FATE OF A WET MONTANE FOREST DURING SOIL AGING IN HAWAII

1 We chose seven sites across the Hawaiian archipelago differing only in substrate age (400 years to 4.1 x 10(6) years). All sites were at 1 200m elevation, and mean annual rainfall was greater than or equal to 4000 mm, This chronosequence reflects long-term ecosystem development from basaltic lava parent material under a humid climatic regime. 2 Li ve above-ground biomass of woody species greater than or equal to 1 m tall changed unimodally along the chronosequence from 217 tons ha(-1) at the youngest site to a peak of 406 tons ha(-1) at the 5000-year sit e, before declining to 75 tons ha(-1) at the oldest site. 3 The size o f the soil organic C pool above sub-surface lava or hardpan initially followed the pattern of above-ground biomass, increasing from the youn gest site to the 5000-year site, and declining at the 9000-year site. However, it then steadily increased to the oldest site. The proportion of above-ground biomass C to the total C (above-ground biomass + soil ) decreased linearly against logarithmic age from 74% at the youngest site to 8% at the oldest site. 4 Net soil N mineralization rate increa sed from the youngest site to the 5000-year site, and then declined wi th age to a nearly constant value except for an outstandingly high val ue at the oldest site. Exchangeable Ca and available P in topsoil incr eased from the youngest to the 5000-year site, before declining at old er sites. 5 Soil redox potential (Eh7) was invariably high (greater th an or equal to c., 500 my) at the sites less than or equal to 9000 yea rs, but declined at two old sites (410000 years and 4100000 years). 6 Live fine-root biomass in the topsoil increased steadily with substrat e age. The distribution of fine roots in the soil profile was positive ly correlated with redox values. 7 High precipitation rates appear to lead to the development of iron hardpan during pedogenesis. This in tu rn initiates a positive feedback that promotes waterlogging and anaero biosis, resulting in reduced organic matter mineralization and increas ed soil C accumulation. Reduction of biomass with age can be explained by increasingly restricted root penetration, as well as by the reduct ion in available soil P, N and Ca as a result of geochemical immobiliz ation, leaching and/or reduced mineralization.