Forest Structure, Composition, Basal Growth, Hydrology, and Salinity in the Lower Mobile-Tensaw Delta

Abstract

Coastal wetlands, particularly tidal freshwater forested wetlands (TFFWs), are among the most vulnerable ecosystems to climate change. They face direct impacts from global sea level rise and extreme weather events, compounded by indirect anthropogenic disturbances like urban development and hydrologic alterations. This research investigates the ecological responses of TFFWs to tidal influence and salinity intrusion within the Mobile-Tensaw River Delta (MTRD), an internationally significant deltaic region along the northern Gulf of Mexico. This work is intended to serve as a baseline for current forest conditions and an initial indication of resilience within the study area. This study consisted of vegetation surveys (n = 47) conducted in forested wetland stands across a tidal gradient. Results revealed five distinct canopy communities that corresponded with river distance to Mobile Bay and plot elevation. Multivariate analyses highlighted a strong response of tidal influence on forest composition and structure, indicating community-level sensitivity to estuarine influences. Forested areas located near Mobile Bay exhibited lower basal area, species richness, higher shrub stem density, and a higher proportion of visually stressed canopy trees. To assess species-level responses, over 50 Taxodium distichum (bald cypress) trees were monitored over two growing seasons using low-cost dendrometer bands and continuous hydrologic measurements. Results showed that inundation was the best determiner of tree basal growth, and that, surprisingly, floods with low salinity levels also acted as a subsidy for basal growth across the tidal gradient. However, tidal influence did not account for differences in growth among our long-term forest transects (n = 8). In addition, during the monitoring period, a major saltwater intrusion event following Hurricane Francene in September 2024 was documented near the end of the study. Although this event was near the end of the growing season, it further emphasized the vulnerability of these forests to extreme climatic events. Together, these findings demonstrate how tidal hydrology and salinity gradients influence forest structure, composition, and productivity in TFFWs. This research underscores the importance of site-specific monitoring to inform adaptive management and conservation strategies in the face of accelerating climate change. As sea levels continue to rise and saltwater intrusion events become more frequent and intense, understanding the nuanced responses of coastal forest communities will be critical for land managers looking to predict ecosystem trajectories and mitigate coastal forest loss.