Root system architecture of Quercus pubescens trees growing on different sloping conditions

Abstract

Background and aims: Plant roots' growth direction has important implications for plant development and survival; moreover it plays an effective and vital role in stabilizing weathered soil on a steep slope. The aim of this work was to assess the influence of slope on the architecture of woody root systems.

Methods: Five mature, single-stemmed Quercus pubescens trees growing on a steep slope and five on a shallow slope were excavated to a root diameter of 1 cm. A very precise numeric representation of the geometry and topology of structural root architecture was gained using a low-magnetic-field digitizing device (Fastrak, Polhemus). Several characteristics of root architecture were extracted by macros, including root volume, diameter, length, number, spatial position and branching order.

Key results: The diameter at breast height (dbh) was the best predictor of the root volume but had no correlation with length and number of roots. The slope affected the root volume for each branching order, and the basal cross-sectional area (CSA), number and length of the first-order roots. Number and length of the second- and third-order laterals were closely related in both conditions, although this relationship was closer in the shallow trees, suggesting the influence of a genetic control. Sloping trees showed a clustering tendency of the first- and second-order lateral roots in the up-slope direction, suggesting that the laterals rather than the taproots provide much of the anchorage. In a steep-slope condition, the taproot tapering was positively correlated with the asymmetry magnitude of first-order roots, indicating compensation between taproot and main lateral roots' clustering tendency.

Conclusions: These results suggest that on a slope, on clayey soils, root asymmetry appears to be a consequence of several environmental factors such as inclination, shallow-slides and soil compactness. In addition, this adaptive growth seems to counteract the turning moment induced by the self-loading forces acting in slope conditions, and as a consequence improves the tree stability.

Fig. 1.

Regressions of root volume components versus diameter at breast height (dbh) for five Quercus pubescens trees grown in shallow-slope (•) and five in steep-slope conditions (○), calculated within a radial distance of 1 m from the stump centre. Regression equations are as follows, where trv = total root volume; v1–v3 = volume of first-, second- and third-order roots. Shallow slope: (A) trv = 17810·80 + 2560·36dbh; (B) v1 = −14252·30 + 2084·68dbh; (C) v2 = −2743·30 + 391·47dbh; (D) v3 = −763·86 + 80·38dbh. Steep slope: (A) trv = −10912·58 + 2380·12dbh; (B) v1 = −6549·63 + 1644·68dbh; (C) v2 = −3707·82 + 634·39dbh; (D) v3 = −655·12 + 101·04dbh.

Fig. 2.

Number of roots per branching order for Q. pubescens trees growing on a shallow-slope and a steep-slope. Bars are means of five replicates + s.e. Means significantly different at ^*P < 0·05 and ^**P < 0·01 (Student's t-test).

Fig. 3.

Basal cross-sectional area (CSA) of the first-order roots for each Q. pubescens tree grown in shallow (•) and steep (○) slope conditions. Values are weighted for the total CSA for each tree. Symbol size is proportional to the dbh (scale indicated on figure). Total first-order number is reported for each tree. The dotted line is a reference line indicating the maximum value shown by the shallow slope trees.

Fig. 4.

Orientation (circular direction) and weight (radial direction) of all the first-order lateral roots together with the orientation of the centres of volume (COV) for the first- (cross) and second-order (triangle) roots in trees of Q. pubescens. Weight values are expressed as a percentage of the total volume per each tree calculated at the 60 cm reference hemisphere. The slope direction is indicated on the figure.

Fig. 5.

Asymmetry magnitude expressed as the ratio between the horizontal distance and the hemisphere ray of the centre of volume (COV) for the first-order lateral roots at different radial distances from the stump centre. Values are means of five replicates analysed by one-way ANOVA. The bars indicate the LSD (P < 0·05) used for all pairwise comparisons.

Fig. 6.

Cross-section of an up-slope lateral root sample of Q. pubescens. The section shown was cut at a distance of 20 cm from the root base. The circle indicates the portion of wood characterized by a visible circular symmetry. The arrows indicate the directions in which wood has been added, probably in response to the mechanical stimulus.