There is a renewed interest in temperate agroforestry systems because of their potential to increase biodiversity, sequester carbon and diversify the landscape while maintaining productivity. Little quantitative information is available about the interaction between trees and the crop for water, especially in temperate climate and for tree ages towards the end of an agroforestry cycle. With this study, we quantified the effect of mature poplar trees on soil moisture dynamics in space and time in an agricultural field sown with maize during one growing season. We confirmed the ability of electrical resistivity tomography to study tree-crop interactions for water under field conditions and we delimited an area of influence of the 40-year old trees on the crop of about 15 m. In order to do this, we installed four 30 m electrode transects perpendicular to the field border. Three transects were located next to a tree-bordered part of the field and one reference transect was located along the same border, but without any tree present. We performed seven electrical resistivity tomography (ERT) measurements during the maize growing season and compared the soil moisture distribution and dynamics with and without tree border as a proxy for a mature agroforestry system. We showed that the ERT tomograms in a tree-bordered zone are significantly different from a reference zone without trees along the 30 m of the transect using a single and segmented linear regression analysis. This article shows the potential of ERT to quantify tree-crop-soil interactions for water in agroforestry systems.