Cordgrass (Spartina alterniflora) was introduced to China from the USA. It grows vigorously in China and has spread over much of the Chinese coast. Surveys in estuaries of southeast China have revealed a fast expansion of mangrove-Spartina ecotone, from Leizhou Peninsula to Yueqing, a range of more than 7 degrees of latitude. Few studies have explored the latitudinal variation in interspecific interactions between exotic S. alterniflora and native mangroves in China. We used a combination of monitoring and experimental transplants to study the interactions of mangrove species and S. alterniflora at 4 sites across latitude within mangrove-Spartina ecotone in southeast of China. We asked: (1) What are the current distribution patterns of S. alterniflora and mangroves across latitude? (2) How does S. alternifloraimpact the establishment of mangrove seedlings across the latitudinal gradient?
Results/Conclusions
From north to south, the growth of S. alterniflora generally decreased across latitude. Monospecific stands of S. alterniflora covered nearly all of the low-elevation and most of the mid-elevation area at high latitude site, but only occurred as small patches on the margins of mangrove forests and bare mudflat at low latitude site. On the contrary, the growth of transplanted mangrove seedlings generally increased with the decrease of latitude. S. alterniflora competitively inhibited growth of mangrove seedlings in the growing season. The strongest relative interaction intensity between S. alterniflora and mangrove seedlings was found at the middle latitude site. At high latitude site, S. alterniflora had no competitive effect on Kandelia obovata seedlings, but competitive reduces biomass of Avicennia marina seedlings to 45.4% over a period of 6 months. These results illustrate the complexity of natural gradients and plant species in mediating plant growth and competitive interactions between mangroves and S. alterniflora.