Is it possible to manipulate root anchorage in young trees?

Abstract

International audienceThe optimal root system architecture for increased tree anchorage has not yet been determined and in particular, the role of the tap root remains elusive. In Maritime pine (Pinus pinaster Ait.), tap roots may play an important role in anchoring young trees, but in adult trees, their growth is often impeded by the presence of a hard pan layer in the soil and the tap root becomes a minor component of tree anchorage. To understand better the role of the tap root in young trees, we grew cuttings (no tap root present) and seedlings where the tap root had (-) or had not (+) been pruned, in the field for 7 years. The force (F) necessary to deflect the stem sideways was then measured and divided by stem cross-sectional area (CSA), giving a parameter analogous to stress during bending. Root systems were extracted and root architecture and wood mechanical properties (density and longitudinal modulus of elasticity, E L ) determined. In seedlings (-) tap roots, new roots had regenerated where the tap root had been pruned, whereas in cuttings, one or two lateral roots had grown downwards and acted as tap roots. Cuttings had significantly less lateral roots than the other treatments, but those near the soil surface were 14% and 23% thicker than plants (+) and (-) tap roots, respectively. Cuttings were smaller than seedlings, but were not relatively less resistant to stem deflection, probably because the thicker lateral roots compensated for their lower number. Apart from stem volume which was greater in trees (+) tap roots, no significant differences with regard to size or any root system variable were found in plants (-) or (+) tap roots. In all treatments, lateral roots were structurally reinforced through extra growth along the direction of the prevailing wind, which also improved tap root anchorage. Predictors of log F/CSA differed depending on treatment: in trees (-) tap roots, a combination of the predictors stem taper and %volume allocated to deep roots was highly regressed with log F/CSA (R 2 = 0.83), unlike plants (+) tap roots where the combined predictors of lateral root number and root depth were best regressed with log F/CSA (R 2 = 0.80). In cuttings, no clear relationships between log F/CSA and any parameter could be found. Wood density and E L did not differ between roots, but did diminish with increasing distance from the stem in lateral roots. E L was significantly lower in lateral roots from cuttings. Results showed that nursery techniques influence plant development but that the architectural pattern of Maritime pine root systems is stable, developing a sinker root system even when grown from cuttings. Anchorage is affected but the consequences for the long-term are still not known. Numerical modelling may be the only viable method to investigate the function that each root plays in adult tree anchorag

Understanding the Impact of Root Morphology on Overturning Mechanisms: A Modelling Approach

“…Therefore, if able to reach its full potential with regard to rooting depth, the tap root acts as a rigid stake held in place with lateral roots holding it in position like guy ropes (Ennos, 1994). In young trees, where the vertical growth of the tap root is not usually hindered, such a root system is effective in preventing stem movement and toppling (Khuder et al, 2007). However, in older trees where rooting depth is often arrested by seasonal waterlogging or hard pans in the soil, tap roots are short and thick compared with the lateral roots and play only a small role in tree anchorage (Danjon et al, 2005).…”

“…Therefore, the tap root becomes defunct with regard to anchorage if unable to grow vertically. It would be prudent in such cases to encourage the growth of lateral roots, although in practice this may be difficult to achieve (Khuder et al, 2007).…”

“…Variables then measured can include stem size and mass, root -soil plate dimensions (Nicoll et al, 2005(Nicoll et al, , 2006b) and root system architecture (Khuder et al, 2007). However, although winching tests provide useful information, they are time consuming and can be difficult or even dangerous to carry out.…”

Root reinforcement and slope bioengineering stabilization by Spanish Broom (<i>Spartium junceum</i> L.)

“…Cuttings do not generate laterally and vertically with the same facility, at least in young plants. Cuttings uprooting is easier than seedlings uprooting at the same age, but these differences may disappear after several years (Khuder et al, 2007).…”

“…The essential difference between seedlings and cuttings is that the latter can develop a taproot only after five years (Khuder et al, 2007). Plants which were generated from cuttings are usually smaller and have a lower number of roots than the seeds grown ones.…”