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 lava flows that differed in age, but not in cl imate or in accessibility to pioneer plants. The data were then used t o infer the patterns of forest development over 3000 yr on a single fl ow. Stand structure was measured for populations of Metrosideros polym orpha, other canopy tree species, and tree ferns on a chronosequence o f lava flows aged 47, 137, almost-equal-to 300, almost-equal-to 400, a nd almost-equal-to 3000 yr, in the montane rain forest zone on Mauna L oa, Hawaii. The upper stratum of vegetation at all sites was dominated by M. polymorpha; populations of other trees were relatively sparse t hroughout the study area. For M. polymorpha populations, the basal are a per hectare increased, at an ever-decreasing rate, as flow age incre ased; population densities of seedlings, saplings, 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. pol ymorpha population density on older flows was accompanied by an increa se in the density of tree ferns, which formed a closed subcanopy on th e 3000-yr flow, and may have inhibited regeneration of M. polymorpha. Among M. polymorpha trees there was a successional transition from pub escent varieties on the three youngest flows to glabrous varieties on the oldest flow. A comparison between plant populations on the chronos equence and on one other flow, aged almost-equal-to 1400 yr but contai ning some volcanic ash, indicated that ash may alter some aspects of f orest development.