Decomposition of fallen timber in mangrove forests at Missionary Bay, Hinchinbrook Island, north Queensland

Between December 1986 and August 1987, surveys were made of wood detritus from the floor of two stands of mixed Rhizophora spp. mangrove forest in Missionary Bay, Hinchinbrook Island. The mature forest stand consisted of a mixture of Rhizophora stylosa, Rhizophora apiculata, Rhizophora x lamarkii and Bruguiera gymnorhiza of heights between 10 and 15m. The younger forest stand consisted of a mix of even sized Rhizophora stylosa and Rhizophora apiculata of heights between 15 and 20m.

Surveys were conducted along 6 randomly placed 50m straight line transects within each forest. Observers moved along each transect, recording the length and diameter of each dead trunk, prop root, branch, and twig which lay within 1m of either side of the transect line. Each piece of fallen dead wood was assigned to one of three decay classes: moderate (M), very (V), and extremely (E), which was assessed on the ease of penetration with a pointed stick.

In the laboratory, samples from each type and decay class of fallen wood were used to determine dry weight to volume ratios for converting wood volume (calculated by assuming each piece of wood was a cylinder) to mass equivalents. The total mass of trunks, prop roots, branches, and twigs was then calculated for each transect.

Subsamples of wood from which all fauna and walls of shipworm tubes had been removed was dried at 80°C for 4 weeks, ground to a powder and analysed for carbon and nitrogen using a Leco 600 CHN analyzer.

In the field, numbered bundles of twigs and branches from a freshly felled Rhizophora apiculata were weighed and layed out randomly on the forest floor in the low intertidal zone to measure rates of decomposition. Five bundles were retained for calculation of fresh weight to dry weight conversion factors. Four or five bundles were sampled after .2, 1.0, 1.5, 1.8 and 2.3 years in the field.

Decomposition rates of trunks and large prop roots were determined from trees that had been felled for research purposes between 1977 and 1986 and trees that had been felled during a cyclone in 1971 at Magnetic Island. The length and radius of each log was measured, weighed. If partially decomposed, the original diameter was estimated. A subsample fo each was taken for CHN analysis.