Root growth and biomass partitioning of nine juvenile Sahelian agroforestry tree species under drought and irrigation treatments

Abstract Background and aims Root traits of tree species are of great importance for selection of trees in dry zone agroforestry systems. Climate models predict increased frequency and severity of drought in the Sahel but our understanding on the strategies of root growth of tree species under drought is limited. To increase our knowledge of species differences in rooting patterns we investigated root growth of nine dryland tree species in West Africa during one year when exposed to drought stress. Methods Seedlings were planted in 2 m tubes inserted into the soil and subjected to irrigation and drought (no irrigation during the dry season). After 4, 8 and 12 months, we assessed root growth and root biomass, supplemented with assessments of water potential, leaf phenology, and above-ground growth. Results Tree species showed significant differences in rooting profile. Drought induced deeper root growth, especially in deciduous species. Adansonia digitata, Sterculia setigera and Anacardium occidentale had shallow roots when irrigated. Acacia senegal, Acacia nilotica and Faidherbia albida had deeper root growth and showed clear differences in leaf water potential between treatments while A. digitata and S. setigera appeared to be water conserving under drought. Conclusions Our results link root growth with phenology, physiology and above-ground growth and provide an attempt to group dryzone tree species in functional types based on their root growth.