COS 54-1 - Evaluating restoration options in coastal ecosystems impacted by sea level rise and extreme freeze events

Coastal vegetation is subject to chronic and acute effects of climate-related drivers. In coastal south Texas, mangroves and salt marsh occupy the terrestrial-aquatic interface (TAI). Mangroves are expanding at the expense of salt marsh with climate warming, but extreme freeze events cause mangrove dieback, creating opportunities for salt marsh reestablishment. These areas are simultaneously experiencing added stress from rising sea levels and sometimes they remain barren, leading to erosion, conversion to open water, and loss of coastal wetlands. Prior to the November 2022 freeze, we initiated a long-term assisted colonization experiment, perpendicular to the shore of Cayo del Oso, to test the potential for establishment of salt marsh (Spartina alterniflora) and mangrove (Avicennia germinans) dominated vegetation back into their “home zones” and in locations upslope of their current TAI distributions. Plots (0.25 m²) were randomly located in salt marsh, mangrove stands, and in three upslope zones, including upland mesquite forest. Sods (5-10 cm in depth) from the target vegetation types, including above- and below-ground material, were excavated and transported to similarly excavated plots in “recipient zones”. In February 2023, plant survival and condition of transplanted vegetation were assessed in each plot. The first monthly community composition survey was conducted in May 2023; vascular plant species were identified and their above-ground cover estimated.

Results indicate high survival (100%) of transplanted salt marsh vegetation in home and upslope recipient zones; transplanted vegetation was in excellent (23/25 plots) or good condition (2/25 plots). By May, salt marsh transplants had expanded, with cover exceeding 65% in some plots. Although initial mangrove survival was high (100%), vegetation condition was poor, except in its home zone. Mangrove cover was variable, due to slow recovery in some plots and mortality in others. Early results suggest salt marsh is amenable to restoration plantings in all zones across this coastal transition, from TAI to upland mesquite forest. Poorer performance by mangroves may reflect susceptibility to the freeze, life stage at planting, and possibly other constraints to upslope establishment. We will continue to track compositional changes and monitor establishment and trajectories of these de novo ecosystems. The results of our research could prove useful for managing leverage points at opportune times, such as freeze events that influence the dynamic between mangrove and salt marsh, and for getting ahead of long-term sea level rise impacts that continually push the TAI upslope.