Mangroves are coastal woody plants with complex canopies that generally outcompete salt marsh species. Mangroves are freeze intolerant, which severely limits their growth and distribution, therefore mangrove dominance is primarily restricted to the tropics. At their poleward limits, mangroves are intermixed with temperate dominating salt marsh species creating marsh-mangrove ecotones. Global changes are driving shifts in plant species distribution and dominance, particularly within ecotones, such as warmer winter temperatures are facilitating mangrove poleward expansion and subsequent encroachment into salt marshes. Over the last 30 years, black mangrove (Avicennia germinans) stands have been expanding into smooth cordgrass (Spartina alterniflora) dominated salt marshes in the Northern Gulf of Mexico, with the reduction of freezing event frequency, duration, and severity invoked as the leading driver. In January 2018, Winter Storm Inga brought severe freezing conditions to the southern United States causing widespread damage to the local vegetation. We are investigating how this freeze event is impacting the black mangrove population within the Northern Gulf of Mexico marsh-mangrove ecotone. We are monitoring freeze damage and recovery at eight locations along the Texas coast.
Results/Conclusions
The northern four of the eight study sites spanning the Texas coast, had mangroves with major freeze damage, whereas the stands within the other, southern study sites, had minimal to no freeze damage. Mangroves at freeze damaged sites experienced partial to complete defoliation. Some seedlings and propagules (this year’s newest recruits) survived in even the heavily freeze damaged sites, though all showed various levels of damage. These data give insight into how freezing temperatures affect black mangrove growth, recruitment, and survivability at their range limits, and the subsequent effects on mangrove range expansion or contraction. For example, light gaps created by mangrove defoliation can facilitate marsh species recruitment. Additionally, major freeze damage may reduce reproductive output in mature trees in following years, thereby reducing mangrove recruitment. Therefore, a major storm event such as Winter Storm Inga may have the potential to pause mangrove expansion within the marsh-mangrove ecotone.