Figure 1.
3D reconstruction of one “average” root system per population, chosen from the PCA.
(a) control tree n° 24 (b) Slope at 45° tree n° 7 (c) flexed tree n°29 (d) Slope+flexed tree n°19. Segments were coloured as a function of their compartment: grey = (1) stump, black = (2) taproot, dark blue = (3) zone of rapid taper (ZRT), light blue = (4) horizontal shallow roots beyond ZRT, green = (6) deep roots, yellow = (7) intermediate depth roots and magenta = (8) oblique roots. Size is arbitrary but proportional. The black frame is the container wall (0.3×0.3 m width) and the soil surface. North/Upslope is on the left.
Figure 2.
Overplotting of all root systems of each treatment for exploratory data analysis.
Colour coding according to Fig. 1. North/Upslope is on the left. Above: top view, perpendicular to the soil surface. Below, side view, perpendicular to the slope. The black line below is the soil surface. The grey dashed frame is the container side and bottom wall (0.3×0.3×0.11 m).
Table 1.
Shoot and root characteristics of the seedlings.
Figure 3.
Schematic representation of 26 week old Robinia pseudoacacia root systems grown in containers.
Colour coding according to Fig. 1. Lateral roots growing perpendicular to the slope direction are shown as dots on the taproot. Root characteristics (e.g. root number, root size) are the average in each group.
Figure 4.
Principal Component Analysis (PCA) of tree characteristics. Scores for PC1 and PC2.
(a) Loadings for the 16 original variables. Blue: General tree and root system characteristics and branching variables. RPC = root partitioning coefficient, DW1D = cubic root of seedling dry weight, RnAxes = number of root axes divided by root dry weight, muaxeVolZRT = mean axis volume in the ZRT, SRLaterals = specific root length of laterals, qbShallow = Qb in the shallow root compartment, ILLtapRoot = interlateral length on the taproot, SRLstumpTapv = specific root volume of the first order root. Orange: relative root volume (RV) by compartment; RVhzbeyond is shallow root beyond ZRT, RVtaproot is for the taproot, RVdeep is for deep roots, RVstump is for the stump, RVinterm is for intermediate depth roots and RVZRT is for the ZRT relative root volume. Red: circular distribution of root volume: RvolUpZRT is the volume of root in the ZRT upslope (or north) divided by the whole volume in the ZRT. RvolPerpShallow is the proportion of perpendicular to slope (or east and west) shallow roots out of the total shallow roots. (b) Loadings for the 34 trees of the sample. Black = control; blue = slope; red = flexed; orange = slope+flexing.
Table 2.
Branching characteristics.
Table 3.
Root volume, length and number proportion in the six compartments.
Table 4.
Circular distribution of various root system characteristics in the three discontinuous slope oriented sectors.
Figure 5.
Schematic representation of hypotheses concerning the modifications in response to flexing of seedlings on 0° or 45° slope.
All flexed seedlings have a large rigid vertical stump (grey) and taproot (black) and finer lateral roots parallel to the soil surface (blue arrows), analogous to guy ropes around a vertical stake. The 1st order root tapers and lateral roots (yellow and green arrows) become shorter and thinner with depth. The 1st order root undergoes bending, whereas fine roots act in tension. The volume of soil in which the 1st order roots can be potentially embedded is a truncated cone (black vertical hatched zone). The potential volume of soil which can be explored by the guying lateral roots is shown by the grey oblique hatched zones. Above: On 0° slope, most of the hatched zones are filled with soil (orange shading). Lateral roots are both horizontal and perpendicular to the 1st order root. A design with fine and evenly distributed lateral roots is efficient for keeping the stem vertical when lateral forces are dominant. Below: On 45° slope: the above mentioned design is no longer efficient, the resulting root system pattern does not allow for the stump to be held in place, nor for lateral roots to firmly anchor the stump and taproot. As the 1st order root is no longer perpendicular to the soil surface, there is no soil in a large part of the hatched zones downslope and lateral roots are oblique and not perpendicular to the stump and taproot. However, the hatched zone upslope is completely filled with soil (orange shading). Therefore, rotation of the stump is prevented by the thicker upslope shallow laterals which literally hold the stump in place. There is also a progressive shift in the circular distribution of deep laterals downslope as a function of soil depth (violet shading). These deeper roots will help the taproot to be strongly anchored in the soil.