Effects of removal of alien Spartina densiflora and restoration of native S. maritima on succession and zonation in European salt marshes

Although ecological zonation and succession are critical concepts to ecology and environmental management, little is known about their development in previously invaded ecosystems after ecological restoration. This present study analyzed plant zonation and succession and the maturation of the abiotic environment of restored Spartina maritima marshes compared to unrestored marshes invaded by Spartina densiflora during a period of 126 months in the Odiel Marshes (Southwest Iberian Peninsula). These restored marshes hosted seven native halophyte species, which were growing in three distinct bands parallel to the tidal line with contrasting abiotic and biotic development during their rapid maturation: a S. maritima non-successional monospecific band, a lower successional band characterized by increasing abundance of Sarcocornia perennis and an upper successional band characterized by rising abundance of the hybrid Sarcocornia perennis x fruticosa and Atriplex portulacoides. The restoration of these European marshes using S. maritima modified their sedimentary conditions, and the establishment of the native ecological zonation and the development of the ecological succession was successfully achieved. In contrast to this, in unrestored marshes, invaded by S. densiflora, only four native halophytes could be found, and their maturation was very slow. Most of the halophytes occupied a narrower range along the intertidal gradient in unrestored marshes than in the restored marshes. The invasion by S. densiflora altered the native zonation pattern, but this invasion seemed to have been limited by healthy growth areas of S. maritima at low elevations, and also by healthy growth areas of native Amaranthaceae species, at middle elevations.