Fate of a wet montane forest during soil ageing in Hawaii

1. We chose seven sites across the Hawaiian archipelago differing only in substrate age (400 years to 4.1 x 10⁶ years). All sites were at 1200 m elevation, and mean annual rainfall was ≤ 4000 mm. This chronosequence reflects long-term ecosystem development from basaltic lava parent material under a humid climatic regime. 2. Live above-ground biomass of woody species ≤ 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 site, before declining to 75 tons ha^-1 at the oldest site. 3. The size of the soil organic C pool above sub-surface lava or hardpan initially followed the pattern of above-ground biomass, increasing from the youngest 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 increased from the youngest site to the 5000-year site, and then declined with age to a nearly constant value except for an outstandingly high value at the oldest site. Exchangeable Ca and available P in topsoil increased from the youngest to the 5000-year site, before declining at older sites. 5. Soil redox potential (Eh7) was invariably high (≤ c. 500 mv) at the sites ≤ 9000 years, but declined at two old sites (410 000 years and 4100 000 years). 6. Live fine-root biomass in the topsoil increased steadily with substrate age. The distribution of fine roots in the soil profile was positively correlated with redox values. 7. High precipitation rates appear to lead to the development of iron hardpan during pedogenesis. This in turn initiates a positive feedback that promotes waterlogging and anaerobiosis, resulting in reduced organic matter mineralization and increased soil C accumulation. Reduction of biomass with age can be explained by increasingly restricted root penetration, as well as by the reduction in available soil P, N and Ca as a result of geochemical immobilization, leaching and/or reduced mineralization.