Influence of salt marsh canopy on black mangrove (Avicennia germinans) survival and establishment at its northern latitudinal limit

As annual minimum temperatures increase due to climate change, species once constrained by minimum temperatures are expanding poleward. Avicennia germinans (black mangrove), a freeze-intolerant tree, has been expanding northward into salt marsh-grass-dominated communities. Distribution and colonization dynamics of A. germinans are crucial for understanding changes in coastal habitats and ecosystem structure and function along the Gulf of Mexico Coast (USA). We transplanted A. germinans seedlings and propagules into salt marsh plots of S. alterniflora, half of which had the canopy removed, along a latitudinal gradient that spanned locations within and outside of A. germinans' current range limits (29 degrees 7 ' 20 '' N to 30 degrees 23 ' 41 '' N). Plot microclimate and transplant survival and growth were monitored for 2years. Canopy removal resulted in lower minimum temperatures and longer cumulative freeze duration. Seedling survival was greatest at the southernmost site; however, seedling growth was reduced in plots with the canopy intact, as hypothesized. Seedling survival at northern sites was limited to plots with the S. alterniflora canopy intact. Propagules survived and established at all sites in the second year, although there was no S. alterniflora canopy effect on propagule survival and establishment. Our results illustrate the complexity of ecological interactions between herbaceous marsh species and mangroves at the species' range limit.