Expansion of Phragmites australis in the Liaohe Delta, north-east China

P>Based on satellite images and historical vegetation maps, Phragmites australis (common reed) in the Liaohe Delta, in north-east China, has expanded from 78 078 ha in 1984 to 91 262 ha in 2006, averaging over 600 ha year-1. This study also attempted to elucidate the reason for this expansion, using both historical documents and field surveys. The results suggested that the expansion of P. australis was driven mainly by human activities. Freshwater irrigation reduced soil salinity; irrigation of land blocks divided by canals and ditches (square-land irrigation) maintained optimal temperature, water and oxygen for the growth of P. australis; deep freshwater irrigation and seawater irrigation eliminated some weeds; combined irrigation and burning eliminated an insect pest. These examples in the Liaohe Delta showed that water, salinity, oxygen, temperature and insect pests were important ecological factors that affected the growth of P. australis. By harmonising these ecological factors, optimal conditions were created for the natural expansion of P. australis and promoted the formation of monospecific stands of P. australis.