The use of demographic studies in mangrove silviculture

Abstract

The Matang Mangrove Forest Reserve in Malaysia has been managed for timber production since the beginning of the century and is reputedly the best managed mangrove forest in the world. The present management plan is a 30-year rotation period with two thinnings, at 15 and 20 years. However, there has been a decline in yield from 299 t ha⁻¹ of green-wood from virgin stands to the second generation yields of 158 t ha⁻¹ in 1967–69 to an even lower 136 t ha⁻¹ in 1970–77.

This study on the demography of the forest was conducted to try to determine ways to improve the silviculture and management system. The species of the tree, whether it was living or dead, and the girth at breast height were recorded for all trees in selected representative plots covering a range of ages (5, 8, 13, 18, 23 and 28 years). The standing biomass of these plots was calculated using previously obtained allometric regressions.

The high density of 15 030 Rhizophora apiculata trees per hectare in the 5 year-old stand and the sharp decrease to 9810 in the 8 year-old stand indicate that the initial stocking was too high. We suggest that artificial regeneration should be carried out at 1.2 m intervals only if the natural regeneration is less than 50% (rather than 90% as is the present practice). Extremely high mortality occurred in the 13 year-old and the 18 year-old stands where 43% and 29% respectively of the Rhizophora trees were dead. We therefore suggest that the thinnings be carried out earlier — at 12/13 and 17/18 years (instead of 15 and 20 years) to reduce this wastage due to natural thinning. An additional silvicultural thinning could be carried out at 8–9 years to remove non-Rhizophora trees and to reduce stand density to around 8000 ha⁻¹ to allow better growth. The standing biomass of the trees did not increase from 23 years (155 t ha⁻¹) to 28 years (153 t ha⁻¹). Based on biomass, we suggest that a rotation of 25 years be used instead of the present 30. This is also supported by size distribution of the stems which showed slow increase in the girth after 18 years.