PS 23-39 - Do Established Adult Mangroves Facilitate Further Mangrove Encroachment in Herbaceous Salt Marshes?

Background/Question/Methods

Rising sea levels and shifts in temperature and precipitation, caused by climate change, are expanding the global distribution of mangroves poleward, causing them to encroach on what is currently herbaceous salt marsh. Areas that have historically been considered within mangrove range limits, like the Florida Gulf Coast, are experiencing recent within-range expansions of mangrove forests, also at the expense of salt marsh areas. Within-range shifts in mangrove distribution are likely to be influenced by several interrelated factors that may act to inhibit or facilitate mangrove establishment, growth, and/or survivorship, so predicting changes in mangrove distribution involves more than forecasting changes in climate and sea level. For instance, established mangrove trees in the area may alter environmental properties, act as a propagule source, and compete with seedlings. This study investigates the role of established adult mangrove trees in facilitating or inhibiting further mangrove expansion in Florida Gulf Coast salt marshes in the early stages of mangrove takeover. This study uses Arc GIS pro and existing satellite imagery of salt marshes from 2006 to 2020 to map the spatial pattern exhibited by encroaching mangroves. Two sites have been chosen at different latitudes and degrees of present-day mangrove encroachment.

Results/Conclusions

Both sites historically consisted entirely of salt marsh, but currently, the southern site is completely overtaken by mangroves, and the northern site is mostly herbaceous salt marsh with some established mangroves. Preliminary results in the northern site show a significant (p < 0.05) clustered pattern of new trees, with clustering becoming slightly less strong over time. (Nearest neighbor ratio 0.496 in 2006 and 0.565 in 2017. Data for the southern site is pending) This indicates that established trees have a positive effect on the growth and survivorship of new mangroves, especially in the early stages of marsh-to-mangrove conversion. Thus, the presence of a few established mangroves in a salt marsh may increase the rate of mangrove encroachment. Because mangroves are expanding their territory poleward, successful documentation of a spatial pattern of encroachment over time has important implications for forecasting wetland response to climate change. If the historic spatial patterns of mangrove encroachment at these sites match early stages of encroachment in more northern marshes, it can be inferred that the northern marshes will continue the same pattern as the climate continues to warm. This information can be used in efforts to model future wetland responses to climate change.