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.
Permanent plots were established to check the validity of previous estimates of mangrove forest production in the Fly River delta. Earlier estimates were based on a rapid survey technique, which is thought to underestimate productivity. Determining the growth rates of marked trees in these plots allows for a more accurate estimation of the production of trunk and branch wood in the major forest types.
A larger number of Nypa plots were chosen to examine the potential for aging stem sections of Nypa for investigations of recent metal histories.
This research project was a component of a larger project "The influence of fluvial discharge on the pelagic and benthic ecology and biogeochemistry of the Fly Delta and Gulf of Papua". It follows on from an earlier project "The influence of freshwater and detrital export from the Fly River system on adjacent pelagic and benthic systems".
Rhizophora-Bruguiera forests dominate the seaward end of the Fly Delta and Nypa palm forms extensive forests in the mid intertidal zone in the mid- to low-salinity regions. Sonneratia-Avicennia forests occurr on accreting mud banks throughout the delta, but Sonneratia lanceolata is most abundant in the near-freshwater regions of the upper delta and extended some 240 km upstream from the sea.
The rapid survey techniques used in the 1989/90 surveys are described in:
Robertson AI, Daniel PA, Dixon P (1991) Mangrove forest structure and productivity in the Fly River estuary, Papua New Guinea, Marine Biology 111: 147-155.
This publication is also referred to in the metadata record:
Mangrove forest structure and productivity in the Fly River estuary, Papua New Guinea
Robertson, Alistar I, Dr (Principal Investigator)
Statement: Each plot was marked by a permanent metal post on the corner nearest the adjacent waterway and tape measures were laid to mark out a square or rectangular plot. Trees were tagged with a numbered aluminium tag attached to a stainless steel wire looped around the trunk. In Nypa plots, all fronds on a sub-sample of Nypa palms, as well as all canopy emergent species other than Nypa were tagged.
The angle-count cruising method used to survey Nypa is described in:
Cintron G and YS Novelli (1984) Methods for studying mangrove structure. In: Snedaker S and Snedaker M (eds.) The mangrove ecosystem: research methods. UNESCO. Paris. p91-113.
The allometric relationships used for species other than Nypa are described in:
Clough, BF and Scott, K (1989) Allometric relationships for estimating above-ground biomass in six mangrove species. Forest Ecology and Management 27:117-127).
The allometric relationship calculated for Nypa, based on harvesting and weighing Nypa fronds in the Fly Delta during this study is:
log10 DW = 2.20 log10 DBH - 1.52, r² = 0.99, where DW = dry weight of fronds and DBH = frond diameter at breast height.