Résumé
Background and Aims: Characterising the spatial distribution of tree fine roots (diameter ≤ 2 mm) is fundamental for a better understanding of tree functioning in agroforestry ecosystems. The absorptive function of fine roots is closely associated to aboveground tree performance and the amount of competition experienced by a tree partly depends on how root spatial distribution varies with soil depth in mixed systems. We therefore compared root interception densities (RID) and fine root orientation of trees grown in an alley cropping agroforestry stand (AF) with those in a monocropped (M) tree stand. Methods: Fieldwork was conducted in two 17 year old hybrid walnut (Juglans regia × nigra cv. NG23) stands in southern France: one was an alley cropping agroforestry stand with durum wheat (Triticum turgidum L. subsp. durum) intercropped in strips between tree rows and the second was a monocropped stand with a natural understorey. One pit was dug in each stand to a depth of 1.6 m, and an additional pit was dug in the agroforestry stand to a depth of 4.0 m to examine the tree root distribution below the crop maximum rooting depth. Root intercepts were mapped on trench walls and soil cubes were sampled to assess 3D root orientation, and to establish a predictive model of root length densities (RLD).Results: In the monocropped stand, root mapping evidenced a very high RID in the top 0.5 m and a slight decrease with increasing soil depth, whereas in the agroforestry stand, RID was much lower for the same depth, but roots occupied a higher volume of soil. In the agroforestry stand, RID and RLD were higher within the tree row than in the inter-row at deeper depths. Tree fine roots in both stands were not preferentially oriented from the soil surface until a depth of 1.5 m, beneath which fine roots in the agroforestry stand were significantly more vertically oriented.Conclusions: Fine roots of intercropped walnut trees went significantly deeper indicating strong plasticity of root distribution. Deeper root systems would reduce direct competition from the crop and enable trees to access deeper water tables not available to the crop.