Nitrogen Addition Increases Freeze Resistance in Black Mangrove (Avicennia germinans) Shrubs in a Temperate-Tropical Ecotone

Low temperature stress is the primary factor determining the latitudinal limits of tropical plants. As the climate warms, tropical species are migrating poleward, displacing native species and modifying ecosystem structure and function. Changes are particularly evident along latitudinal gradients with the highest velocity of change occurring in wetlands. In coastal wetlands, saltmarshes dominate at latitudes above 30°, whereas mangroves occur mostly in the tropics because most species are intolerant of freezing temperatures, but others, like Avicennia germinans (black mangrove), do tolerate freezing temperatures. In response to a warmer climate and fewer killing freezes, mangroves are currently expanding into saltmarshes. However, the speed of the transition from saltmarsh to mangrove can also be modified by extreme events and nutrient subsidies. In a fertilization experiment along the Atlantic coast of North America, we found that nitrogen addition altered plant traits in Avicennia , which increased their resistance to freezing temperatures. This trait shift resulted in negligible freeze effects during a January 2018 extreme freeze event compared to unfertilized plants, which lost more than 80% of the leaves and more than 40% of the wood in their canopies. The freeze-killed litter from unfertilized plants provided a nutrient pulse that influenced recovery, growth and mangrove cover for three years following the freeze. Nutrient enrichment and recovery from the freeze effects led to increased growth and structural complexity of the mangrove canopy, which further enhanced freeze tolerance, shrub growth form and the ability of Avicennia to displace the saltmarsh in the temperate–tropical ecotone.