Maternal Nutrient History Enhances Black Mangrove (Avicennia germinans) Seedling Growth After Propagules Experience a Hard Freeze

Climate change is a primary driver of tropical mangroves colonizing temperate salt marshes in the Gulf of Mexico. Studies indicate that threshold temperatures of adult and sapling Avicennia germinans survival range from -7 to -10 degrees C and survival can depend on life history stage during a freeze event. We performed a growth chamber study to explore responses to varying freezing temperature regimes when produced at two different latitudes with varying salinity and nutrient conditions. Propagules were collected from Corpus Christi and South Padre Island, Texas. In Corpus Christi Bay, propagules were harvested from fertilized maternal shrubs (nitrogen, phosphorus, and controls) in an ongoing fertilization experiment (2017 to present). A 2-h hard freeze treatment (-8 degrees C), resulted in 20% survival across latitude and nutrient treatments. The lower latitude propagules, which were harvested from the hypersaline South Padre Island A. germinans, had higher survival rates after -2 degrees C exposure, grew significantly taller as seedlings over 20 weeks, and produced more leaves after -8 degrees C exposure. Propagules from nitrogen-treated shrubs grew taller following -2 degrees C exposure. In all treatments, propagule weight was a significant covariate predictor of seedling height and leaf production. Avicennia germinans populations may not be decimated by a single -8 degrees C freeze, and populations could possibly experience rapid recruitment from surviving propagules after a freeze, leading to local adaptation. Growth benefits from fertilized parent plants indicate that propagules from high nutrient areas may be hotspots of future A. germinans expansion.

Automated summary

Climate change drives the expansion of tropical mangroves into temperate salt marshes. The survival and growth of mangroves are influenced by freezing temperatures, latitude, salinity, and nutrient conditions. Survival rates and growth performance of mangroves vary with the latitude, salinity, and nutrient conditions of the source area.