Physical crop parameters â group A: to calibrate (grey field) and fixed (green field) .... Description of the 44 different land units and the respective assumed tree.
SILVOARABLE AGROFORESTRY FOR EUROPE – Final Report
WP5: Modelling below-ground tree-crop interactions WP5 produced two deliverables: Deliverable 5.1 describes the below-ground competition modules for water and nitrogen, and deliverable 5.2 includes the scientific papers produced about belowground competition between trees and crops.
Field evidence calling for a new approach to below-ground competition modelling in silvoarable systems Most of the results below are obtained from the two following papers produced by the SAFE project Moreno G., Obrador J.J., Cubera E., Dupraz C., 2005. Root distribution in Dehesas of CentralWestern Spain. Plant and Soil, accepted for publication Mulia R., Dupraz C., (2005). Unusual fine root distributions of two deciduous tree species in southern France: what consequences for modelling of tree root dynamics? Submitted to Plant and Soil
The spatial distribution of fine roots of two deciduous and one evergreen tree species was investigated in contrasting growing conditions in southern France and western Spain. Hybrid walnut trees (Juglans nigra x J. regia cv. NJ203) and hybrid poplars (Populus euramericana cv. I214) were both cultivated with or without annual winter intercrops for 10 years on deep alluvial soils. Holm oak (Quercus ilex) was either intercropped or intergrazed. Fine root distribution of both trees and crops was measured by soil coring down to 2-3-metre depth at several distances and orientation from the tree trunks. The observed root systems of trees were very patchy, and unexpected root profiles were found. In the tree-only stands, fine root profiles followed the common decreasing pattern with depth and distance from the tree trunk. But most intercropped tree root profiles were Results- Page 76
SILVOARABLE AGROFORESTRY FOR EUROPE – Final Report
uniform with depth (Figure 37, Figure 37), and sometimes-inverse profiles were found (i.e. significantly more roots at depth than next to the surface).
Figure 36: variation of Holm oak and herbaceous plant (oats and natural grasses) root length densities (RLD) by soil depth in Dehesas of central Western Spain
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Figure 37: Root distribution profiles of walnut (A) and poplar (B) in the agroforestry and forestry plots. The vertical bars indicate one standard error. The shaded areas indicate the 1.1 m deep sand horizon and the 2.5 m – 3 m deep gravel layer in the poplar stand.
In the cropped alleys, the root distribution of the trees was significantly modified by the crop (Figure 38 for poplars, Figure 39 for walnut trees).
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SILVOARABLE AGROFORESTRY FOR EUROPE – Final Report
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Figure 38: Fine roots in a poplar agroforestry stand in July 2003. Left: Cumulative fine roots of poplars at various distances from the tree stem; Right: Lrv on the tree row and in the centre of the cropped alley.
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Figure 39: Absolute and cumulative root distribution profiles of walnut trees with soil depth within tree row and in the cropped alley based on the root observations in year 2002 – 2003 (A) and 2004 (B).
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These distributions may result from a high degree of plasticity of tree root systems to sense and adapt to fluctuating and heterogeneous soil conditions. Heterogeneous 3D soil conditions resulted from both vertical gradients due to soil properties and horizontal gradients due to the varying extraction dynamics by the plants. The distortion of the tree root system was more pronounced for the oak trees and the walnut trees that only partially explored the soil volume: in the tree-only stand, the walnut rooting pattern was very superficial, but in the intercropped stand walnut trees developed a deep and dense fine root network below the crop rooting zone (Figure 40). The larger poplars explored the whole available soil volume, but the intercrop significantly displaced the root density from the topsoil to layers below 1 m depth. distance from tree trunk (m) -1
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Figure 40: Compared root distributions of durum wheat and trees in the walnut intensive agroforestry plot observed in spring 2002 (left, Grazzia variety) and in the poplar agroforestry plot observed in spring 2004 (right, Allure variety). The bubble sizes are proportional to the observed root length densities.
Most tree root growth models assume a decreasing fine root density with depth and distance from the tree stem. These models would not predict correctly tree-tree and tree-understorey competition for water and nutrients in 3D heterogeneous soil conditions that prevail under low-density tree stands. To account for the integrated response of tree root systems to such transient gradients in soils, we need a dynamic model that would allow for both genotypic plasticity and transient environmental local soil conditions.
An innovative dynamic model of tree fine roots These results are based on the following paper produced by the SAFE consortium: Dupraz C., Mulia R., van Noordwijk M., (2005) A 3D model with voxel automata to simulate plant root growth in a heterogeneous soil condition. In preparation for New phytologist. Prior simulation models of plant root growth in heterogeneous soil conditions where based on root architecture and often-required detailed parameterisation, but no parsimonious 3D simulator with continuum representation was available as yet. Simplified models usually assume a fixed pattern that is not relevant for our silvoarable systems (Figure 41).
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Figure 41: The classical hypothesis of decrease in root length density with depth and distance that we could not include in our model of tree-crop interactions
During the SAFE project, we proposed to model the dynamic three-dimensional root growth of trees with a voxel automata approach. If root expansion is allowed to occur via the flat faces of the central voxel, only six other voxels are truly adjacent and therefore available for colonisation
If root expansion is allowed to occur also via the linear edges and corners of each voxel, then a further 20 directions are possible, giving 26 total accessible voxels for expansion
Figure 42: Voxel colonization by tree root growth in two different voxel automata. The simple approach with 6 neighbour voxels was retained in Hi-sAFe
Root system grows within voxel spaces according to an automaton mechanism and responds to local soil conditions. Both fine and coarse root growth are simulated. Six parameters are involved including two parameters, which describe preferential growth directions.
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SILVOARABLE AGROFORESTRY FOR EUROPE – Final Report Parameter A weighting factor to regulate the effect of local water uptake A weighting factor to regulate the effect of root dry matter sourcevoxel distance Threshold of root colonisation Proliferation rate Plagiotropism factor Geotropism factor
Symbol and value -∞
