1. Regeneration of Metrosideros polymorpha (Myrtaceae) in dieback stands of a montane tropical rain forest Hans Jürgen Boehmer*, Grant C. Gerrish #, James D. Jacobi §, Dieter Mueller-Dombois ** *Technical University of Munich, Department of Ecology, Landscape Ecology (LOEK), Am Hochanger 6, D-85350 Freising-Weihenstephan, Germany; e-mail: neobiota@web.de # University of Hawaii at Hilo, Biology Department, 200 W. Kawili Street, Hilo, HI 96720, U.S.A.; e-mail: grant@hawaii.edu § US Geological Survey, PIERC Kilauea Field Station, Bldg. 344 P.O. Box 44, Hawaii National Park, HI 96718, U.S.A.; e-mail: jim_jacobi@usgs.gov ** University of Hawaii at Manoa, Department of Botany, 3190 Maile Way, Honolulu, HI 96822, U.S.A.; e- mail: amdhawaii@aol.com Metrosideros polymorpha (Myrtaceae) is the dominant tree species in the montane rain forests on the island of Hawaii. During the 1960s and 1970s, about 50,000 hectare of Metrosideros dominated rain forest experienced a decline of the canopy ('ohia dieback’). The forests affected were located on the windward side of the volcanic mountains Mauna Loa and Mauna Kea between 700 and 1,500 m elevation (Fig. 1). References: Jacobi, J. D., Gerrish, G., Mueller-Dombois, D. & L. Whiteaker (1988): Stand-level Dieback and Metrosideros Regeneration in the Montane Rain Forest of Hawaii. - GeoJournal 17 (2): 193-200; Mueller-Dombois, D. (1987): Natural Dieback in Forests. - BioScience 37 (8): 575-583. Mueller-Dombois, D. & F. R. Fosberg (1998): Vegetation of the Tropical Pacific Islands. New York. After a decade of intensive research, a regeneration model was created, predicting a recovery of the canopy due to recruitment of young, healthy individuals into the tree layer (Fig. 3). Today, the primary objective of study is to test these predictions and to document the regeneration of Metrosideros polymorpha. Metrosideros population structure and tree vigor were resampled in 1990, 1995, and 2002/2003 (Fig. 5-7). The results indicate that in many dieback sites a new cohort of Metrosideros trees has been established about 30 years after the dieback started. 11 plots show a distinct canopy improvement. The number of seedlings strongly decreased in dieback plots since the mid-1970´s. Fig. 1: Study area and location of the permanent plots Key words: cohort senescence, dieback, Hawaii, Metrosideros polymorpha, rain forest, vegetation dynamics In 1976, 25 permanent plots were established to assess tree vigor, composition and structure of the vegetation, and to describe substrate characteristics. At that time, 14 plots showed heavy or severe dieback symptoms (dieback plots; Fig. 4); 11 plots showed no, slight or moderate dieback symptoms (non-dieback plots). Attention focused on a uniform age and stand structure ('cohorts') brought about by macro-disturbances through volcanic eruptions and resulting in the simultaneous ageing and finally death of the stands ('cohort senescence'), triggered by climatic anomalies like extreme rainfall events and other forms of abiotic stress. Fig. 3: Generalized model of the population structure of Metrosideros polymorpha A) before dieback and B) after dieback (Mueller-Dombois 1987) Fig. 4: Dead and dying Metrosideros trees in the Saddle Road area Fig. 2: Healthy Metrosideros forest with native understorey (Cheiroden- dron trigynum, Cibotium glaucum) Fig. 5: In the mid-1970’s, an exceptionally high number of Metrosideros seedlings (0.1 - 0.5 m tall) was counted in dieback plots Fig. 6: The seedlings flush resulted in a wave of saplings (0.5 m - 5.0 m tall Metrosideros) which had its peak in the early 1980´s Fig. 7: Today, the density of healthy trees per hectare has almost reached the level of healthy Metrosideros stands again