Root Anchorage

In many cases, failure due to mechanical loading often occurs in the root system. Thus, an understanding of root biomechanics is of crucial interest, not only because the anchoring capacity of a plant is an important factor for survival with regards to external abiotic stresses, such as wind loading or animal grazing, but also because roots are a major component in the reinforcement of soil. Whereas many studies have been carried out on the morphological development of roots with regards to their absorption capacity [65-67], very few investigations have focused on the mechanical role of roots [68,69]. Nevertheless, these pioneer studies have provided a sound base for a better understanding of root anchorage efficiency in both plants and trees.

Root anchorage has largely been investigated at the single root level [70,71] or at the scale of whole root systems [72-76], whereas soil reinforcement by roots has generally been considered at the population scale [77-79]. To better understand the biomechanical role of specific root elements and in particular plant adaptation to mechanical stresses, a distinction must be made between small roots, i.e., roots that resist tension but which have a low bending stiffness, and large roots, i.e., roots that can resist both tension and bending. The first category can be compared to "cable" structural elements, whereas the second type can be considered as "beam" elements. This latter category is mainly encountered in adult trees or shrubs and the former in herbaceous species. Such a distinction between these two categories of roots is necessary to avoid confusion when considering the consequences of root mechanical properties on uprooting efficiency, as discussed in the next paragraph.

Over the last 30 years, an increasing awareness of the role of fine roots (defined as less than 25 mm in diameter) in soil reinforcement has led to several studies being carried out on the mechanical properties of roots [80-83]. Soil shear strength is enhanced by the presence of roots due to the increase in additional apparent cohesion [71,84,85]. When roots are held in tension, such as pull-out or soil slippage on a slope, root tensile strength is fully mobilized and roots act as reinforcing fibers in the surrounding soil matrix [86,87]. In studies where the tensile strength of small roots has been measured, it is usually shown that the strength, as well as the modulus of elasticity, decreases with increasing diameter d, following an exponential law of the type |3 exp(-ad) (Figure 1.3) (values of root resistance in tension, bending, and compression are given for different woody species in [72]). This decrease in tensile strength is due to a lower quantity of cellulose in small roots ([83]; see Figure 1.3). Although this type of information is invaluable when studying the mechanism or root reinforcement, especially on slopes subject to instability problems [77,86,88], it is also of extreme interest to researchers trying to understand the specific role of small roots on tree anchorage. It could be suggested that for a fixed amount of invested biomass, a network of several small roots is more resistant in tension than a few large structural roots [89,90]. However, a large number of small roots may be also detrimental to anchorage because a group effect could result in more failure occurring in the soil [89,91].

Diameter (mm)

FIGURE 1.3 Tensile strength increased significantly with decreasing root diameter (y = 28.96x<0-57), R2 = -0.45, P <0.05) and cellulose content (y = 0.47x<-1442), R2 = 0.23, P <0.005) in roots of sweet chestnut (Castanea sativa Mill.) (after [83]).

Fine roots have often been ignored when investigating the root anchorage of forest trees. This neglect is mainly due to the difficulty in extracting them from the soil, in particular those roots far away from the trunk. Nevertheless, these distal roots determine the boundary conditions of the whole structure and can be very important from the biomechanical point of view. However, this observation cannot be applied to all plants, e.g., it has been shown in leek (Alliumporrum L.) seedlings that the distal part of a long, single, fine root is not stressed before failure of its proximal part [70]. Therefore, the failure mechanism in trees is probably significantly different than that observed in herbaceous species.

The difficulty of investigating root anchorage is not only due to the complexity of the mechanisms occurring in both roots and soil, but also to their multifactorial aspect [88,89]. A good alternative to difficult and time-consuming field experiments can be found in numerical modeling. Dupuy et al. [89] carried out such numerical analyses using the finite element method. These authors determined the mechanical response of small ramified roots to pull-out in tension. Parametric studies showed that the number of lateral ramifications and their diameter were both major components affecting the resistance to pull-out for a given soil pressure.

Plant anchorage efficiency must be investigated taking into consideration not only the mechanical behavior of single roots, but more importantly, the whole root architecture. A number of studies were carried out in the 1990s on annual or herbaceous plants [70,76]. Ennos and Fitter [92] proposed an alternative hypothesis concerning root system shape and function. These authors suggested that creeping and climbing plants develop fibrous root systems because the only mechanical stress transferred to the roots is tension. However, root systems of single-stemmed, freestanding plants tend to develop a more plate-like or tap-like morphology [93]. Based on mechanical assumptions, Ennos and Fitter [92] also showed that these different anchorage strategies can have an impact on the biomass allocation ratio between above- and below-ground parts during plant growth. Other studies on dicots have since been carried out to quantify the root biomass investment according to the external loading on the plant [94].

Investigating the anchorage of adult trees is rather more complicated because of the morphological complexity encountered in tree root systems. Several structural elements of importance for mechanical stability can coexist. Root topology, i.e., the way branches are linked together, is of major interest when trying to understand how external forces are transmitted throughout the whole system and into the soil [90,91,95,96]. Root system depth and the number of root branches have also been identified as highly significant components of tree anchorage [88,96]. Coutts [97,98] identified the main components that play a role in root anchorage of Sitka spruce (Picea sitchensis Bong) by order of importance, i.e., the weight of the root-soil plate, the windward roots in tension, the soil cohesion, and the bending strength of leeward roots. A further component to consider in shallowly rooted species is the presence of buttressing around the stem. Although several hypotheses exist concerning the development and function of buttresses in tropical trees [99-102], in temperate species at least, buttresses tend to develop in trees with shallow, plate-like systems [103]. The presence of such buttresses will help external loading forces be transmitted more smoothly along the lateral roots and into the soil, thereby improving anchorage [104]. Particular attention has also been paid to tree species that develop tap root systems [88,101,105,106]. Specific experiments carried out by Mickovski and Ennos [107] on Scots pine (Pinus sylvestris L.) showed that in tap root systems, lateral roots are not a major component of root anchorage. However, in separate studies, Niklas et al. [108] illustrated the lack of efficiency of a massive tap root if not associated with thick lateral roots, and Tamasi et al. [109] showed that in oak (Quercus robur L.) seedlings subjected to artificial wind loading, lateral root growth was increased at the expense of tap root length.

Contradictory results are often encountered in the literature concerning the relationship between root architecture and anchorage efficiency, and one explanation may lie in the underestimated role of soil characteristics on uprooting [110]. Numerical analyses may help fill this gap in knowledge [95,111]. Fourcaud et al. [95] and Dupuy et al. [96] developed methods allowing morphological data from real or simulated root systems to be subjected to virtual uprooting tests. Soil mechanical properties could be changed easily, therefore, allowing a rapid assessment of root architecture efficiency in different soils [112]. In a clay soil, the root and soil system of a heart root system rotates around an axis that is situated directly beneath the stem, whereas in sandy soil, the same system rotates around an axis that is shifted leeward. Heart and tap root systems [93] also behave similarly in clay soil but are over twice as resistant to overturning than plate or herringbone [66] systems in the same soil. However, between the four root types that were studied, anchorage in sandy soil was less variable between the four root types; the most efficient anchorage in sand was found in the tap-rooted system and the least, the plate root system [112].

In conclusion, although the study of root biomechanics has been neglected until recent years, a large number of studies exist that elucidate the mechanisms by which roots are anchored in the soil. Modifications in root system architecture due to external loading will have consequences not only for anchorage efficiency but for the ability of root systems to absorb nutrients [66,67]. Future studies need to incorporate both root and soil mechanical properties into numerical models, which are in turn validated by field experiments.

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