Population ecology of Metrosideros Polymorpha and some associated plants of Hawaiian volcanoes

Abstract

Metrosideros polymorpha is the dominant tree of Hawaiian rain forests and lava flows. Research was undertaken to explain: the pattern of Metrosideros forest development on lava flows; the role seed ecology plays in maintaining Metrosideros populations; and the relationships between Metrosideros and associated species. Long-term rain forest development was inferred from measurements of population structure for Metrosideros, other trees, and tree ferns on a chronosequence of five lava flows (aged 47-3000 yr; Mauna Loa). Metrosideros comprised ≥ 70% of the tree basal area at each site. Although Metrosideros basal area increased with flowage, population densities peaked on the 137 yr flow, then declined, apparently through self-thinning. On the older flows, a dense tree fern canopy may have inhibited regeneration of Metrosideros. Dominance shifted from pubescent to glabrous varieties of Metrosideros as flow age increased. Seed dispersal data, collected on a lava flow downwind of Metrosideros forest, indicated that seed density decreased from 5580 m- 2yr- 1 at the forest edge, to 20 m-2yr-1 250 m away. However, seedling density did not decrease across the flow, suggesting that recruitment is not limited by seed rain. In the laboratory, Metrosideros seeds germinated over a wide range of temperatures and light qualities. Germination in the dark and emergence after burial were poor. The light requirement was not overcome by a fluctuating thermoperiod. Seeds from glabrous and pubescent plants differed in their germination characteristics. In a Kilauea forest, abundances were quantified for seed plants in the vegetation, seed rain, and seed bank. The seed rain and vegetation were more similar to each other than either was to the seed bank. Metrosideros dominated the vegetation and seed rain, but formed only a pseudo-persistent seed bank. Native species dominated the vegetation and seed rain, but alien species dominated the seed bank, suggesting that aliens may replace many of the natives if the forest is disturbed. Metrosideros seeds are produced in great numbers, are widely dispersed, and can germinate under diverse environmental conditions. These attributes contribute to the success of Metrosideros as an early colonizer of new substrates and should also promote regeneration in established forests.