The End Ever Nigh: Contemporary population change on Pitcairn island

Approximately 50,000 ha of native ‘ōhi’a (Metrosideros polymorpha) rain forest on the windward side of the island of Hawaii experienced a pronounced dieback of the tree canopy during the 1960s and early 1970's. The forests affected were located between 600 and 1,500 m elevation where the median annual rainfall was greater than 25000mm. Dieback stands were found on a variety of different substrates ranging from 500–10,000 years in age. Most of the stands that experiencel dieback had canopy cover greater than 60% prior to 1960. In 1976 and 1977 we established 43 relevés in dieback and non-dieback forest stands to assess canopy tree vigor, composition and structure of the vegetation, and to describe substrate characteristics.Metrosideros population structure and tree vigor were resampled for 25 of these relevés in 1982 and 1985–1986. The results of air photo analysis and ground sampling showed that dieback has not expanded very much within the study area since 1977. However, 5 of the relevés sampled in 1977 continued to decline in tree canopy vigor between 1977 and 1982 while 2 others showed a slight recovery in vigor over this same period. Seedling and sapling regeneration has been extremely vigorous in most of the sites that experienced a breakdown of the canopy while, in stands with an intact, dense tree canopy no such regeneration occurred. It appears that most of the stands which experienced canopy dieback have the potential to become closed forest communities again, dominated byMetrosideros. The natural recovery process may be disrupted in some areas due to additional competition for light and nutrients from invading populations of both native and alien plant species that have become established following canopy dieback.