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

Abstract

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 x 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 stemmapped 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.