The Effect of Temperature and Light on Metrosideros polymorpha Seed Germination

Seeds of a Hawaiian rain forest tree species, Metrosideros polymorpha, were germinated at temperatures ranging from 5 to 35°C and under photosynthetic photon flux densities ranging from 0 to approximately 2000 IlE·m-2·s-1. Results after 30 days indicate that 25°C is the optimal temperature and 170 IlE·m-2·s-1 or about 4-15 percent relative irradiance is the optimal light intensity for M etrosideros germination. Declining germination at higher irradiances was probably due to excessively high temperatures and intermittent desiccation. No seeds germinated at temperatures less than 12°C. Light was not found to be strictly required but improved germination by up to four times. Only 14 percent of the seeds sampled appeared to have intact embryos, hence the poor (15 percent) germination achieved even under optimal conditions. Low temperatures (generally less than 17°C) must curtail germination success on the floor of montane rain forests. The higher temperatures associated with increased light intensity are probably more beneficial than light itself in increasing germination success in forest clearings. These relationships to light and temperature may partly explain why Metrosideros seedlings are often infrequent beneath dense rain forest canopies. THE MOST COMMON CANOPY-FORMING tree species in the Hawaiian Islands is Metrosideros polymorpha Gaud. (Myrtaceae). Canopy dieback of this species on the island of Hawaii and the role of this phenomenon in stand maintenance is currently under investigation (Mueller-Dombois 1981). An assessment of the regeneration requirements of a species must consider the effects of environmental conditions on its germination success. This investigation specifically addresses the issue I This work was supported by National Science Foundation grant no. DEB 79-10993 to D. MuellerDombois. This paper is part of a thesis submitted to the Graduate Division of the University of Hawaii in partial fulfillment of the requirements for the M.S. degree in Botanical Sciences. Manuscript accepted 4 December 1981. 2 University of Hawaii, Department of Botany, 3190 Maile Way, Honolulu, Hawaii 96822. Present address: Monenco Consultants Limited, 900 One Palliser Square, 125 9th Avenue S.E., Calgary, Alberta, Canada T2G OP6. of optimal temperature and light conditions required for maximum germination of M etrosideros seeds. On the island of Hawaii, Metrosideros can be found growing naturally from sea level to an elevation of 2700 m (9000 ft) (Rock 1913). This implies a range in annual mean air temperature from approximately 9 to 23°C (Doty and Mueller-Dombois 1966) and that Metrosideros must be able to germinate across this temperature range. It has been suggested that M etrosideros cannot maintain itself in mature rain forests due to the fact that its seedlings cannot become established in the dense forest shade (Mueller-Dombois et al. 1980). Corn (1979) states that light is required for the germination of this species. Sarvas (1950), Evenari (1964), Black (1969), and Toole (1973) cite many examples of species with seeds that do not germinate in the absence of light or for which germination success is increased by the presence of light. Environmental conditions of light and tem-