Created mangrove forest succession, pioneer species, and chronosequence assessment in Tampa Bay, Florida

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Sofia M. McNally, Jeannine Lessmann and Chelsea Duca, Marine Science, Eckerd College, St. Petersburg, FL

Background/Question/Methods
Mangroves form important habitat for wildlife, filter runoff, are highly productive, and ameliorate coastal flooding. Anthropogenic damage in the U.S. has led to declines in mangrove forest area which have been mitigated in part by mangrove habitat creation; however, these restoration projects are rarely monitored annually and long-term, a necessity to understand the success of created mangrove forests. This study monitored the vegetative succession of created mangrove forests in Cockroach Bay Aquatic Preserve, Ruskin, FL (Tampa Bay) using a chronosequence. A chronosequence uses differently-aged sites with similar attributes which are measured at the same time. Data are presented by site age to create a long-term prediction of successional patterns. Our study consisted of three differently-aged, created mangrove forests sampled annually from 2015 through 2020. Typical studies that monitor restoration are unable to commit long-term to an annual-sampling chronosequence technique like the one employed in this study and instead use one-time sampling of differently-aged sites. To compare this technique to ours, single-year sampling chronosequences were created with data collected in the year 2020 only. Each site was initially planted with marsh grasses, and then mangroves were recruited naturally. Five replicate plots within each site contained five quadrats along a 14 m transect placed perpendicularly to the creek. Mangrove seedlings, juveniles, young trees, and adult trees of the three neotropical species were measured, and their densities were calculated.
Results/Conclusions
This study supports others’ findings that created mangrove forest development is slow, taking decades. At this study location, Laguncularia racemosa acted as the pioneer species, dominating the other species in the young forest as seedlings, juveniles, and young trees. In the oldest site age (21 years), classic mangrove zonation had not yet been established. For this and similar studies, chronosequences are the most realistic option for long-term monitoring due to the labor and cost needed for decades-long assessment but depend on the necessary and sometimes false assumption that different sites follow similar successional pathways. This study of annual-sampling chronosequences was compared to single-year sampling chronosequences and found that single-year sampling across sites may be misleading. We suggest long-term annual sampling using a chronosequence to obtain the resolution needed to fully assess created mangrove forest succession.