VEGETATION DECLINE FOLLOWING RECENT ERUPTIONS ON WHITE-ISLAND (WHAKAARI), BAY-OF-PLENTY, NEW-ZEALAND

Vegetation decline following recent (1976-90) eruptions on White Island (Whakaari) was assessed during 1986 and 1990 using historical accounts (1959 and 1967) as a baseline. More than two-thirds of the Metrosideros excelsa (pohutukawa) forest and scrub and all of the gannetry vegetation had declined markedly and several species had become locally extinct. The most probable causes of death of M. excelsa were toxic fumes, wet ash coating leaves, and ''acid rain''. For the small herbaceous species, complete burial by tephra was probably the main cause of death. Size class structures of three forest plots and M. excelsa growth ring counts suggested several phases of forest development, with the most advanced forest older than 180 years. Data from M. excelsa forests on other volcanic islands indicate that mature M. excelsa forest develops in 250 years. The depauperate flora of White Island compared with other Bay of Plenty islands probably results from the extreme soil conditions and continuing volcanicity. With the current eruption regime on White Island, further vegetation decline is expected.