Observation and Simulation of Root Reinforcement on Abandoned Mediterranean Slopes

The mechanics of root reinforcement have been described satisfactorily for a single root or several roots passing a potential slip plane and verified by field experiments. Yet, precious little attempts have been made to apply these models to the hillslope scale pertinent to landsliding at which variations in soil and vegetation become important. On natural slopes positive pore pressures occur often at the weathering depth of the soil profile. At this critical depth root reinforcement is crucial to avert slope instability. This is particularly relevant for the abandoned slopes in the European part of the Mediterranean basin where root development has to balance the increasing infiltration capacity during re-vegetation. Detailed investigations related to root reinforcement were made at two abandoned slopes susceptible to landsliding located in the Alcoy basin (SE Spain). On these slopes semi-natural vegetation, consisting of a patchy herbaceous cover and dispersed Aleppo pine trees, has established itself. Soil and vegetation conditions were mapped in detail and large-scale, in-situ direct shear tests on the topsoil and pull-out tests performed in order to quantify root reinforcement under different vegetation conditions. These tests showed that root reinforcement was present but limited. Under herbaceous cover, the typical reinforcement was in the order of 0.6 kPa while values up to 18 kPa were observed under dense pine cover. The tests indicate that fine root content and vegetation conditions are important factors that explain the root reinforcement of the topsoil. These findings were confirmed by the simulation of the direct shear tests by means of an advanced root reinforcement model developed in FLAC 2D. Inclusion of the root distribution for the observed vegetation cover mimics root failure realistically but returns over-optimistic estimates of the root reinforcement. When the root reinforcement is applied with this information at the hillslope scale under fully saturated and critical hydrological conditions, root pull-out becomes the dominant root failure mechanism and the slip plane is located at the weathering depth of the soil profile where root reinforcement is negligible. The safety factors increase only slightly when roots are present but the changes in the surface velocity at failure are more substantial. Root reinforcement on these natural slopes therefore appears to be limited to a small range of critical hydrological conditions and its mitigating effect occurs mainly after failure.     

Penetration of very strong soils by seedling roots of different plant species

The abilities of seedling roots of twenty-two plant species to penetrate a strong growth medium were compared under controlled conditions. Seedlings were grown for 10 days in compression chambers filled with siliceous sandy soil at 0.2 kg kg^–1 water content and mean penetrometer resistance of 4.2 MPa. Root elongation and thickening were measured after growth. The results show that soil strength reduced the elongation of roots of all plant species by over 90% and caused the diameters of the roots to increase compared with control plants grown in vermiculite (0 MPa resistance).Differences in both root elongation and root diameter were observed among plant species. Generally, the roots of dicotyledons (with large diameters) penetrated the strong medium more than graminaceous monocotyledons (with smaller diameters). There was a significant positive correlation (r=0.78, p<0.05) between root diameter and elongation over all the species in the stressed plants. The species were ranked according to the relative root elongation and relative root thickening. Based on this ranking, lupin (Lupinus angustifolius), medic (Medicago scutelata) and faba bean (Vicia faba) were the species with the greatest thickening and elongation while wheat (Triticum aestivum), rhodesgrass (Chloris gayana) and barley (Hordeum vulgare) had the least. The weight of the seeds did not seem to influence either the thickening or elongation of the roots.     

Comparison of Fine Root Dynamics in Scots Pine and Pedunculate Oak in Sandy Soil

In this study, we investigated the relationship between the seasonality of vegetation cover and that of fine root processes in a man-made forest in northern Belgium. Due to their contrasting foliar development, we expected different seasonal patterns of fine root growth and standing biomass between Pedunculate oak (Quercus robur L.), and Scots pine (Pinus sylvestris L.). Biomass and necromass of fine and small roots were estimated by repeated core sampling in February, April, June, August and October 2003. Measurements showed that Pedunculate oaks maintained more live fine roots in winter than Scots pines. However, Scots pines produced more than twice as much fine roots in spring, such that in summer both species had similar root mass. Scots pine root production started before-, but declined during leaf unfolding. Pedunculate oak roots, in contrast, started elongating only after bud break. For both species, fine root production peaked in JuneJuly, but was more than offset by drought-induced mortality at the end of July and early August. Summer drought in 2003 was exceptionally long and intense, significantly reducing leaf area, killing most new roots, and inhibiting root decomposition, such that the obtained results cannot be typical for this forest.     

