Limited Mangrove Propagule Retention at a Latitudinal Range Limit: Spatiotemporal Patterns at the Patch Scale

Dispersal and establishment dynamics are critical in understanding shifts in species' ranges. We seek to illuminate patch-level dispersal dynamics by examining the shifting salt marsh-mangrove ecotone. Specifically, we ask the following: (1) How are mangrove propagules dispersed, retained, and exported within a discrete patch? (2) How do differences across a flooding gradient influence propagule dispersal dynamics? (3) How does the distribution of established seedlings compare to propagule movements?Avicennia germinansis the most temperate mangrove species in the northern Gulf of Mexico forming an ecotone withSpartina alternifloramarshes in coastal Louisiana. Sets of 500 distinctively marked mangrove propagules were placed at five different elevations. After their release, we observed dispersal dynamics for 1 month. Retention was limited in the study area (< 10%) with similar to 70-80% of propagules exporting out of the system and similar to 20% propagule predation. Retained propagules largely remained at their original elevations and were generally found at the highest elevation. Seedling establishment was also studied and unlike propagule dispersal distributions, peak seedling density occurred at elevations flooded 20-40% of the time. Our study highlights the mass export of mangrove propagules, the disparity between dispersal and establishment dynamics, and the need to explore dispersal at biologically relevant temporal and patch-level spatial scales. By understanding dispersal and establishment dynamics within the ecotonal boundary, we provide one of the first studies on dispersal at a temperature-controlled latitudinal limit for mangroves and highlight some of the drivers needed to better connect plot-, patch-, and landscape-level dynamics at this and other range limits.

Automated summary

The dispersal and establishment dynamics of mangrove propagules in a salt marsh-mangrove ecotone were studied, revealing significant export of propagules and disparity between dispersal and establishment dynamics. Most retained propagules stayed at their original elevations, while peak seedling density occurred at elevations flooded 20-40% of the time.