March of the mangroves: Drivers of encroachment into southern temperate saltmarsh

Abstract

Tidal wetlands are experiencing climate change driven shifts in their community composition, distribution, and functions. Mangroves have expanded into saltmarshes where their ranges overlap on five continents. Yet questions remain about the spatial variability of climate change drivers and subsequent local ecological feedbacks on these distributional shifts. Along the southernmost range of mangroves, we studied shifts between Avicennia marina and threatened saltmarsh communities. We used semi-automated object based image analysis to classify wetlands in aerial imagery from 1970 to 2017. Saltmarsh coverage decreased at the expense of mangrove encroachment. Landward migration of saltmarsh was negligible. Unlike other mangrove-saltmarsh ecotone studies, neither changes in seasonal extreme temperatures nor sea level rise were associated with the observed transitions. Rather, mangrove encroachment correlated with increasing annual maximum temperature and was more extensive at sites with lower mean elevations, shorter mean distances to earliest known mangrove-saltmarsh boundary, and higher densities of tidal creeks. Accounting for local variation in mangrove encroachment is informative for managing the provision of desirable ecosystem services. As climate change impacts the southernmost mangrove-saltmarsh ecotone, areas with higher mean elevations and fewer tidal creeks could be prioritized to mitigate the loss of saltmarsh.