Allelopathic potential of five agroforestry trees, Botswana

Agroforestry systems have the potential to enhance agricultural production and buffer rural livelihoods against drought in semi-arid countries such as Botswana. However, allelopathy constrains the range of tree species that can be used. Concentrations of allelopathic compounds can become particularly high in semi-arid soils because of low leaching and high evaporation rates, leading to a reduction in sub-canopy vegetation biomass. In this study, allelopathic potential was evaluated for five multi-purpose trees that are currently promoted for agroforestry in Botswana: Acacia erubescens, Acacia tortilis, Combretum imberbe, Sclerocarya birrea and Terminalia sericea. It was established that leaves from each species contained a variety of phenolic compounds (flavones, flavonols, flavonones, anthocyanins, leucoanthocyanins, coumarins and tannins) and alkaloids (tertiary and quaternary alkaloids) in varying concentrations. Although the implications for crop production are still uncertain, these results suggest that agroforestry extension in Botswana should be wary about recommending the use of these species before further research.