Effect of light on the germination of forest trees in Ghana.

Seed germination in the light and dark, and responses to irradiance and light quality, were tested in shadehouse experiments for 19 West African tropical forest tree species representing a wide range of ecological types. Germination in artificial forest gaps of different sizes (adjusted to give ÷15, 30 and 50% of unshaded irradiance, compared with the control closed canopy irradiance of 2%, and the outside forest irradiance of 100%), was tested for 11 species (10 of them the same as used in the shadehouse studies) during the rainy season in 1992 and 1993, in the Tinte Bepo Forest Reserve, Ghana. Percentage germination was reduced in the dark only for 3 small-seeded species that are common in forest soil seed banks - Musanga cecropioides, Nauclea diderrichii and Milicia excelsa. Percentage germination of the other 16 species, including 4 widely regarded as pioneers, was unaffected. Effects of different irradiances in shadehouses, where the seeds were watered, were significant for some species, but there was no consistent pattern. Irradiance effects 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. Effects of light quality (neutral vs. green shade; red:far-red 0.83 and 0.43, respectively) were insignificant at 5% irradiance in shadehouses for all species except N. diderrichii. In growth chamber experiments, the low energy response was only evident at 1.0 æmol m-2 s-1 (.LT.1% of unshaded forest irradiance) in M. cecropioides and N. diderrichii. The speed of germination was affected by irradiance in many species, but the effect was small compared with differences between species, in which time to complete germination varied between 3 wk and .GT.6 months. 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 seedling biomass and subsequent death in deep shade. Light-mediated germination is relatively rare among these forest trees, even among pioneers, so that the working definition of a pioneer should be seen to depend more on a species' ability to survive in forest shade. The effects of canopy opening on seed germination were small except in the largest openings, which severely depress germination in a number of species, including some species with strongly light-demanding seedlings.