The importance of maternal shrub nutrient history, seedling soil nutrients, and freezing temperatures on avicennia germinans recruitment

Abstract

Human-driven climate change is increasing the global range of Avicennia germinans (L.) L., especially in communities in the Gulf of Mexico. Avicennia germinans propagules establish in salt marshes along the Texas coast with high maternal energy invested in propagules that can withstand abiotic environmental fluctuations. As these systems shift in dominance from herbaceous to woody plants, biotic interactions, including energy transfer from a maternal plant to its offspring, can affect the rate of change. Modifications in dispersal patterns and propagule recruitment drive the population biology and subsequent community interactions within mangrove-marsh ecotones. Fluctuations in nutrient availability, particularly with nitrogen and phosphorus as limiting factors, influence the reproductive output of A. germinans. This research analyzed the effects of maternal A. germinans shrub fertilization, soil fertilization in early propagule and seedling growth stages, and the combination of maternal and soil fertilization on propagule establishment, survival, and growth. Higher propagule weights significantly increased percent survival and height across all treatment types. Nitrogen (N) additions to the soil increased percent survival, and the combination of N maternal-shrub fertilization and N-seedling soil fertilization increased growth of seedlings in height and leaf production. Seedlings growing in phosphorus (P) soil had increased leaf production and the combination of maternal P and soil NP greatly increased leaf production. Propagule response to varying freezing temperatures in Corpus Christi and South Padre Island was analyzed using maternal-shrub fertilized and unfertilized propagules. Experiencing a harsh freeze (-8°C) for two hours, significantly reduced survival across all maternal treatments, with a 20% overall survival. Thus, A. germinans populations will not be wiped out by a single - 8°C freeze and could possibly experience rapid recruitment from surviving propagules, leading vi to local adaptation. Propagules harvested from South Padre Island, a more southern location on the Texas coast, grew taller as seedlings and had higher survival rates during a lighter freeze (- 2°C), and produced more leaves during a harsher freeze (-8°C). Nitrogen maternal-shrub fertilization increased growth of Corpus Christi propagules, but only during the -2° freeze treatment. Assessing mangrove survival and growth in salt marshes under varying nutrient conditions will provide insight into where A. germinans will become dominant as their global livable range expands. Growth benefits from fertilizer additions parent plants and the substrate during the seedling stage could indicate areas of future A. germinans “hotspots” where the propagules produced may have increased viability, a larger growth form, and be more able to withstand freezing temperatures as populations spread farther north.