Litter production, decomposition and nutrient return of uplifted coral reef tropical forest

Four habitat types in a 10-ha permanent plot of an uplifted coral reef forest in southern Taiwan were chosen and subjected to systematic litterfall related processes. Ebenaceae and Euphorbiaceae were the dominant families. The common species in the four habitats were Diospyros maritime, Ficus benjamina and Melanolepis multiglandulosa. Habitat I on the flat terrace was dominated by Bischofia javanica and Palaquium formosanum; habitat II on the ridge of exposed coral reef was dominated by Aglaia formosana and Pouteria obovata; habitat III on the sedimentary basin was dominated by Macaranga tanariu; habitat IV at the bottom of valley was dominated by Pisonia umbellifera. Litter productions, decomposition processes and nutrient returns were monitored over a 12-month period in the four habitats. Bulk litter was gathered from traps for monthly accession of litterfall. Mixed-species litter bags containing equal portions of the individual species were used to measure the decomposition constants of the leaf litter. Fresh and decomposing litters were analyzed for C, N, P, Ca, Mg, K and Na. Experimental results indicated that mean monthly litterfall in all the habitats displayed a marked seasonal pattern, with spring troughs and summer, autumn and winter peaks associated with the typhoon and monsoon seasons. The annual litterfall ranged from 6.98 to 9.13 Mg ha−1 year−1, is within but in the higher range for tropical forests. The litterfall production in habitats I and IV was significantly (p < 0.05) exceeding that in habitats II and III. This phenomenon probably occurs because the dominant tree species of habitat II are relatively adapted to strong wind, and the leaves survived for a long time on the exposed coral reef; meanwhile, habitat III comprised secondary forest which was dominated by early succession tree species with lower leaf area index and basal area than those in other habitats. Significant differences in initial C concentration in the leaf litter were observed in the four habitats. Carbon release from the leaf litter was fastest in habitat IV, while the thin and less rigid leaves without prominent skeletal tissues of Pisonia umbellifera are easily decomposed. The mass and nutrients in the leaf litter could be arranged sequentially in order of decreasing mobility: Ca > Mg = N > Na > K > P > mass > C. Carbon was returned to the forest floor in the highest amount, and the next element was Ca which ranged from 196 to 324 kg ha−1 year−1. Surprisingly, the annual returns of Ca significantly exceeded those of N, and differed significantly from those of other tropical forests.