Nutrient capital in different aged forests of the mangrove Rhizophora apiculata

Abstract

Partitioning and storage of mineral nutrients and trace elements were examined in tree components and soils of differently aged (3-, 5- and 25-year-old) forests of the mangrove Rhizophora apiculata in southern Thailand. Despite lack of replication of forests, three patterns of element partitioning and forest nutrient capital emerged: (1) concentrations of most (but not all) elements in various tree parts declined with increasing tree age; (2) the soil pool size of most elements decreased with increasing stand age; and (3) the proportion of C, N, Na, Mn, Zn and Mo in total living biomass increased with increasing forest age. Most elements were stored in soils and dead roots, supporting the concept that wet tropical forests do not store proportionally more nutrients in their biomass than in soils, as compared to temperate and boreal forests. The large element pools stored in dead roots may serve as a conservation mechanism. Our data suggest that the proportion of element capital stored in forest biomass increases, but decreases in soils as forests age, suggesting net accumulation with forest maturity. Mean residence time for total forest N increased from 0.3 years for the 3-year-old forest to 0.5 years for the 5-year-old forest to 0.6 years for the 25-year-old forest. Compared with other forests, N turnover is very rapid, mirroring the tight coupling between production and decomposition processes in these wet tropical ecosystems.