The fundamental and realized vertical niches of <i>Avicennia germinans</i> in a Florida marsh-mangrove ecotone

In Northeast Florida, Avicennia germinans are colonizing saltmarshes that have been free of mangroves for decades. We examined the relationship between elevation and survivorship along a creekbank in a saltmarsh-mangrove ecotone to better understand the elevational niche of A. germinans. We combined a mangrove planting experiment with measurements of natural mangrove colonization to assess how elevation-related biophysical variables influence seedling establishment. We then used habitat suitability modeling to project mangrove expansion across the broader study area based on these findings. We found that A. germinans did not occupy its full fundamental vertical niche along the creekbank. Some seedlings survived throughout the study across a broader elevational range (from 10% to 82% survival from −0.1 to 0.8 m NAVD88) than where natural colonization occurred (95% of volunteer mangroves were found from 0.3 to 0.7 m NAVD88). The optimal elevation for survivorship of planted seedlings was 0.45 m NAVD88, corresponding to a hydroperiod of 19% of daily inundation and survival of 77.8% of transplanted seedlings. Mangrove growth was highest at 0.12 m NAVD88, however, this may be due to abiotic and biotic stressors that select for taller, or faster growing seedlings. We predict that the realized vertical niche of A. germinans will expand in the ecotone as dispersal limitations decrease. Understanding the vertical niche of A. germinans improves our ability to predict local mangrove distribution shifts and better plan for restoration.