Complete mechanical impedance increases the turgor of cells in the apex of pea roots

In this paper we describe an experimental approach which allows turgor (p) in an impeded root to be measured without the need to remove the root from the impeding environment. The maximum axial growth pressure (σ_max) generated by completely impeded pea (Pisum sativum L.) roots was measured using a novel apparatus incorporating a force transducer. The apparatus was designed so that it was possible to gain access to the impeded root with the microcapillary of a pressure probe and so obtain in situ measurements of P. Turgor in cells in the apical region of impeded roots was 0.78 MPa, compared with 0.55 MPa in unimpeded roots. In impeded roots, σ_max was 0.52 MPa, showing that the pressure component resulting from cell wall tension (W, where W=P–σ) decreased from 0.55 to 0.26 MPa as the roots became impeded. When impeded roots were removed from the apparatus, there was no decrease in P over the following 90 min. Impedance did not cause P to change in the non-elongating part of the roots further from the apex.     

Biomass and distribution of fine and coarse roots from blue oak (Quercus douglasii) trees in the northern Sierra Nevada foothills of California

This research adds to the limited data on coarse and fine root biomass for blue oak (Quercus douglasii Hook and Arn.), a California deciduous oak species found extensively throughout the interior foothills surrounding the Central Valley. Root systems of six blue oak trees were analyzed using three methods — backhoe excavation, quantitative pits, and soil cores. Coarse root biomass ranged from 7 to 177 kg per tree. Rooting depth for the main root system ranged from 0.5 to 1.5 m, with an average of 70% of excavated root biomass located above 0.5 m. Of the total biomass in excavated central root systems, primary roots (including burls) accounted for 56% and large lateral roots (> 20 mm diameter) accounted for 36%. Data from cores indicated that most biomass outside of the root crown was located in fine roots and that fine root biomass decreased with depth. At surface depths (0–20 cm), small-fine (< 0.5 mm diameter) roots accounted for 71%, large-fine (0.5–2.0 mm) for 25%, and coarse (> 2 mm) for 4% of total root biomass collected with cores. Mean fine root biomass density in the top 50 cm was 0.43 kg m⁻³. Fine root biomass did not change with increasing distance from the trees (up to approximately 5 m). Thus, fine roots were not concentrated under the tree canopies. Our results emphasize the importance of the smallest size class of roots (<0.5 mm), which had both higher N concentration and, in the area outside the central root system, greater biomass than large fine (0.5–2.0 mm) or coarse (> 2.0 mm) roots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Vegetation Succession and its Consequences for Slope Stability in SE Spain

The effect of land abandonment as a result of changing land-use policies is becoming more and more important throughout Europe. In this case study, the role of vegetation succession and landslide activity on steep abandoned slopes was investigated. The influence of vegetation succession on soil properties over time, as well as how developing root systems affect soil reinforcement was determined. The study was carried out in the Alcoy basin in SE Spain, where the marl substratum is prone to landsliding along steep ravines. The bench-terraced slopes have been abandoned progressively over the last 50 years and show various stages of revegetation. The study was carried out at two scales; at the catchment scale long-term evolution of land-use, vegetation succession and slope failure processes were investigated. At a more detailed scale, vegetation cover, soil properties and rooting effects on soil strength were determined. Results showed that the soil has changed over a period of 50 years with respect to soil properties, vegetation cover and rooting, which is reflected in the activity of geomorphological processes. Vegetation succession progressively limits surface processes (sheet wash and concentrated overland flow) over time, whereas slopes affected by mass wasting processes increase in number. The spatial heterogeneity of infiltration increases over time, leading to increased macro-pore flow towards the regolith zone, enhancing the potential risk of fast wetting of the regolith directly above the potential plane of failure, as was concluded from rainfall simulations. In situ experiments to determine soil shear strength in relation to rooting indicated that roots contributed to soil strength, but only in the upper 0.4 m of the soil. Most failures however, occur at greater depths (1.0–1.2 m) as anchorage by deeper roots was not effective or absent. The observed initial increase in mass wasting processes after land abandonment can therefore be explained in two ways: (1) the limited contribution of anchorage by root systems at potential slip planes which cannot counterbalance the initial decline of the terrace walls, and (2) the fast transfer of rainfall to the potential slip plane by macro-pores enhancing mass movements. However, after approximately 40 years of abandonment, mass wasting processes decline.     

