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 US Forces led to the decimation of mangrove forests in the Mekong Delta. Mangrove trees Avicennia alba were sampled across the Mekong Delta; their age was assessed using models based on internode growth and samples were genotyped for 6 microsatellite loci. The evolution of genetic diversity over 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 the A. alba population is still increasing in the Mekong Delta 3 decades after the end of the Vietnam 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 intraspecific 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. KEY WORDS: Recolonization genetics · Genetic recovery · Demographic recovery · Local extinction · Mangrove · Deforestation · Agent OrangeResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 390: 129-135, 2009 by US Forces of a highly toxic defoliant named Agent Orange (Stellman et al. 2003) over Southern Vietnam, which led to the decimation of mangrove forests in the Mekong Delta. A major unprecedented reforestation program was led, following the end of the US-Vietnam war (1974), by the Vietnamese government to recover the lost mangrove area (Hong & San 1993, Hong 1996. This effort, however, was based on the use of a single genus, Rhizophora, for which nurseries were available and which local communities (including scientists and local farmers) believed to be the most valuable component of mangrove forests. As a result of these efforts, Rhizophora stands have partially recovered (Hong 1996). In contrast, the recovery of other mangrove species in the community, such as the hermaphroditic Avicennia alba, was dependent on propagule dispersal from external sources, but tree cover and density were recovered about 2 decades later (Hong 1996). However, because potential sources of propagules in the Mekong Delta were few and distant, due to the thorough herbicide devastation of the vast mangrove forest area, it is likely that most colonizing propagules were delivered from a few sources. As a consequence of this extinction-recolonization process, the recovery of the plant population may have involved initial decimation of genetic diversity relative to the original stands, a genetic bottleneck effect, eventually followed by recovery at an unknown rate. Whereas the rates and patterns of recovery of plant communities have been extensively studied, including...

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