Studies of Mineral Cycling in a Montane Rain Forest in New Guinea: I. The Distribution of Organic Matter in the Vegetation and Soil

Abstract

(1) As a preliminary to studies on mineral cycling in a tropical montane system, where slow release of minerals from organic matter may be an important feature of the environment for olants, estimates have been made of the amounts of organic matter in a Lower Montane Rain forest and its soil at 6 ?S and altitude c. 2500 m in New Guinea. (2) A detailed destructive study was made of a plot 20 x 20 m, and derived estimates of biomass were made for various other plots using a regression of dry weight on the square of trunk diameter at breast height. On a 0-24 ha plot considered representative of the forest at large the total biomass was estimated at c. 350 t ha-1, made up of c. 40 t ha-' in roots, 295 t ha-' in above-ground parts of trees of gbh 3 30 cm, c. 4 t ha-' in climbers and scramblers, c. 2 t ha-' in epiphytes, c. 2 t ha-' in the 0-1 m layer and c. 7 t ha-' in the other plants. (3) Determinations of specific leaf area were made for seventeen tree species (range 37-127 cm2 g-', mean 58 cm2 g1), and for four common climbers (45-81 cm2 g') and a scrambling bamboo (216 cm2 g-1). The leaf area index was estimated as. c 5 5 m2 m 2 on the plot of 20x20m. (4) The ash contents of 140 representative plant samples were determined; they ranged from c. 10% for the 0-1 m layer to 2% in the trunks. The organic matter content of the soil was taken to be twice the organic carbon content, which was c. 19% dry weight at 0-2 cm and c. 13% at 2-10 cm, falling to c. 9% at 100 cm depth. The organic matter content of the plants on the 0-24 ha plot was estimated at c. 340 t ha 1, and that of the soil at c. 1200 t ha-'. About 1 t ha-' of soil (4% ash) had accumulated in the crowns of the larger trees, formed largely from the remains of epiphytes. (5) Comparisons are made with other tropical forests, and the inadequacy of sampling in most studies is emphasized. (6) The ratio of organic matter in the soil to that in the plants is 3-4: 1 in the New Guinean forest. This may be compared with probable values of 0-7-4-0:1 in other Lower Montane forests and measured values within the range 0-2 0-6:1 in Lowland forests. The very large amount of organic matter in the soil of the New Guinean forest is tentatively related to fixation by amorphous clays.