Influence of temperature and water potential on root growth of white oak

Root growth of white oak ( Quercus alba L.) was observed under field conditions using a rhizotron. The effects of temperature, soil water potential, and leaf water potential were evaluated on three measures of root growth and development: root elongation rate, number of growing roots, and root growth intensity (sum of projected root area compared to the total root viewing area). Root elongation rate was linearly related to changes in soil temperature and soil water potential. At soil temperatures less than 17deg;C, temperature was the dominant factor affecting rate of growth, bat at temperatures greater than 17°C soil water potential became the important factor. Unlike root elongation rate, the number of growing roots and root growth intensity increased at cold soil temperatures (8°C) and at soil water potentials of-0.3 to -0.8 MPa. At high soil water potentials (-0.1 MPa) root elongation rate reached a maximum while the number of growing roots and root growth intensity were low. These differences showed that root growth and development were not exclusively affected by the soil environment. In addition, the relationship between root growth and predawn leaf water potential suggested that root growth was a contributing factor to the drought resistance of white oak.     

Periodicity in the development of the root system of young rubber trees (Hevea brasiliensis Müell. Arg.): relationship with shoot development

The growth pattern of the root system of young rubber trees (Hevea brasiliensis) was studied in relation to shoot development over a period of 3 months. Temporal and spatial variations in elongation and branching processes were examined for the different root types, by means of root observation boxes. Shoot growth was typically rhythmic. Root development was periodic and related to leaf expansion. Root elongation was depressed during leaf growth, whereas branching was enhanced. Consequently, highly branched areas with vigorous secondary roots alternated along the taproot with poorly branched areas with shorter roots. Root types were not affected to the same degree by shoot competition: during leaf expansion, taproot growth was just depressed but remained continuous, the emergence and elongation rates of secondary roots were significantly affected and the elongation rates of tertiary roots fell to zero. These results were consistent with the hypothesis that root growth is related to competition for assimilates and to the sink strength of the different root types, whereas root branching appeared to be promoted by leaf development.     

Below‐ground herbivory and root toughness: a potential model system using lignin‐modified tobacco

Plants exploit an array of defences against insect herbivores based on chemical and biomechanical properties. There is increasing evidence that plant toughness comprises a particularly effective defence against herbivory, yet studies to date have focussed exclusively on leaf toughness and folivore behaviour. The relationship between root mechanical properties and the chewing behaviour of a root‐feeding insect, the Agriotes spp. wireworm (Coleoptera: Elateridae L.), feeding on tobacco (Nicoiana tabacum) is investigated. Root toughness is manipulated using introduced transgenes for the down‐regulation of key enzymes in the lignin biosynthesis pathway: cinnamoyl‐CoA reductase (CCR line) and caffeate O‐methyltransferase and cinnamyl alcohol dehydrogenase (CO line). Two biomechanical analyses (i.e. conventional cutting and notched tensile) are conducted to quantify root toughness on both lines. Roots from the CO line are significantly tougher than those of the CCR line in terms of fracture toughness and fracture energy, although not for cutting energy or stiffness. Bioassays that compel wireworms to chew through roots demonstrate that only 30% can penetrate roots of the CO line compared with 90% on the CCR line. It takes wireworms over twice as long to penetrate roots from the CO line (8 h) compared with CCR roots (3.5 h). There is a statistically significant positive relationship between penetration time and fracture toughness evaluated with tensile tests, although not with cutting energy from cutting tests. Using this exploratory model system, it is concluded that root toughness derived from tensile tests is a practical indicator of the ability of root‐feeding insects to penetrate roots.     

