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 [10.sup.6] years). All sites were at 1200m 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 Live above-ground biomass of woody species [greater than or equal to] 1 m tall changed unimodally along the chronosequence from 217 tons [ha.sup.-1] at the youngest site to a peak of 406 tons [ha.sup.-1] at the 5000-year site, before declining to 75 tons [ha.sup.-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 ([greater than or equal to] c. 500 mv) at the sites [less than or equal to] 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. Keywords: above-ground biomass, anaerobiosis, nutrient availability, redox potential, soil organic carbon