The ecology of mangrove vegetation in Bintuni Bay, Irian Jaya, Indonesia

Abstract

The ecology of mangrove forests on Potential Acid Sulphate soils at Bintuni Bay (132° 55' - 134° 02' E, and 2° 02' - 2° 97' S), Irian Jaya, Indonesia was studied. The annual rainfall is 3000mm and there is mixed semi-diurnal tide of 1-5.6m amplitude. The water has a varying salinity of 0-27%. Forest structure was studied in plots of 10m x 10m along three transects across Sikoroti Island and in three 50m x 50m plots in mixed Rhizophora - Bruguiera forest. All trees > 10cm trunk diameter were enumerated, measured and identified. Of the nine tree species, Rhizophora apiculata was the most dominant, followed by Bruguiera gymnorrhiza and Ceriops decandra. There was good regeneration seven years after clear-felling in plots on Amutu Besar and Amutu Kecil Islands. Small litterfall production, measured using ten 1m x 1m litter traps in each of the three 50m x 50m plots, was 11.09 t ha-1 y-1, and greatest during the wet season (December, 1.29 t ha-1 ) and least during the dry season (July, 0.61 t ha-1). The annual litterfall mineral element accession was (kg ha-1 y-1): N 240.4, P 6.1, K 43.2, Na 136.2, Ca 204.4 and Mg 48.7. Leaf decomposition of five species was studied in litterbags on the forest floor under trees of the same species. Sonneratia alba decomposed quickly with a half-life of 24 days, and Bruguiera parviflora was the slowest with a halflife of 124 days. Decomposition rates of all species followed a single exponential decay model. Leaf herbivory of young stands of Rhizophora apiculata and Bruguiera gymnorrhiza was significantly different among sites, species, plant height and leaf-age. Seed predation on six species was studied in three different sites. Twenty propagules of each species were secured by string in each of six 10m x 10m sub-plots within each of five 10m x 60m plots. On average 62.1% of the seeds were dead after 36 d. Predation was higher in the lower intertidal zone, and Avicennia alba and Bruguiera parviflora were the most predated. The results suggested that the Bintuni Bay mangroves can be sustainably managed but the best silvicultural system needs to be determined.