Mangrove diversity is more than fringe deep

Abstract

Mangroves form coastal tropical forests in the intertidal zone and are an important component of shoreline protection. In comparison to other tropical forests, mangrove stands are thought to have relatively low genetic diversity with population genetic structure gradually increasing with distance along a coastline. We conducted genetic analyses of mangrove forests across a range of spatial scales; within a 400 m² parcel comprising 181 Rhizophora mangle (red mangrove) trees, and across four sites ranging from 6-115 km apart in Honduras. In total, we successfully genotyped 269 R. mangle trees, using a panel of 677 SNPs developed with 2b-RAD methodology. Within the 400 m² parcel, we found two distinct clusters with high levels of genetic differentiation (F_ST = 0.355), corresponding to trees primarily located on the seaward fringe and trees growing deeper into the forest. In contrast, there was limited genetic differentiation (F_ST = 0.027-0.105) across the sites at a larger scale, which had been predominantly sampled along the seaward fringe. Within the 400 m² parcel, the cluster closest to the seaward fringe exhibited low genetic differentiation (F_ST = 0.014-0.043) with the other Honduran sites, but the cluster further into the forest was highly differentiated from them (F_ST = 0.326-0.414). These findings contradict the perception that genetic structure within mangroves forests occurs mainly along a coastline and highlights that there is greater genetic structure at fine spatial scales.

Figure 1

Structure outputs of Rhizophora mangle stands in Honduras. (a) Four principal sampling sites, CS Cuero y Salado, EC Elijah canal, OB Oyster bed lagoon, and FC Fort cay; (b) 400 m² parcel of forest at Fort Cay; and (c) Fort Cay divided into two sites, FC-K1 and FC-K2.

Figure 2

Geographic distribution of individual Rhizophora mangle trees within the 400 m² parcel of forest on Fort Cay as assigned by K-means clustering analysis. Black outlines demark individual trees and gray squares represents no tree present (A13) or samples that did not sequence (I10, Q16, T17).

Figure 3

Spatial autocorrelation at 2 m spatial distance classes within the 400 m² parcel of forest on Fort Cay. Gray dashed lines denote the upper and lower null confidence interval. Asterisk denotes significant correlations.

Figure 4

Map of (a) the Honduran north shore, (b) Fort Cay and the fine-scale sampling area, marked by the yellow square with the orange peg indicating the initial sampling square A1. (c) Schematic of the 400 m² sampling grid and the position of the 181 Rhizophora mangle trees within the parcel. Black outlines demark individual trees; Light green—trees dominant within a 1 m² area; Dark green—trees dominant in 2 m² or greater; Gray represents no tree present (A13) or samples collected that did not sequence (I10, Q16, T17). Maps were created with R Studio using satellite images provided by Google Maps.