Black mangrove recovery from freeze disturbance accelerated by nutrient enrichment

Due to climate change, mild winters and fewer extreme freeze events at the temperate-tropical transition zone are allowing some species to migrate poleward, causing shifts in species dominance. In coastal Texas and some other sections of the GOM, woody encroachment by mangrove shrubs and trees into low latitude salt marsh leads to reduced salt marsh dominance likely through competition for light. A recent catastrophic (i.e., mangrove-killing) freeze event in Texas influenced the present regime shift by causing extensive mortality of mature, sapling, and seedling mangroves, alleviating salt marsh vegetation from competition for light. A key topic for study is assessing rate and extent of mangrove recovery following the freeze as a crucial factor impacting the direction, timing, and extent of the regime shift. Additionally, anthropogenic fertilization is predicted to increase and enrich estuaries and bays, resulting in various consequences, such as increased mangrove colonization and dominance, adding another potential impetus to the regime shift. To better understand how nutrient enrichment affects recovery following a catastrophic freeze, fertilization plots were used in a field experiment to ascertain how nitrogen (N) and phosphorus (P) affect black mangrove survival and growth, as well as, changes in the salt marsh vegetation. Other observational sites, with different high and low nutrient conditions, were employed to identify how the findings from the fertilization experiment translate to a larger scale and expand the inference space of the study. Results indicate that nutrient enrichment increased above-ground growth and reproductive output of mangroves, thus enhancing population recovery. Salt marsh species (mainly Batis maritima) reduced growth of mangrove seedlings via inferred competition, although marsh plants did not respond to fertilizer treatments with enhanced above-ground growth. Rather salt marsh was influenced by elevation and shading by mangrove canopy. The findings suggest that nutrient enrichment alters wetland plant dynamics and accelerates a regime shift from a salt marsh to a mangrove-dominated ecosystem, depending on the frequency and magnitude of future freeze events that can reset succession.