Nutrient enrichment effects on co-occurring Avicennia germinans and Spartina alterniflora: Implications for mangrove expansion within the Northern Gulf of Mexico
Mangroves have complex canopies and generally outcompete salt marsh species. However, mangrove dominance is restricted to the tropics because mangroves are freeze intolerant, which severely limits their growth and distribution. Global climate changes, particularly warmer winter temperatures, are predicted to facilitate mangrove poleward range expansion. Over the last 30 years, black mangrove (Avicennia germinans) stands have been expanding into temperate smooth cordgrass (Spartina alterniflora) dominated salt marshes in the Northern Gulf of Mexico. Climate-induced shifts in dominant vegetation may be further complicated when paired with anthropogenic stressors such as nutrient input. However, little is known about how co-occurring smooth cordgrass and black mangrove respond to nutrient enrichment. To understand how nutrient addition may influence mangrove expansion, it is necessary to perform enrichment experiments within the marsh-mangrove ecotone. We fertilized in situ mixed smooth cordgrass and black mangrove stands in Port Aransas, TX, USA with nitrogen and phosphorus slow-release fertilizer.
After 28 months of enrichment, black mangrove leaf total nitrogen and chlorophyll a content were 31% and 13% higher, respectively, in enriched plots compared to ambient plots; smooth cordgrass leaf metrics were not significantly different between treatments. Plant density did not change significantly with enrichment for either species, but black mangrove maximum height was 35% taller in enriched plots compared to those in ambient plots. A stronger nutrient response suggests that enrichment may give black mangroves a competitive advantage that could lead to increased mangrove coverage and subsequent exclusion of smooth cordgrass. Consequently, areas with higher anthropogenic nutrient input within the marsh-mangrove ecotone may experience faster mangrove expansion rates. These data provide a baseline for understanding how nutrient enrichment may facilitate mangrove expansion and will inform restoration and management decisions in the context of future climate change.