Factors influencing mangrove forest recruitment in rehabilitated aquaculture ponds

Abstract

Aquaculture development was the major reason for mangrove loss in many Asian countries during the previous century. With increasing recognition of the importance of mangroves, rehabilitating aquaculture ponds back to mangrove forests has become a popular initiative. However, factors influencing mangrove recruitment, in particular natural recruitment in pond rehabilitation remain unclear. Here we surveyed the vegetation, elevation thresholds of mangroves, soil properties and the distance to open water in 17 rehabilitated ponds across different geomorphologic settings in Dongzhai Harbor, Hainan, China five years after the rehabilitation project was initiated. We found that mangroves colonized within an elevation range from 10 cm to 161 cm above mean sea level. The lower and higher elevation thresholds for mangrove recruitment varied greatly across ponds and were positively correlated. The thresholds were not related to the distance to open water, salinity or soil properties. Natural mangrove recruitment occurred in all ponds and non-planted species dominated in 14 ponds, among which non-planted exotic Sonneratia apetala dominated or co-dominated with other species in ten ponds. Total ground trunk area of mangroves and the relative dominance of non-planted mangroves were negatively related to soil salinity and positively related to soil nitrogen and the fraction of soil clay+silt (particles < 0.02 mm). These results suggest that elevation may not be a good proxy for hydrological conditions at restored sites and there are no unique elevation thresholds for mangrove recruitment within the same area due to the spatial heterogeneity of hydrodynamics or the varying inundation tolerance of mangroves along environmental gradients. Natural recruitment is a cost-effective way of mangrove rehabilitation at fluvial sites with more favorable soil properties, but planting is helpful to speed up mangrove rehabilitation and increase species diversity at oceanic sites with less favorable conditions. However, invasion of exotic species should be taken into consideration during pond rehabilitation.