Regeneration and self-thinning processes in a restored Rhizophora apiculata plantation in southern Thailand

Degraded mangrove sites, such as abandoned shrimp ponds, are usually restored through the establishment of even-aged Rhizophora apiculata stands with an initial spacing of 1.5 m × 1.5 m. It has been observed that under good site conditions, densely planted R. apiculata trees compete from an early age onward. However, it is unknown whether early competition and self-thinning occur in recently restored mangrove plantations. The study was conducted in a 16-year-old R. apiculata stand established on an abandoned shrimp pond in Nakhon Si Thammarat province, southern Thailand. All trees were stem-mapped and their basal area was determined. Point process modeling was used to simulate the impact of self-thinning and regeneration processes on the spatial distribution of trees. Spatial statistics were applied to assess the goodness-of-fit of the simulated tree distribution compared to the observed distribution. Tree density declined from initially 4444 trees/ha to 3566 trees/ha, while the distribution of tree stems became more regular. The simulations indicated that 50% of all planted R. apiculata trees died due to self-thinning and 38% of all mapped trees regenerated naturally in close proximity to their mother tree. Moreover, planted trees tended to have a smaller basal area. The results suggest that the density of planted R. apiculata seedlings could be lowered in order to reduce the costs of mangrove restoration, as a large number of seedlings is lost due to density-dependent mortality.