Ecological studies on a lowland evergreen rain forest on Maraca Island, Roraima, Brazil. II. Litter and nutrient cycling

1. The Maraca rain forest occurs on sandy soils which have very low concentrations of nutrients, but it has none of the structural features which are often held to be associated with forests on nutrient-poor soils. 2. The present paper reports tests of three hypotheses which were proposed to account for these unexpected features of the Maraca forest: (a) that the forest is unproductive and has a low nutrient demand; (b) that the trees are particularly efficient at withdrawing foliar nutrients before leaf abscission; and (c) that decomposition is rapid and the nutrients are quickly recycled. 3. Small litterfall (leaves, wood less than or equal to 2 cm in diameter, reproductive parts, and trash) was measured for 12 months in three replicate 0.25-ha plots for quantity, seasonal distribution and concentrations of nitrogen, phosphorus, potassium, calcium and magnesium. 4. The mean total small litterfall was relatively high with a mean value for the three plots of 9.28 t ha-1 year-1 (range 8.85 - 9.52). 5. The nitrogen concentrations in the Maraca small litterfall were at the lower end of the range reported for other tropical forests whilst the concentrations of phosphorus, potassium, calcium and magnesium were relatively high. 6. Small-litter layer mass was measured in the plots on four occasions over a year. It showed changes with season and the mean values for total small-litter for the three plots were least (4.00 t ha-1) in the wet season and highest (5.14 t ha-1) in the dry season. 7. The concentrations of nitrogen, magnesium and potassium were lower in the small-litter layer than those of the litterfall, whilst phosphorus and calcium concentrations were similar. 8. Mean large-wood (greater than or equal to 2 cm in diameter) litter (including standing dead trees) had a mean value of 5.80 t ha-1 (range 3.54 - 7.37) for the three plots. 9. Decomposition quotient (kL) values were relatively high with mean value of 2.94 for the leaf fraction and 2.01 for total small litterfall. 10. The quotients of litterfall content/litter layer content ranged from 1.99 for calcium to 5.91 for potassium and are indicative of the rapid cycling and an important source of nutrients to the trees. 11. A comparison of nutrients in fresh leaves of six species with those in their litterfall showed retranslocations (on a unit-calcium basis) ranging from 17% to 73% for nitrogen and from 41% to 82% for phosphorus. This wide range of values, suggests no strong selection for nutrient retranslocation and that neither nitrogen nor phosphorus are limiting growth in this forest. 12. It is concluded that the Maraca forest shows no indications of nutrient limitation in spite of its occurrence on sandy soils with low concentrations of nutrients and contradicts several generalizations about rain-forest nutrition.