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 cedrata, Heritiera utilis, Lophira alata, Mansonia altissima, Milicia excelsa, Pericopsis elata, Pterygota macrocarpa, Strombosia glaucescens, Terminalia ivorensis and Triplochiton scleroxylon, which represent different ecological guilds (pioneers, non-pioneer light-demanders and non-pioneer shade-bearers) and distributions on a rainfall and soil fertility gradient were tested for their response to differences in irradiance and soil fertility. 2. Seedlings were grown for 4 months at a common site on contrasting forest soils from a wet evergreen forest and a moist semi-deciduous forest and at five irradiances, which mimicked variation in irradiance from single tree falls to cleared land. 3. Pioneer species responded differently to irradiance by showing higher maximum relative growth rates (>30 mg g -1 day -1 ) and unit leaf rates (>3.0 g m -2 day -1 ) than non-pioneer shade-bearers ( 220 cm g -1 ) and specific leaf areas (>400 cm g -1 ) compared to shade-bearers. Non-pioneer light-demanders showed intermediate responses to differences in irradiance. 4. The soil from the wet evergreen forest was more acid (pH ≤ 4.87) than the soil from moist semi-deciduous forest (pH ≥ 6.12). It also had lower concentrations of total N and available P, K, Ca and Mn, particularly in the topsoil. Seedlings grown in wet evergreen forest soil showed reduced leaf foliar concentrations of P, Ca and Mg and to a lesser extent N, compared with those grown in moist semi-deciduous forest soil, but the response varied between species. Foliar nutrient concentrations, particularly of P and Mg, were lowest in species naturally restricted to wet evergreen forest. 5. The non-pioneer light-demander 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 increased 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. Suggestions for further research are made.