Supply-Side Ecology in Mangroves: Do Propagule Dispersal and Seedling Establishment Explain Forest Structure?

Abstract

Abstract. Theory and empirical evidence suggest that spatial and temporal variation in propagule availability can have as great or greater an influence on community structure as post-recruitment biotic interactions, a phenomenon known as supply-side ecology. One of the first theories to invoke supply-side dynamics, D. Rabinowitz's Tidal Sorting Hypothesis (TSH), attributed the tidal zonation of mangrove tree species to the interacting effects of water depth and propagule size on dispersal and establishment. According to the TSH, smaller propagules are carried farther inland by flood tides than larger ones, stranding and establishing in greater numbers at upper tidal elevations. In contrast, larger propagules are better able to establish in deeper, more seaward areas because their size affords greater access to the soil surface and resistance to buffeting by moving water. Here we present results of the first comprehensive investigation of the TSH. In a series of experiments conducted at Punta Galeta on the Caribbean coast of Panama, we quantified patterns of propagule dispersal and establishment of the three dominant species: Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle.To measure dispersal patterns, we monitored movements (directions and distances) of marked propagules released at three elevations along each of three transects. On all transects, regardless of species or elevation, propagules moved seaward rather than being carried inland by tides, as predicted by the TSH. Rates of seedling establishment were monitored in experiments conducted at different tidal elevations. Contrary to the TSH, all three species established best in the lower intertidal, where they were in prolonged contact with the soil surface during low tides, and established more poorly in the upper intertidal basin, where standing water made rooting difficult. Seedling establishment was lowest in the wave-exposed, back reef habitat, but in accordance with the TSH, the large propagules of Rhizophora rooted and persisted better in this turbulent habitat than did the smaller propagules of the other species.We conclude that the TSH does not explain the vertical distributions of mangrove species on Punta Galeta. Rather, a different form of supply-side dynamics imposed by the strong seaward movement of dispersing propagules is important in structuring our study forests.

Modelling drivers of mangrove propagule dispersal and restoration of abandoned shrimp farms

“…We do however not fully support the TSH. Like the reassessment of the latter theory by Sousa et al (2007) indicated, propagule dispersal is not always directed towards areas more inland, but can be strongly concentrated towards the edges of lagoons and channels, as confirmed by the present study. This directional dispersal pattern is typical for areas with a small tidal range where high seasonal rainfall causes an additional flow of runoff that overwhelms any tendency for incoming tidal flow to carry propagules inland (Sousa et al, 2007).…”

“…Like the reassessment of the latter theory by Sousa et al (2007) indicated, propagule dispersal is not always directed towards areas more inland, but can be strongly concentrated towards the edges of lagoons and channels, as confirmed by the present study. This directional dispersal pattern is typical for areas with a small tidal range where high seasonal rainfall causes an additional flow of runoff that overwhelms any tendency for incoming tidal flow to carry propagules inland (Sousa et al, 2007). Moreover, species located along the edges of lagoons and channels, like in this case Avicennia officinalis, can drop their propagules directly into adjacent tidal creeks, allowing for dispersal over larger distances by currents if they remain buoyant.…”

“…The majority of propagules remained within the lagoon and its adjacent channels, and often near the parental trees. Field experiments support these findings as modal or average propagule movement of Rhizophora and even Avicennia was found to be limited and concentrated near the parental trees, especially within mature forest stands (Clarke and Myerscough, 1993;De Ryck, 2009;McGuinness, 1997;Sousa et al, 2007). Water-buoyant propagules may set out to colonise and establish new stands but like in most plant species, they will rather strand in the vicinity of the parent trees to replenish existing stands (Duke et al, 1998;Harper, 1977;Levin et al, 2003;Sousa et al, 2007) with a higher chance of suitable environmental conditions.…”

“…We therefore emphasise the importance of additional field experiments to quantify the dispersal distances and directions of each species in different environmental settings (see Sousa et al, 2007), as well as propagule retention by vegetation (see Chang et al, 2008).…”

Application of remote sensing and GIS for detection of long-term mangrove shoreline changes in Mui Ca Mau, Vietnam

“…Sediment accretion and erosion evidently influences the ability of propagules to disperse and establish, which therefore influences the spatio-temporal changes in mangroves (Clarke et al, 2001;Sousa et al, 2007). Most of propagules were found to disperse over a short distance up to tens of metres (McGuinness, 1997;Breitfuss et al, 2003;Sousa et al, 2007;De Ryck et al, 2012).…”