SEED DISPERSAL OF METROSIDEROS-POLYMORPHA (MYRTACEAE) - A PIONEER TREE OF HAWAIIAN LAVA FLOWS

Wind dispersal of seeds of Metrosideros polymorpha, the dominant tree of rain forests and the main pioneer of volcanic substrates in Hawaii, was measured on a 20-yr-old lava flow downwind of a M. polymorpha forest. Seed rain was concentrated in December and January, when 74.5% of the annual total was dispersed. Annual seed rain at the forest edge was 63,893 seeds/M2, of which 5,580 (8.7%) contained embryos and were potentially viable. Beyond the forest edge, the density of embryo-containing seed decreased to 363, 137, 37, 25, and 20/m2 at distances of 25, 50, 100, 150, and 250 m, respectively. In contrast, the density of established M. polymorpha seedlings remained relatively constant beyond 25 m, a fact suggesting that the density of safe sites for germination and establishment was also constant over the same distance. It was concluded that colonization of the lava flow was limited by the density of safe sites, i.e., conditions for establishment, rather than by seed rain, i.e., dispersal, which was more than sufficient to saturate the available safe sites.