Propagule establishment and seedling growth in a shifting mangrove population: performance in novel tidal conditions

Survival and establishment of mangrove propagules at tidal elevations beyond the landward margin of their distribution underlie continued existence of mangroves in response to sea-level rise. Yet few studies have empirically examined mangrove performance at higher intertidal positions (novel sites) into which mangroves disperse. We compared propagule establishment and seedling growth at 2 intertidal elevations for black mangroves Avicennia germinans displaying landward expansion. Propagule survival, establishment success, and growth rate of seedlings were monitored for 158 d during a reciprocal transplant experiment targeting 2 tidal elevations: one within the current niche of the mangrove population (LOW) and one representing marginal habitat where mangroves have begun migration upland (MID). We introduced propagules to 4 treatments: (1) originating (site of parent tree) at LOW, stranding at MID; (2) originating at MID, stranding at LOW; (3) and (4) stranding and originating at the same position (MID or LOW). Resident and transplant propagules of MID origin were 22 times more likely to die during the first 33 d than propagules of LOW origin. Resident and transplant propagules in MID positions were 6.5 times more likely to die over the first 33 d than individuals in LOW positions. After seedling establishment, no mortality was observed in LOW or MID positions. Thus, environmental conditions of novel intertidal areas into which black mangroves migrate had a greater deleterious impact on propagules than on seedlings. Identifying processes underlying success of mangroves introduced into novel environments, such as those operating as sea level rises, will require increased attention to the fate of propagules.