Population Development of Rain Forest Trees on a Chronosequence of Hawaiian Lava Flows

The purpose of this study was to compare forest structure on a series of geologically similar larva flows that differed in age, but not in climate or in accessibility to pioneer plants. The data were then used to infer the patterns of forest development over 3000 yr on a single flow. Stand structure was measured for populations of Metrosideros polymorpha, other canopy tree species, and tree ferns on a chronosequence of lava flows aged 47, 137, °300, °400, and °3000 yr, in the montane rain forest zone on Mauna Loa, Hawaii. The upper stratum of vegetation at all sites was dominated by M. polymorpha; populations of other trees were relatively sparse throughout the study area. For M. polymorpha populations, the basal area per hectare increased, at an ever—decreasing rate, as flow age increased; population densities of seedlings, samplings, and trees peaked on the 137—yr flow, then declined, apparently through self—thinning and wind—throw, with further increases in flow age. The decrease in M. polymorpha population density on older flows was accompanied by an increase in the density of tree ferns, which formed a closed subcanopy on the 3000—yr flow, and may have inhibited regeneration of M. polymorpha. Among M. polymorpha trees there was a successional transition from pubescent varieties on the three youngest flows to glabrous varieties on the oldest flow. A comparison between plant populations on the chronosequence and on one other flow, aged °1400 yr but containing some volcanic ash, indicated that ash may alter some aspects of forest development.