Changing disturbance regimes and woody species dynamics in coastal wetlands

Background/Question/Methods

During the 21^st century, coastal wetlands will be impacted by rising sea levels and increased tropical storm activity.  Coastal areas are densely populated and fire suppression and landscape fragmentation have altered fire regimes, enabling woody species encroachment into historically herbaceous-dominated wetlands.  Prescribed fire has been used to restore herbaceous dominance and could promote landward expansion of herbaceous species, enabling freshwater marshes to keep pace with increasingly saline conditions.  Changes in interacting disturbance patterns will undoubtedly cause compositional shifts, though the functional impacts of shifts remains unclear.  This study examines a fire-suppressed wetland complex along the Northern Gulf of Mexico before and after three major, sequential, landscape-scale disturbances (Hurricanes Ivan and Katrina in 2004 and 2005, and a prescribed fire in 2010). We investigated how these disturbances differentially affected plant communities across an elevation gradient (saltmarsh to marsh-scrub ecotone) and whether disturbances favored migrations of entire assemblages or differentially affected species, resulting in novel communities.  The gradient was divided into sections 10 cm in elevation (saltmarsh, low brackish marsh, high brackish marsh, intermediate marsh, fresh marsh, sparse marsh-scrub ecotone, and dense marsh-scrub ecotone); five 100 m² plots were established in each zone (n=35) and sampled for vascular plant species incidence.

Results/Conclusions

Following the disturbances, overall saltmarsh species richness and understory wood species richness declined significantly.  NMDS ordination and ANOSIM testing revealed that significant compositional shifts occurred in the saltmarsh, brackish marshes, and the sparse and woody ecotones.  Vector-fitting and indicator species analyses suggested that shifts between years were not driven by unidirectional movements of whole assemblages up or down the gradient but rather by the disappearance of species that were present in 2004, the appearance of novel species in 2011, and species-specific range expansions, contractions, and migrations.  Compositional shifts in intermediate and fresh marsh areas were not significant.  These findings suggest that changes in disturbance patterns during the next century may maintain herbaceous-dominated oligohaline wetland ecosystems while simultaneously driving successional pathways towards novel species assemblages (i.e., assemblages of species whose ranges do not currently overlap) at the upper end of the coastal gradient and species-poor saltmarsh assemblages.  Further study is needed to determine whether compositional shifts are indicative of coastal wetlands able to keep pace with climate change or are concomitants of critical shifts in ecosystem function ultimately on a trajectory towards degradation.