Germination of seeds of tropical rainforest species: responses to time and light quality

Abstract

There is scant information available about the seed ecology of species of the tropical rainforests of Australia. This is the first known investigation of a large number of species from the rainforests of north east Queensland in relation to time and to light quality. A total of 136 seed lots representing 130 species were randomly collected and germinated in four different light treatments - white, red and green light and darkness. For the sake of convenience, darkness is at times referred to as black light in this paper. Time to germination was assessed in white light, and recorded over 12 or more weeks. Results were obtained for 112 seedlots. Sixty percent germinated within the first six weeks after sowing, 23 percent took 7-12 weeks, and the remaining species germinated after 12 weeks, the longest recorded period being 31 weeks. Species with rapid germination tended to have highly uniform germination and a high germination capacity. Over 60 percent of the species germinating in the first two weeks had highly uniform germination, and this fell to 28 per cent for species germinating in the following four weeks. Germination in the three ecologically relevant light treatments (white, green and black light or darkness) is presented in a conceptual framework (a ternary plot) that allows for easier interpretation of the relative light responses of the test species. It is visually apparent that the largest group (57 per cent of the species which yielded results) is made up of species which show no germination response to light quality. Other recognisable groups are species for which germination is facilitated by, or occurs only in white light, and those demonstrating inhibition of germination in green light. Significantly, there are no species with maximum germination in green light, which simulates canopy shade. This indicates that while many species are capable of germinating in shade, germination levels are not maximised in this light environment for the tropical rainforest species in this study. For all species there is a weak negative correlation between percentage of total germination in white light and seed size. Within related groups of species, smaller seed size is associated with a germination response to light quality. Early successional species, as expected, generally germinated better in white light, and distribution patterns of some species suggest that dispersal mechanisms could be important in transporting the seed to a suitable light environment for successful germination.