Fatou Gning

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.

Understanding the root traits of tree species is of great importance for selection of trees in dry zone agroforestry systems. We investigated root growth of nine dryland tree species in West Africa and assessed the effect of drought stress on biomass allocation to root systems as function of soil depth. Seedlings were planted in 2m tubes inserted into the soil and subjected to three watering treatments: top irrigation, low irrigation at 50 cm depth, and drought. After 8 and 12 months, we assessed root growth and architecture, and supplemented with assessments of water potential, leaf phenology, and above-ground growth. Results showed large differences in rooting profile of the tree species. Drought induced faster root growth and all species reached the maximum depth after 12 months. Rooting depth was reduced under top irrigation for Adansonia digitata, Sterculia setigera and Anacardium occidentale that concentrated root biomass in the upper soil layers, while Acacia senegal, Acacia nilotica and Faidherbia albida had deeper root systems. Acacia species showed clear differences in leaf water potential between treatments while A. digitata and S. setigera appeared to be water conserving under drought. We discuss results in terms of isohydry/an-isohydry and evergreen vs. deciduous habits.