Species composition of patches influences mangrove recruitment in a saltmarsh mosaic

Spatial distributions of marine and coastal plant populations are shifting in response to climate change, and interactions among organisms at ecotones may shape patterns of range expansion, such as mangrove encroachment into saltmarshes. Depending upon the environmental context, saltmarsh plants can facilitate or suppress mangrove recruitment. A field study was conducted within a mosaic saltmarsh at the edge of a mangrove forest fringing Tampa Bay, FL, USA, to investigate whether mangrove recruitment is influenced by the species composition of saltmarsh patches within which mangroves develop. Propagules of Avicennia germinans were emplaced into experimental plots at the same tidal elevation representing 3 saltmarsh treatments: (1) Distichlis littoralis in monoculture, (2) Sporobolus virginicus in monoculture, and (3) polycultures containing D. littoralis, S. virginicus, and at least one other species. Significantly fewer seedlings established in S. virginicus monocultures than in D. littoralis monocultures. Seedling establishment within the saltmarsh polyculture was intermediate to the grass monoculture treatments. Herbivory was the most commonly observed source of mortality for mangrove propagules, and propagules were more susceptible to herbivory within monocultures of S. virginicus relative to D. littoralis monocultures. Our results suggest that patch-scale heterogeneity in saltmarsh groundcover may influence spatial patterns of mangrove herbivory and recruitment at landward forest margins. This study builds on prior investigations and demonstrates that interactions between the same mangrove and saltmarsh species may differ between sites. Understanding these context-dependent interactions among saltmarsh plants and mangrove recruits can be useful for forecasting distributional shifts of mangrove populations and for implementing mangrove restoration.