Rapid, global, and anthropogenic alternation of species distribution in ecosystems necessitates a better understanding of how biotic interactions regulate natural communities. In southern coast of China, invasive saltmarsh grass of Spartina alterniflora, exotic mangrove species of Sonneratia apetala, mixed with native mangroves and formed a novel coastal wetland ecosystem in recent few decades. This provides opportunities to study the community dynamic and succession of the ecotone between mangroves and salt marsh for understanding the responses of coastal wetland vegetation to global change, while few studies have tested the relative and interactive role of competition and consumption in affecting the native and exotic mangrove species establishment varies across environmental gradients. We worked at two sites, Zhangjiang Estuary (high latitude) near the northern limit of mangroves, and Leizhou Bay (low latitude) near the southern distribution limit of S. alterniflora in China. We transplanted seedlings of two native mangrove species (Kandelia obovata, Avicennia marina), and the exotic mangrove (S. apetala) into three intertidal habitats (native mangroves stand, Spartina meadow, and mudflat), monitored the plant growth, herbivory, and abiotic factors for two growing seasons.
Results/Conclusions
Survival of the transplanted mangrove seedlings were lower in high latitude site than those in low latitude, and varied with habitat and species. In high latitude site, survival of all three mangrove species were low in the low light habitats where shading by tall-form native Kandelia forest and vigorous Spartina meadow, and rodent grazing strongly suppressed the survive of native Kandelia in Spartina meadow and unvegetated mudflat habitat. In low latitude site, rodent grazing suppressed the survive of native Kandelia in Spartina meadow and unvegetated mudflat habitat, and insect leaf herbivory exacerbated the mortality of native Avicennia under canopy of mangrove forest and unvegetated mudflat habitat, while exotic mangrove had high survival in all three habitats. After two growing seasons, surviving exotic mangrove grows more rapid than the two native mangrove species, had begun to emerge from the canopy of Spartina and dwarf-form Avicennia forest. Synergistic interaction between light competition and consume control suppressed native but not exotic mangrove species, suggesting that S. apetala natural regeneration will be widespread than our expectation. Moreover, rapidly encroachment of S. apetala may transform the native mangroves and monospecific Spartina grasslands into exotic mangroves in southern coast of China.