The Influence of Cellulose Content on Tensile Strength in Tree Roots

Root tensile strength is an important factor to consider when choosing suitable species for reinforcing soil on unstable slopes. Tensile strength has been found to increase with decreasing root diameter, however, it is not known how this phenomenon occurs. We carried out tensile tests on roots 0.2–12.0 mm in diameter of three conifer and two broadleaf species, in order to determine the relationship between tensile strength and diameter. Two species, Pinus pinaster Ait. and Castanea sativa Mill., were then chosen for a quantitative analysis of root cellulose content. Cellulose is responsible for tensile strength in wood due to its microfibrillar structure. Results showed that in all species, a significant power relationship existed between tensile strength and root diameter, with a sharp increase of tensile strength in roots with a diameter <0.9 mm. In roots >1.0 mm, Fagus sylvatica L. was the most resistant to failure, followed by Picea abies L. and C. sativa., P. pinaster and Pinus nigra Arnold roots were the least resistant in tension for the same diameter class. Extremely high values of strength (132–201 MPa) were found in P. abies, C. sativa and P. pinaster, for the smallest roots (0.4 mm in diameter). The power relationship between tensile strength and root diameter cannot only be explained by a scaling effect typical of that found in fracture mechanics. Therefore, this relationship could be due to changes in cellulose content as the percentage of cellulose was also observed to increase with decreasing root diameter and increasing tensile strength in both P. pinaster and C. sativa.     

Differences in Anatomical Structure and Lignin Content of Roots of pedunculate Oak and Wild Cherry-Tree Plantlets During Acclimation

The lignin contents and anatomical structure of roots of wild cherry (Prunus avium L.) and pedunculate oak (Quercus robur L.) plantlets were compared to explain differences in response during transfer from in vitro to ex vitro conditions. Lignification of cell walls increased significantly in both oak and cherry roots during the period of acclimation and finally lignin content of root tissues of in vitro propagated plantlets reached the levels not significantly different from seedlings grown in soil. Later on when secondary tissues appeared, lignified secondary xylem constituted most of the tissues of both species. The most conspicuous interspecific difference in root structure was the presence of phi-thickenings in cortical layers just outer to endodermis in cherry roots cultivated ex vitro. Formation of phi-thickenings was avoided in vitro and their presence thus seems to be under environmental control. Suberised well established exodermis was present in roots of oak but not detected in those of cherry. Very early development of exodermis in oak roots, preceding suberisation of endodermis, was recorded in vitro but not in well aerated soil. While multilayered and well-developed cork occurred in oak, only thin walled and less suberised secondary dermal tissues were found in cherry.     

喀斯特坡地2种地埂篱根-土复合体抗剪和抗冲性能综合评价

分析喀斯特地区不同地埂篱根系的形态和力学特性，量化其根-土复合体抗剪和抗冲性能的强弱，探寻该地区地埂篱根系固土抗蚀性能的评价因子，为喀斯特坡地水土流失治理中植被恢复措施的科学应用提供参考。选取重庆酉阳龙潭槽谷为研究区，分上、中、下坡分别布设拉巴豆和光叶苕子2种地埂篱，采用根系扫描仪和电子万能试验机测定其根系形态和力学参数，应变控制式直剪仪测定复合体抗剪强度，原状土冲刷水槽法测定复合体抗冲指数。结果表明：(1)抗剪复合体中，拉巴豆平均根长密度和根表面积密度分别高出光叶苕子59.32%和16.86%;抗冲复合体中，拉巴豆平均根长密度、根表面积密度和根体积密度较之光叶苕子高出30.48%、57.78%、92.98%;拉巴豆根系极限抗拉力和抗拉强度均显著高于光叶苕子。(2)2种地埂篱根系均能增强土壤的抗剪和抗冲性能，其中拉巴豆和光叶苕子复合体粘聚力较之对照土体分别增强了113.06%—124.37%和51.56%—87.12%,抗冲指数最高达到对照土体的2.81倍和2.45倍。(3)不同坡位，下坡2种植物的根长密度显著高于上、中坡；拉巴豆根系抗拉特性在下坡表现最优，光叶苕子在上坡表现更好；拉巴...     

