traits that underpin salt marsh resilience, (2) quantified resilience of marshes across different environmental contexts, and (3) explored whether spatial variation in resilience is explained by local to large-scale differences in external forcing and internal resilience traits. Three experimental chapters combined observational and experimental approaches at the patch to geographical scale across 1 to 20 salt marshes in the UK.
Chapter 2 investigated how bio-physical feedbacks between vegetation density, sediment vertical accretion and wave forcing interact to affect plant survival and patch lateral expansion. Vegetation density has feedback effects on sediment accretion and thus patch growth, but it is not known how feedbacks are affected by variation in wave forcing. This study planted out 3 levels of vegetation density across 3 levels of wave forcing, to test how bio-physical feedbacks depended on density-force interactions. The results showed vegetation density interacted with wave forcing to impact on plant survival, growth and lateral expansion. At the wave-exposed site, plant survival was highest inside dense patches, as plant density ameliorated erosive forcing; yet the diversion of water generated erosion gullies at the patch perimeter that prevented lateral patch expansion. The wave-sheltered site had no gully formation around dense patches, but plant competition had a negative effect on patch survival. This study shows that plant interactions can switch from positive to negative across stress gradients, according to the stress-gradient hypothesis. Furthermore, the study demonstrates that bio-physical processes occurring at the small, patch-scale have the potential to influence larger, landscape-scale patterns of marsh resilience. Plant interactions across erosive gradients should be considered in future restoration planting designs to increase marsh growth success. For example, salt marsh locations with higher levels of erosive forcing might require moderate vegetation density to permit resilience and growth at the patch scale.
Chapter 3 investigated the drivers and bio-physical properties of salt marsh resilience at a geographical, cross-UK scale. There is indication that marsh down-shore extent varies geographically, from north-west to south-east regions of the UK, potentially indicating that marsh resilience to erosion varies systematically over the same scales. Yet, there is limited empirical evidence to support this, and thus the causes for these large-scale geographical patterns are unclear. Marsh down-shore extent is the degree to which the lower marsh edge protrudes into the intertidal and is a product of the strength of external hydrological forcing balanced against the intrinsic properties of the marsh that enable it to withstand erosion. By observing patterns of down-shore extents of marshes across six UK regions, Chapter 3 aimed to identify which environmental contexts and resilience traits best explained the observed patterns in resilience. The results showed that wave forcing explained the variation in down-shore extents across the UK as marshes in the south-east extended further down-shore than marshes in the north-west, where forcing was greatest. At more local scales (i.e. within the same region), intrinsic marsh properties such as sediment and vegetation characteristics became more important in promoting marsh resilience by acting to increase erosion resistance. The study confirmed that marsh resilience can vary over large- geographical scales as a product of large-scale external drivers such as wave forcing, which increases from south-east to north-west regions of the UK. Despite large-scale drivers regulating patterns of marsh resilience across the UK, intrinsic sediment and vegetation properties mediated cliff erosion to increase marsh resilience at more local, regional scales. The study highlights the importance of considering the scale at which resilience is observed before making assessments of salt marsh resilience.
Chapter 4 tested marsh resilience directly by observing how vegetation recovery to experimental disturbance depended on large-scale variation in environmental context and marsh-intrinsic properties across the UK, as large-scale variation in marsh resilience and their underlying mechanistic causes are imperative to marsh conservation and restoration, but are poorly understood. Chapter 4 aimed to determine which environmental gradients and bio-physical properties, which varied systematically across the UK, best explained variation in vegetation recovery after disturbance. The study experimentally disturbed above- and below-ground patches of salt marsh vegetation in twelve salt marsh locations of six regions in the UK, to test how vegetation recovery was affected by environmental contexts. In general, marshes in the south-east recovered better than marshes in the north-west. Variation in marsh recovery was explained by a combination of temperature and intrinsic marsh properties, including sediment characteristics and above-ground vegetation biomass. These variables varied systematically with a latitudinal gradient from the north-west to the south-east, which shows that marshes have geographical resilience contexts that are driven by large-scale variation in climate and geology. Large-scale variation in resilience is likely to be a common trend across other ecosystems, and although the intrinsic bio-physical properties behind resilience contexts will be system specific, climate is likely to be a common driver.
Overall, the thesis demonstrates how system inherent properties are key to understanding small to large-scale variation in salt marsh resilience to erosive forcing. It showed that resilience varied systematically across latitudinal gradients in the UK, and that smaller-scale patterns at patch (tussock) and local (regional) scales should not be overlooked because they can affect larger scale patterns of marsh change. This thesis emphasises that marsh resilience is specific to the temporal and spatial scales at which we observe it, and in a time of climatic uncertainty, it addresses a pressing need to understand what increases the vulnerability of salt marshes to disturbance. This thesis suggests that by observing the drivers and properties of resilience across different scales and contexts, we might be able to gain useful insights into understanding the mechanisms of resilience in salt marshes and other ecosystems.