Interactions with marsh species will become increasingly important as mangroves expand their range with global warming. In a field experiment in the Indian River Lagoon, Florida we hypothesized that colonizing mangrove seedlings would be affected differently by various marsh species and combinations of marsh species. In 2008 we established different salt marsh treatments (8 replicates each of bare unvegetated plots, monocultures of each species, and 2, 3 and 4 groupings of species) using Spartina alterniflora, S. patens, Sesuvium portulcastrum, and Distichlis spicata. A target red mangrove (Rhizophora mangle) was planted in the middle of each experimental plot 8 months later. We followed marsh plant clonal growth, growth and leaf production of the target seedling, and new recruitment by mangrove seedlings of all species initially for a year, and later (2014) returned to assess long-term consequences of the treatments. Both salt marsh clonal growth and R. mangle growth was generally less at lower elevations.
Results/Conclusions
Year 1: monocultures and combinations of marsh species promoted growth of R. mangle compared to bare plots. S. alterniflora and S. portulcastrum monocultures had the greatest on mangrove growth, and S. patens and D. spicita, the least. S. alterniflora increased stem growth by 2.5x over that of bare plots. S. patens increased R. mangle growth 2x over that of bare plots, and the two-species combination, S. alterniflora - S. patens had an even greater + effect. The four-marsh-species combination increased the growth of R. mangle by 2.3x. Recruitment of other mangrove species was also affected.
Year 5: mangroves up to >3 m dominated most of the site, although marsh plants still occurred in varying abundance depending on mangrove canopy. In plots subjected to the original marsh treatments S. alterniflora-D. spicata, S. alterniflora- S. patens, and S. alterniflora- S. portulcastrum, the abundance of A. germinans trees was lower than in originally-bare treatments. Abundance of L. racemosa trees were substantially greater in plots that had been planted with S. portulcastrum, with marginally significant + effects also of S. patens, and S. patens- S. portulcastrum treatments, and the four-marsh-species treatment. No salt marsh treatments significantly influenced the eventual abundance of R. mangle trees.