Two years old peach trees intercropped with a grass mixture can grow through a lithic discontinuity to access water the grass cannot

The hypothesis of niche complementarity for the use of soil resources is crucial for the performance of agroforestry systems. Our aim was to test the hypothesis that roots of 2 years old peach trees intercropped with grass can extract deep soil water below a shallow lithic discontinuity, composed of a layer of pebbles in a powdery calcareous matrix and that hardens in dry conditions. A peach tree orchard was planted on a soil with an average 0.5 m deep lithic discontinuity. Soil water content was measured every fortnight on the row and the inter-row with a neutron probe every 0.2 m up to 3 m depth. The contribution of each soil layer to trees and grass transpiration was simulated with a dynamic water balance model. Results show that tree roots grew through the lithic discontinuity and accessed a water refilled soil layer beneath it only 2 years after plantation, whereas grass roots did not grow below 0.5 m. Soil water content decrease up to 2 m depth after the cessation of irrigation was analysed as the result of tree water uptake. Tree water uptake simulations showed that soil layers below 1 m can contribute up to 17% of the total water uptake of the 2 years old trees. Our results suggest that a lithic discontinuity such as the one in our study may not be an impediment to the perennial trees root growth allowing them to access deep soil water. This suggests that soils with such a lithic discontinuity may be suitable for dryland agroforestry in which deep soil water available to the trees may help in reducing competition for water.