Survival and establishment of mangrove propagules at higher tidal elevations beyond the landward margin of their distribution is a requirement for the continued existence of mangrove populations in response to rising sea-level. Despite the growing body of literature that discusses mangrove recruitment patterns, few studies have empirically examined establishment and post-establishment growth success of propagules at the higher intertidal positions into witch mangrove populations are migrating. Using an experimental field approach, this study compares establishment and post-establishment growth success of propagules at three positions across a tidal elevation gradient within a landward-transgressing mangrove population of SW Florida (USA). I observed black mangrove, Avicennia germinans, recruitment as adults of this taxon may occupy low to high tidal elevations of the intertidal zone in SW Florida, indicating that propagules are capable of successfully establishing and reaching reproductive age in novel environments. However, establishment and post-establishment growth of A. germinans in encroachment areas landward to that of lower intertidal positions has not been examined. To accomplish this, I began by monitoring movement patterns of marked A. germinans propagules released at three intertidal positions during a high spring tide to confirm that propagule dispersal to encroachment areas located at higher tidal elevations occurred at the selected field site. Propagule survival, establishment success, and post-establishment growth rate of seedlings was monitored during a reciprocal transplant study utilizing two of the three intertidal positions, one representing a lower intertidal area within the mangrove population’s niche and one representing a higher intertidal area beyond the population’s landward margin. Regardless of parental tree origin, A. germinans propagules had greater establishment success in the lower intertidal position. Likewise mean seedling height was consistently greater among established seedlings in the lower intertidal although the difference in mean seedling height between tidal locations decreased linearly over the monitoring period. Propagule mortality was greatest at the higher intertidal position (27.5% of tethered propagules died) when compared to that in the lower intertidal (0.07%). Interestingly, the tidal position of propagule origin significantly influenced survival only during the first 33 days of the reciprocal transplant experiment. After this time interval and establishment as a seedling, no mortality was observed in either treatment position for 125 days. Together, results show that intraspecific variation in A. germinans propagule establishment and post-establishment seedling growth exists in landward transgressing populations across intertidal positions. My findings indicate that abiotic conditions of the higher intertidal environments into which mangroves are migrating may be detrimental for early life stages of A. germinans but not seedlings. Combined, my results suggest that investigations into mangrove success at novel intertidal positions should focus on limitations at the propagule life stage as there was no indication that survivorship varied among tidal elevation once mangrove seedlings were established. Finally, assessing maternal reserves of dispersing propagules may provide additional insight into the importance of mangrove propagule origin on initial survival.
| Identifer | oai:union.ndltd.org:USF/oai:scholarcommons.usf.edu:etd-8497 |
| Date | 28 June 2018 |
| Creators | Grogan, Shannon Victoria |
| Publisher | Scholar Commons |
| Source Sets | University of South Flordia |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Graduate Theses and Dissertations |
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