Interaction between Water and Wind as a Driver of Passive Dispersal in Mangroves

Abstract

Although knowledge on dispersal patterns is essential for predicting long-term population dynamics, critical information on the modalities of passive dispersal and potential interactions between vectors is often missing. Here, we use mangrove propagules with a wide variety of morphologies to investigate the interaction between water and wind as a driver of passive dispersal. We imposed 16 combinations of wind and hydrodynamic conditions in a flume tank, using propagules of six important mangrove species (and genera), resulting in a set of dispersal morphologies that covers most variation present in mangrove propagules worldwide. Additionally, we discussed the broader implications of the outcome of this flume study on the potential of long distance dispersal for mangrove propagules in nature, applying a conceptual model to a natural mangrove system in Gazi Bay (Kenya). Overall, the effect of wind on dispersal depended on propagule density (g l-1). The low-density Heritiera littoralis propagules were most affected by wind, while the high-density vertically floating propagules of Ceriops tagal and Bruguiera gymnorrhiza were least affected. Avicennia marina, and horizontally floating Rhizophora mucronata and C. tagal propagules behaved similarly. Morphological propagule traits, such as the dorsal sail of H. littoralis, explained another part of the interspecific differences. Within species, differences in dispersal velocities can be explained by differences in density and for H. littoralis also by variations in the shape of the dorsal sail. Our conceptual model illustrates that different propagule types have a different likelihood of reaching the open ocean depending on prevailing water and wind currents. Results suggest that in open water, propagule traits (density, morphology, and floating orientation) appear to determine the effect of water and wind currents on dispersal dynamics. This has important implications for inter- and intraspecific variation in dispersal patterns and the likelihood of reaching suitable habitat patches within a propagule's viable period.

A general framework for propagule dispersal in mangroves

“…The second stage, the dispersal or transfer stage, covers the movement of propagules from one site to another. This stage is governed by fluid dynamics with dispersal trajectories being modulated by tides and tidal flow, the effect of near‐shore, coastal, and open‐ocean currents, as well as wind (Rabinowitz, ; Di Nitto et al , ; Van der Stocken et al , ). Propagules can disperse within the same population [short‐distance dispersal (SDD)], to nearby or distant populations [long‐distance dispersal (LDD)], or between a mangrove stand and a new location that lacks established mangroves.…”

“…Given that water and wind currents vary in time, the actual distance of dispersal and the potential of a stranded propagule to establish in a certain area will depend on the time of propagule release (Van der Stocken, López‐Portillo & Koedam, ). While water currents represent the standard dispersal vector in mangroves, wind has been shown to influence dispersal trajectories and probably more so for propagules that protrude more from the water (Van der Stocken et al , ). Therefore, emigration of propagules from various species from a particular stand can result in different trajectories.…”

“…Tonné et al () combined anatomical analyses and buoyancy behaviour experiments to demonstrate that propagules of B. gymnorrhiza , C. tagal , and R. mucronata may lose buoyancy over time because the density of the propagule tissue increases. This process progresses from the plumule (embryonic shoot) towards the radicle (embryonic root) and may also change the floating orientation of a propagule from horizontal to vertical, potentially making it less wind‐sensitive during dispersal (Van der Stocken et al , , ).…”

Monitoring of Eco-Restoration of Mangrove Wetlands through Time Series Satellite Images: A Study on Krishna-Godavari Delta Region, East Coast of India

“…The mangroves have grown not only where they were planted but also spread to the adjacent areas as well. The dispersal mechanism of mangrove propagules by water to wider areas where they anchor the favourable ground (Van der Stocken et al 2015) has apparently led to spread of mangroves in the K-G deltas.…”

Plant dispersal traits determine hydrochorous species tolerance to connectivity loss at the landscape scale

“…In these slow-flowing systems, long-distance dispersal is likely to involve several events of seed-stranding along banks and remobilization following heavy rains and high winds, rather than a single dispersal event (Nilsson et al 2010). This suggests the importance of dispersal traits favouring seed buoyancy, including low seed density or long floating durations (Nilsson et al 2002;Carthey et al 2016), both being related to seed mass (De Ryck et al 2012;Van der Stocken et al 2015). Floating dispersal capacity depends also on some morphological features of seeds: rounder seeds are, for instance, less likely to be trapped by vegetation, and therefore expected to disperse more quickly (Chang et al 2008;Chambert & James 2009;O'Hare et al 2011).…”