Macroclimate controls on tidal wetland ecosystems: Variation in plant community structure across climatic gradients in northern Gulf of Mexico estuaries

Background/Question/Methods

The northern Gulf of Mexico coast spans broad temperature and precipitation gradients. However, the effects of macroclimate on coastal wetland ecosystem structure have not been fully quantified. In tidal wetlands, foundation plant species (e.g., mangrove trees or salt marsh grasses) play important functional roles, including creating habitat and supporting entire ecological communities. This study investigates the following questions: (1) How do macroclimatic factors govern, directly or indirectly, the distribution, abundance, and performance of foundation plant species in tidal wetlands?; and (2) What conditions characterize ecological transition points where major shifts occur in wetland community structure or species dominance? To address these questions, we measured coverage by individual species, vegetation height, biomass, and soil and landscape characteristics in 1019 study plots across elevation and salinity gradients in ten northern Gulf of Mexico estuaries that span broad regional temperature and rainfall gradients. Sites ranged from hot and dry (south Texas), to cool and wet (Louisiana, Mississippi, Alabama), and to hot and wet (south Florida). We developed models using climate, elevation, and plant community data for common species and functional groups, and identified critical ecological transition points along key abiotic gradients. 

Results/Conclusions

We quantified major differences among the estuaries in plant community structure, composition, and zonation that are attributable to macroclimatic drivers. At the local scale (i.e., intra-estuary), we quantified ecological transition points associated with changes in elevation and salinity. At the landscape scale, we quantified transitions associated with variation in temperature and rainfall. Total vegetation cover, biomass and canopy height generally increased with rainfall. These parameters were lowest in south Texas where algal flats dominated and highest in south Florida where older mangrove forests dominated. Prevalence of particular foundation species and functional groups also varied predictably with rainfall. Graminoids (e.g., Spartina alterniflora and Juncus roemerianus) or mangroves (e.g., Avicennia germinans) dominated wetter (higher rainfall) estuaries, succulents (e.g., Batis maritima) dominated drier estuaries, and algal mats dominated the driest estuaries. Temperature influenced aridity and governed whether graminoids or mangroves were locally dominant, with warmer climates favoring mangroves. Intra-estuary transitions in vegetation relate to transitions in local abiotic conditions, which are greatly influenced by macroclimate, especially rainfall and freshwater availability. Our results highlight the importance of accounting for changing macroclimatic conditions within future-focused management and restoration efforts for coastal wetlands.