Coastal Landform Constrains Dispersal in Mangroves

Abstract

Mangrove forests are dynamic ecosystems found along low-lying coastal plains along tropical, subtropical, and some warm-temperate coasts, predominantly on tidal flats fringing deltas, estuaries, bays, and oceanic atolls. These landforms present varied hydrodynamic and geomorphological settings for mangroves to persist and could influence the extent of within-site propagule transport and subsequent local regeneration. In this study, we examined how different landform characteristics may influence local genetic diversity, kinship, and neighborhood structure of mangrove populations. To do so, we considered independent populations of Avicennia marina, one of the most abundant and widespread mangrove species, located in estuarine and coastal bay environments spread across the Western Indian Ocean region. A transect approach was considered to estimate kinship-based fine-scale spatial genetic structure using 15 polymorphic microsatellite markers in 475 adult A. marina trees from 14 populations. Elevated kinship values and significant fine-scale structure up to 30, 60, or 90 m distances were detected in sheltered systems void of river discharge, suggesting a setting suitable for very local propagule retention and establishment within a neighborhood. Slopes of a linear regression over restricted distance within 150 m were significantly declining in each sheltered transect. Contrastingly, such a spatial structure has not been detected for A. marina transects bordering rivers in the estuarine systems considered, or alongside partially sheltered creeks, suggesting that recruitment here is governed by unrelated carried-away mixed-origin propagules. South African populations showed strong inbreeding levels. In general, we have shown that A. marina populations can locally experience different modes of propagule movement, explained from their position in different coastal landforms. Thus, the resilience of mangroves through natural regeneration is achieved by different responses in coastal landforms characterized by different hydrodynamic conditions, which can be important information for their management and protection within the variety of coastal environments.

Avicennia Genetic Diversity and Fine-Scaled Structure Influenced by Coastal Proximity of Mangrove Fragments

“…The restriction on successful dispersal of Avicennia propagules may be attributed to retention of propagules in the pneumatophores of the trees themselves, however low or absent tidal currents of fragmented mangrove patches are primary causes of leaving a trace of elevated kinship values. Such FSGS traces in other studies also estimated within spatial stretches of a few meters up to several hundreds of meters (Mori et al, 2015;Do et al, 2019;Chablé Iuit et al, 2020;Triest et al, 2020) and when populations are sheltered (Triest and Van der Stocken, 2021) or severely fragmented and confined within artificial dikes (Hasan et al, 2018), these kinship values may become enhanced. Rhizophora mangle showed a fine-scale spatial genetic structure up to 90 m in different hydrological estuarine conditions of Caribbean mangroves (Yucatan, Mexico), although up to 240 m along a river (Chablé Iuit et al, 2020).…”

“…Tidal currents and wind action allow mangrove propagules to disperse and settle in various habitats that range from most seaward protruding open systems, over mudflat areas, along dynamic riverbank systems and up to farthest inland sheltered systems that seldom experience high tide or spring tide. Spontaneous processes of propagule dispersal and formation of a mangrove vegetation are expected to result in a different neighborhood size of individual trees in e.g., exposed seaward versus sheltered landward positioned populations (Triest and Van der Stocken, 2021), depending on river flow (Ngeve et al, 2017;Chablé Iuit et al, 2020), channel structures (Triest et al, 2020), or degree of fragmentation (Hasan et al, 2018). For example, Rhizophora mangle L. along a river showed a fine-scaled genetic structure that did became extended (Chablé Iuit et al, 2020) most likely from onward carried-away propagules alongside the bank.…”

“…Fine-scaled genetic structure of populations thus can be indicative of establishment events that happened during most recent generations and therefore, a comparison of mangrove patches in different positions may help in understanding whether local sheltered conditions effectively did promote internal dispersal and allowed cohorts of related propagules to settle within a neighborhood, still detectable from their genetic relatedness (Triest and Van der Stocken, 2021). Alternatively, absence of any fine-scale structure points at a more dynamic system within which mangroves of mixed kinship or unrelatedness became settled.…”

“…Their zonal regeneration primarily depends on the extent of propagule spread and on subsequent establishment in suitable habitats either within close vicinity or beyond. Suffice prior knowledge of polymorphic genetic markers is available of the geographically widespread A. marina and related species (Triest, 2008) to allow resolution within an estuary and at local fine-scale level (Hasan et al,2018;Do et al, 2019;Triest et al, 2020;Triest and Van der Stocken, 2021).…”

Genotypes of Rhizophora Propagules From a Non-mangrove Beach Provide Evidence of Recent Long-Distance Dispersal

“…One phenomenon unifies all these methods: a leptokurtic dispersal kernel describing patterns of mangrove propagule dispersal (Van der Stocken et al, 2019b), where the number of short-distance dispersal (SDD) events far exceeds the number of long-distance dispersal (LDD) events which occur rarely (see review by Van der Stocken et al, 2019b). Yet, LDD events are crucial; e.g., ancient LDD and vicariance explain contemporary distribution of mangrove genera (e.g., Triest, 2008;Lo et al, 2014;Takayama et al, 2021;Triest and Van der Stocken, 2021;Triest et al, 2021a,c). However, more frequent short distance dispersal events are notable for maintaining the connectivity of populations at regional to fine spatial scales (Ngeve et al, 2017a,b;Triest et al, 2020Triest et al, , 2021a, and contributing to range expansions (Saintilan et al, 2014;Giri and Long, 2016;Kennedy et al, 2020;Triest et al, 2021b).…”