Root Strength and Root Area Ratio of Forest Species in Lombardy (Northern Italy)

Forest vegetation is known to increase hillslope stability by reinforcing soil shear resistance and by influencing hydrologic conditions of soil. Although the importance of plant root systems for hillslope stability has received considerable attention in recent years, the quantification of such an effect needs more investigation. In this paper, we present a synthesis of the data gathered in the last 5 years for some species in different locations of the Alps and Prealps of Lombardy (Northern Italy) with the aim to increase our knowledge on root tensile strength and on Root Area Ratio distribution within the soil. Concerning root tensile strength we developed tensile strength–diameter relationships for eight species: green alder (Alnus viridis(Chaix) D.C.), beech (Fagus sylvatica L.), red willow (Salix purpurea L.), goat willow (Salix caprea L.), hazel (Corylus avellana L.), European ash (Fraxinus excelsior L.), Norway spruce (Picea abies (L.) Karst.) and European larch (Larix decidua Mill.). Results show a great variability among the different species and also for the same species. In general, however, root strength (in terms of tension) tends to decrease with diameter according to a power law, as observed by other Authors. Comparing the power law fitting curves for the considered species, it can be observed that they fall in a relatively narrow band, with the exception of hazel, which appears the most resistant. Concerning the evaluation of root distribution within the soil we estimated the Root Area Ratio (the ratio between the area occupied by roots in a unit area of soil) according to its depth for five species (beech, Norway spruce, European larch, mixed hazel and ash) in three locations of Lombardy. Results show that there is a great variability of root density for the same species well as for different points at the same locality. The general behaviour of root density, in any case, is to decrease with depth according to a gamma function for all the studied species. The results presented in this paper contribute to expanding the knowledge on root resistance behaviour and on root density distribution within the soil. The studied location have allowed the implementation of soil–root reinforcement models and the evaluation of the vegetation contribution to soil stability.     

Penetrometer resistance,root penetration resistance and root elongation rate in two sandy loam soils

Root penetration resistance and elongation of maize seedling roots were measured directly in undisturbed cores of two sandy loam soils. Root elongation rate was negatively correlated with root penetration resistance, and was reduced to about 50 to 60% of that of unimpeded controls by a resistance of between 0.26 and 0.47 MPa. Resistance to a 30° semiangle, 1 mm diameter penetrometer was between about 4.5 and 7.5 times greater than the measured root penetration resistance. However, resistance to a 5° semiangle, 1 mm diameter probe was approximately the same as the resistnace to root penetration after subtracting the frictional component of resistance. The diameter of roots grown in the undisturbed cores was greater than that of roots grown in loose soil, probably as a direct result of the larger mechanical impedance in the cores.     

The role of riparian trees in maintaining riverbank stability: A review of Australian experience and practice

