A field experiment in Florida's salt marsh-mangrove ecotone was conducted to determine the impact of nitrogen (N) addition and aboveground competition on mangrove seedling growth and biomass partitioning. At this ecotone, Avicennia germinans (black mangrove) seedlings grow in competition with salt marsh species and provide a natural experiment. The seedlings were subjected to nutrient enrichment and released from aboveground competition via a meshing treatment. Aboveground competition removal decreased leaf area and foliar C:N (by 44.1% and 26.8%, respectively) while N-enrichment increased mangrove leaf production by 41.7%, foliar N by 62.2%, and leaf biomass by 72.1%. These results indicate that A. germinans seedlings significantly modify leaf characteristics in response to changing availability of light and nutrient resources. This work illustrates that mangrove seedlings (1) show large plasticity in both growth and nutrient acquisition, and (2) compete with salt marsh species. Our work also brings to light the interactive effects of biotic and abiotic drivers of mangrove productivity and the need for more multifactorial work in this important ecotone, where range shifts may be occurring.
