Macroclimatic controls on tidal wetland ecosystems: Variation in foundation plant community zonation across abiotic gradients in three 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 ecosystems are poorly understood and much more investigation is needed at the regional scale. In coastal wetlands, foundation plant species (e.g., mangrove trees or salt marsh grasses) play an important functional role; they create habitat and support entire ecological communities. This study investigates the following questions: (1) How do macroclimatic factors govern, directly or indirectly, the distribution and abundance of foundation plant species in tidal wetlands of the northern Gulf Coast? (2) What conditions characterize ecological transition points where major shifts occur in tidal wetland community structure or species dominance? To address these questions, we quantified plant community structure and composition across elevation and salinity gradients in three northern Gulf of Mexico estuaries that span broad regional temperature and rainfall gradients: Grand Bay, Mississippi (cold, wet; n=90 plots); Mission-Aransas Estuary, Texas (intermediate; n=87 plots); and Laguna Madre, Texas (hot, dry; n=107 plots). Sampling was designed to identify transitions between vegetation zones and utilized real-time kinematic GPS elevation data. We fit nonlinear distribution models using elevation and coverage data for common species and calculated transitional elevations using model derivatives.

Results/Conclusions

We quantified major differences among the estuaries in plant community structure, composition, abundance and zonation that are attributable to macroclimatic drivers. At the local scale (i.e., intra-estuarine), we quantified ecological transition points associated with changes in elevation, salinity and/or soil. The hottest, driest estuary (Laguna Madre) had the most unvegetated land (e.g., algal flats) and the lowest total vegetation cover, light interception and canopy heights. In Laguna Madre, thin Spartina alterniflora bands infrequently preceded moderate Avicennia germinans (black mangrove) zones. Inland, vast algal flats bordered by succulent/halophyte zones preceded xeric upland forbs and shrubs. In Mission-Aransas, narrow Spartina alterniflora bands occasionally preceded vast Avicennia zones. Inland, sometimes expansive halophyte zones preceded narrow flats, then Spartina spartinae preceded shrubs/trees. In Grand Bay, typically narrow but sometimes broad Spartina alterniflora bands often preceded vast Juncus roemerianus zones. Inland, Spartina patens grasslands preceded shrubs, then Pinus elliotii woodlands. Succulent zones and/or halophyte-bracketed flats infrequently followed Juncus. Observed transitions in vegetation relate to transitions in local conditions that are greatly impacted by macroclimate, especially rainfall and freshwater availability. Our results highlight the importance of accounting for macroclimatic change within future-focused climate-based management decisions for coastal wetlands.