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Competition with winter crops induces deeper rooting of walnut trees in a Mediterranean alley cropping agroforestry system


Background and aims

Characterising the spatial distribution of tree fine roots (diameter ≤ 2 mm) is fundamental for a better understanding of belowground functioning when tree are grown with associated crops in agroforestry systems. Our aim was to compare fine root distributions and orientations in trees grown in an alley cropping agroforestry stand with those in a tree monoculture.


Fieldwork was conducted in two adjacent 17 year old hybrid walnut (Juglans regia × nigra L.) stands in southern France: the agroforestry stand was intercropped with durum wheat (Triticum turgidum L. subsp. durum) whereas the tree monoculture had a natural understorey. Root intercepts were mapped to a depth of 150 cm on trench walls in both stands, and to a depth of 400 cm in the agroforestry stand in order to characterise tree root distribution below the crop’s maximum rooting depth. Soil cubes were then extracted to assess three dimensional root orientation and to establish a predictive model of root length densities (RLD) derived from root intersection densities (RID).


In the tree monoculture, root mapping demonstrated a very high tree RID in the top 50 cm and a slight decrease in RID with increasing soil depth. However, in the agroforestry stand, RID was significantly lower at 50 cm, tree roots colonized deeper soil layers and were more vertically oriented. In the agroforestry stand, RID and RLD were greater within the tree row than in the inter-row.


Fine roots of intercropped walnut trees grew significantly deeper, indicating a strong plasticity in root distribution. This plasticity reduced direct root competition from the crop, enabling trees to access deeper water tables not available to crop roots.

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This study was financed by the French ANR funded project ECOSFIX (Ecosystem Services of Roots - Hydraulic Redistribution, Carbon Sequestration and Soil Fixation, ANR-2010-STRA-003-01), and by the ADEME funded project AGRIPSOL. We thank the farmer Mr Breton, for his authorization to sample roots and open pits. We are very grateful to our French colleagues for their help with field and laboratory work and logistics, including Jean-François Bourdoncle (INRA), Lydie Dufour (INRA), Clément Enard (INRA), Alain Sellier (INRA), and the students (Jordan Chauliaguet, Hugo Fontenille, James Metayer, Floriane Schmith, Aurélien Schüller).

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Correspondence to Christophe Jourdan.

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Responsible Editor: Angela Hodge.

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Figure S1

a) Raw data of walnut fine root intersection densities (RID) within the pit in the tree monoculture. DTR = Distance to the tree row. b) Raw data of walnut fine root intersection densities (RID) within the agroforestry pit. DTR = Distance to the tree row. c) Raw data of the walnut fine root intersection densities (RID) within the 400 cm deep agroforestry pit. DTR = Distance to the tree row. (ZIP 3170 KB)

Figure S2

Linear regressions between walnut fine root length density (RLD) and the mean fine root intersection density (RID) for cubes, for the different pits. Dotted lines: confidence interval of the regression line. (JPEG 3541 KB)

Figure S3

a) Estimated walnut fine root length density (RLD) profiles in the agroforestry and in the tree monoculture to a depth of 150 cm. For the agroforestry stand, profiles from the AF and deep-AF pits were combined for values to a depth of 150 cm. b) Estimated walnut fine root length density (RLD) profiles in the agroforestry stand to a depth of 400 cm as a function of distance to the tree row. (ZIP 3821 KB)

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Cardinael, R., Mao, Z., Prieto, I. et al. Competition with winter crops induces deeper rooting of walnut trees in a Mediterranean alley cropping agroforestry system. Plant Soil 391, 219–235 (2015). https://doi.org/10.1007/s11104-015-2422-8

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  • Deep roots
  • Intercropping
  • Fine roots
  • Juglans sp
  • Root anisotropy
  • Root intersection density
  • Root length density
  • Root mapping
  • Specific root length