Genetic recolonization of mangrove: genetic diversity still increasing in the Mekong Delta 30 years after Agent Orange

Abstract

The widespread use of Agent Orange (a mixture of phenoxyl herbicides) over Southern Vietnam by United States Forces, led to the decimation of mangrove forests in the Mekong Delta. Mangrove trees of Avicennia alba were sampled across the Mekong delta, their age was assessed using models based on internode growth and samples were genotyped for six microsatellite loci. The evolution of genetic diversity with time elapsed since local extinction was reconstructed, and compared with the genetic diversity of an unaffected population from Thailand. The results show that genetic diversity of A. alba population is still increasing in the Mekong Delta three decades following the end of the war, but is reaching an asymptotic level that is comparable to the adjacent non affected population of Thailand. This might be a sign of genetic recovery, but may also reveal a limitation, either of genetic enrichment due to current predominance of auto-recruitment, or of demographic increase due to intra-specific competition in this pioneer species. In any case these results, although encouraging, demonstrate that genetic recovery after complete or almost complete population depletion continues over a longer time-scale than apparent demographic recovery.