Mangrove forest growth in the Fly River delta, Papua New Guinea

During September 1991 and January 1993, permanent plots were established in the three major mangrove forest types in the Fly River delta to follow the growth rates of marked trees.

A total of 13 plots were established in Nypa forests on Puruta, Aibinio and Wabuda Islands in September 1991. In January 1993, 5 plots were established in Rhizophora-Bruguiera forests on Umuda Island and 5 plots in Sonneratia-Avicennia forests on small islets east of Sumagi Island.

The size of plots varied with tree densities in each forest type and ranged from 50 to 500 m². The diameter of each tree in the plot was measured at breast height (DBH = 1.5 m above the sediment). In the Nypa plots all emergent tree species other than Nypa were similarly measured and tagged. For Nypa, the plot was surveyed using an angle-count cruising method to obtain size frequency distribution of Nypa fronds (the palm has a creeping trunk with upright fronds) and their density per hectare. All fronds of at least four trees in each of 12 plots were measured (DBH) and tagged (13-19 fronds per plot).

Size-frequencies and densities of trees and Nypa fronds in plots were converted to biomass per hectare using known allometric relationships between DBH and weight for trees or a measured relationship for Nypa based on harvesting and weighing fronds in the Fly Delta.

All Nypa plots were remeasured 12 months after establishment in September 1992. Significant turnover and growth of Nypa fronds was observed. For the two canopy emergent tree species in the Nypa forests, Heritiera littoralis and Xylocarpus granatum, only Xylocarpus granatum showed significant trunk growth within this period. The plots in the other forest types were not remeasured at this time. However, these plots were revisited in February/March 1995.

Trunk mass increments for individual Xylocarpus granatum trees were calculated by subtracting mass at time zero (calculated from DBH measurements and allometric relationships between DBH and weight) from mass at t = 1 yr. For Nypa, the same procedure was followed for fronds present at t= 0 and also for fronds that were produced during the year. For Xylocarpus granatum, mass increments were summed for all trees in a plot to obtain annual trunk and branch wood production per unit area. For Nypa, annual mass increments per tree were multiplied by tree densities to obtain production on an areal basis.