Success rates of recruited tree species and their contribution to the structural development of reforested mangrove stands

In response to widespread global degradation of mangrove forests, many initiatives have been undertaken to reforest degraded sites. However, many of these replantings are monospecific, raising questions regarding habitat change and reduced ecological functions in mangrove plantations compared to natural mixed mangrove forests. The main objective of this study was to determine whether colonising mangrove juveniles can be recruited into the adult tree layer and thus enhance the structural complexity of previously reforested monospecific stands. Vegetation structure of originally monospecific replanted mangrove stands (Sonneratia alba and Rhizophora mucronata) that were 8 yr old was assessed. Instantaneous juvenile (seedling/sapling) diversity and abundance were determined. Adjacent natural stands were used as reference sites. The R. mucronata stand had the same species richness as its natural reference and thus had a higher Complexity Index due to recruitment of non-planted species into the adult tree population than in a previous assessment when it was monospecific. Juvenile species richness and diversity were similar between the R. mucronata stands (reforested and natural); however, the total juvenile density was significantly higher (p < 0,05) in the natural stand (7390 660 juveniles ha(-1)) than in the reforested stand (2048 +/- 667 juveniles ha-1). In contrast, the adult vegetation layer of the S. alba reforested stand remained monospecific, suggesting 100 % mortality of previously observed non-planted mangrove juveniles. Many replanted mangroves around the globe are monocultures and are thus likely to have low thresholds to perturbations, as they do not benefit from the 'portfolio effect' provided in mixed species stands. Successful recruitment of non-planted species into replanted monospecific mangroves stands may therefore enhance ecosystem productivity and stability. However, more research needs to be conducted on traits of different mangrove species and their contribution to ecosystem productivity and sensitivities to environmental perturbations.