Mangrove forest structure and productivity in the Fly River estuary, Papua New Guinea

Mangrove forests in the Fly River delta were surveyed on three trips between April 1989 and March 1990.

In April 1989, mangrove forests were surveyed from the air by helicopter to assess their areal extent on all islands and on the mainland banks of the river. Several low level (~100m) passes were made over all of the larger islands, and single passes were made over smaller islands. Rough lists of mangrove species were made for each island and sites were identified for later ground-truthing and standing stock measurements. The aerial surveys and later ground visits by boat, facilitated a fairly accurate estimate of the total area of mangrove forest in the delta.

In July and August 1989, detailed ground surveys were made on foot at 26 sites within the delta, while several other areas were surveyed from small boats to assess species distribution patterns. In February 1990, an additional site was surveyed in detail on foot. Survey sites were not evenly distributed over the delta. Most effort was expended in surveying the more diverse forests (as determined from aerial surveys) in the region of the delta, where river salinities ranged from 5-15 ppt. Only a few sites were surveyed in very low salinity.

At each of the ground surveys sites (n=27), between two and four people walked a line through the forest in order to record all mangrove species. In addition, tree densities, basal areas, potential forest productivity (PN) and the presence of understory species were recorded in three plots at each site.

The diameter at breast height (DBH) of all trees in each plot was recorded and used to calculate densities (stems/ha). The DBH of individual fronds of the palm Nypa fruticans, which grow vertically from ground level, was also recorded. Basal areas of all trees in each plot were measured by the method of angle counts. Potential net primary productivity (PN) for each plot was derived from measurements of incident light taken outside the forest at the beginning and end of each survey and 100 light readings taken at random within each plot.