Traits of Resident Saltmarsh Plants Promote Retention of Range-Expanding Mangroves Under Specific Tidal Regimes

Ecosystem engineers physically alter the environment, but their effects vary with abiotic context. Such context-dependent alteration can influence other species, including establishing recruits. In Florida, mangroves are expanding northward with warming climate and replacing salt marshes. We examined how structural traits of marsh and mangrove vegetation engineer mangrove establishment under different tidal conditions. First, we surveyed mangrove seedlings next to adult mangroves in mangrove-dominated fringe habitat characterized by dense, low-stature pneumatophores and in marsh-dominated interior habitats with sparser, taller marsh vegetation. In the interior, we also counted seedlings next to mangrove shrubs and seedlings to examine how stage-specific structure and propagule production influenced seedlings. Seedlings were most abundant beneath adult mangroves, especially in interior habitats, likely due to proximate propagule supply and heightened retention by marsh structure. Second, we quantified propagule retention by different structural microhabitats (pneumatophores beneath adult mangrove, pneumatophore mimic, saltmarsh vegetation, bare sediment) in fringe and interior habitats during neap and spring tide sequences, which have different hydrodynamics that could interact with structure to alter retention. During the low magnitude neap tide, retention was high in interior habitats and invariant across structural treatments in fringe and interior habitats. However, during the deeper spring tide, almost all propagules were lost from fringe habitats regardless of structural treatment, but in the interior, marsh vegetation retained 2-4 times more propagules than other structures; propagule retention was positively correlated with saltmarsh vegetation density and height. Resident engineers best facilitated propagules during spring tides, but these effects were moot during neap tides; thus, facilitation by a competitor's autogenic structure varies with abiotic conditions.

Automated summary

The study in Florida shows that mangrove establishment can be affected by the number of seedlings and nutrient factors in mangrove-dominated fringe habitats and marsh-dominated interior habitats. During spring tides, mangroves may influence the retention of mangrove nutrients, while in fringe habitats, most propagules are lost regardless of structural treatment.