E ect of light on the germination of forest trees in Ghana

Abstract

1 Seed germination in light and dark, and responses to irradiance and light quality, were tested in shadehouse experiments for 19 West African tropical forest tree spe- cies representing a wide range of ecological types. Germination in forest gaps of di erent size was tested for 11 species. 2 Percentage germination was reduced in the dark only for three small-seeded spe- cies that are common in forest soil seed banks: Musanga cecropioides, Nauclea diderrichii and Milicia excelsa. Percentage germination of the other 16 species, including four widely regarded as `pioneers', was una ected. 3 E ects of di erent irradiances in shadehouses, where the seeds were watered, were signi®cant for some species, but there was no consistent pattern. Irradiance e ects in forest gaps, where the seeds received only natural wet season rainfall, were more widespread and substantial, and were most commonly shown as a depression of percentage germination at high irradiance. 4 E ects of light quality (neutral vs. green shade; red : far-red=0.43) were insignif- icant at 5% irradiance in shadehouses for all species except Nauclea diderrichii. In growth chamber experiments, the low energy response was only evident at 1.0 mmol m±2 s±1 (<1% of unshaded forest irradiance) in Musanga and Nauclea. 5 The speed of germination was a ected by irradiance in many species, but the e ect was small compared with di erences between species, in which time to com- plete germination varied between 3 weeks and over 6months. 6 Seeds of Ceiba pentandra and Pericopsis elata planted in deep forest shade (2% irradiance) and in a small gap (30% irradiance) germinated well in both sites, showed exponential biomass growth in the gap but a linear decline in mean seed- ling biomass and subsequent death in deep shade. 7 Light-mediated germination is relatively rare among these forest trees, even among pioneers, so that the working de®nition of a pioneer should be seen to depend more on a species' ability to survive in forest shade. The e ects of canopy opening on seed germination are small except in the largest openings, which severely depress germination in a number of species, including some species with strongly light-demanding seedlings.