RESPONSES OF WEST-AFRICAN FOREST TREE SEEDLINGS TO IRRADIANCE AND SOIL FERTILITY

1. The West African tree species Albizia zygia, Blighia sapida, Celtis mildbraedii, Chrysophyllum pruniforme, Cynometra ananta, Guarea cedra ta, Heritiera utilis, Lophira alata, Mansonia altissima, Milicia excel sa, Pericopsis elata, Pterygoya macro- Strombosia glaucescens, Termina lia ivorensis and Triplochiton scleroxylon, which represent different ecological guilds (pioneers, non-pioneer light-demanders and non-pione er shade-bearers) and distributions on a rainfall and soil fertility g radient were tested for their response to differences in irradiance an d soil fertility. 2. Seedlings were grown for 4 months at a common sit e on contrasting forest soils from a wet evergreen forest and a moist semi-deciduous forest and at five irradiances, which mimicked variatio n in irradiance from single tree falls to cleared land. 3. Pioneer spe cies responded differently to irradiance by showing higher maximum rel ative growth rates (>30 mg g(-1) day(-1)) and unit leaf rates (>3.0 g m(-2) day(-1)) than non-pioneer shade-bearers (<22 mg g(-1) day(-1) an d <2.1 g m(-2) day(-1), respectively). Particularly at the irradiance level below 8% of ambient, pioneers also showed higher leaf area ratio s (>220 cm g(-1)) and specific leaf areas (>400 cm g(-1)) compared to shade-bearers. Non-pioneer light-demanders showed intermediate respons es to differences in irradiance. 4. The soil from the wet evergreen fo rest was more acid (pH less than or equal to 4.87) than the soil from moist semi-deciduous forest (pH greater than or equal to 6.12). It als o had lower concentrations of total N and available P, K, Ca and Mn, p articularly in the topsoil. Seedlings grown in wet evergreen forest so il showed reduced leaf foliar concentrations of P, Ca and Mg and to a lesser extent N, compared with those grown in moist semi-deciduous for est soil, but the response varied between species. Foliar nutrient con centrations, particularly of P and Mg, were lowest in species naturall y restricted to wet evergreen forest. 5. The non-pioneer light-demande r Mansonia altissima and the pioneer T. scleroxylon showed an increase of 300% and 100%, respectively, in maximum RGR when grown in the more fertile soil type. In contrast, RGR of the pioneer L. alata was incre ased by 80% in the less fertile soil. Species-specific differences in growth response, biomass allocation and foliar nutrient concentration may explain why some species are restricted to more fertile soils. Sug gestions for further research are made.