Riverside vegetation is a significant factor influencing the occurrence and progress of streambed and riverbank erosion. Recent riparian management practice in Australia has focussed on re-establishing or maintaining native riparian vegetation in order to control or prevent erosion as well as regenerate or preserve the complex variety of in-stream and riverside habitats. This work presents an integrated review of field and experimental studies conducted in eastern Australia that evaluate native vegetation's role in mass failure of riverbanks. Several results of these studies have general application and include the following: (1) The presence of riparian forest on riverbanks significantly reduces the likelihood of erosion by mass failure due to reinforcement of riverbank soils by tree roots and this reduced likelihood of mass failure enables a narrower channel cross-section than would otherwise be the case for many Australian coastal streams. (2) A number of Australian tree species have apparently evolved roots that seek the permanent, summer water table in order to survive prolonged dry spells, these root systems are particularly effective in mass failure mitigation due to rooting depths that are commonly greater than 5 m and are sometimes well in excess of 20 m. (3) The so-called “Root-Area-Ratio method” of calculating the shear strength of root-reinforced soil using root tensile strength data and Waldron's [Waldron, L.J., 1977. The shear resistance of root-permeated homogenous and stratified soil. J. Soil Sci. Soc. Am. 41, 843–849] and Wu et al.’s [Wu, T.H., McKinnell, W.P., Swanston, D.N., 1979. Strength of tree roots and landslides on Prince of Wales Island, Alaska. Can. Geotech. J. 16, 19–33] simple root model leads to significant overestimation of the actual root-reinforcement due to (a) breakage or pull-out of roots that taper and narrow beneath the shear plane such that individual roots do not achieve the tensile strength calculated on the basis of root diameter at the shear plane; and/or (b) the fact that the soil mass fails progressively along the length of potential shear plane rather instantaneously across the entire shear plane.     

Desirable plant root traits for protecting natural and engineered slopes against landslides

Slope stability models traditionally use simple indicators of root system structure and strength when vegetation is included as a factor. However, additional root system traits should be considered when managing vegetated slopes to avoid shallow substrate mass movement. Traits including root distribution, length, orientation and diameter are recognized as influencing soil fixation, but do not consider the spatial and temporal dimensions of roots within a system. Thick roots act like soil nails on slopes and the spatial position of these thick roots determines the arrangement of the associated thin roots. Thin roots act in tension during failure on slopes and if they traverse the potential shear zone, provide a major contribution in protecting against landslides. We discuss how root traits change depending on ontogeny and climate, how traits are affected by the local soil environment and the types of plastic responses expressed by the plant. How a landslide engineer can use this information when considering slope stability and management strategies is discussed, along with perspectives for future research. This review encompasses many ideas, data and concepts presented at the Second International Conference ‘Ground Bio- and Eco-engineering: The Use of Vegetation to Improve Slope Stability—ICGBE2’ held at Beijing, China, 14–18 July 2008. Several papers from this conference are published in this edition of Plant and Soil.     

An Experimental Investigation of the Resistance of Model Root Systems to Uprooting

The architecture of a tree root system may influence its abilityto withstand uprooting by wind loading. To determine how theroot branching pattern may alter the anchorage efficiency ofa tree, artificial model root systems with different topologiesand branching angles were built. The root systems were embeddedat various depths in wet sand and the pull-out resistance measured.A model to predict the uprooting resistance from the data collectedwas designed, allowing predictions of anchorage strength withregards to architecture. The dominant factors influencing pull-outresistance were the depth and length of roots in the soil. Themost efficient type of branching pattern predicted by the programwas one with an increased number of roots deep in the soil.The optimum branching angle most likely to resist pull-out isa vertical angle of 90° between a lateral and the main axis.The predicted mechanically optimal radial angle between a lateralbranch and its daughter is between 0 and 20°. Values ofbranching angle are compared with those measured in real woodyroot systems of European larch and Sitka spruce. Root architecture; root anchorage; pull-out resistance; windthrow; Picea sitchensis ; Larix decidua     

四种不同生活型树种细根寿命及影响因素

以4种不同生活型树种(常绿阔叶和针叶树种、落叶阔叶和针叶树种)为研究对象,通过微根管法现地观测细根的生长动态,比较不同生活型树种细根寿命在种内和种间的差异,探讨影响细根寿命的主要因子,研究结果对理解和预测森林生态系统碳及养分循环过程具有重要的理论意义。结果表明:(1)细根形态特征(分枝结构和直径)显著影响种内细根寿命,分枝等级越低、直径越小,细根的寿命越短;(2)4个树种的细根寿命表现出明显的土层效应和季节效应,即随土壤深度增加,细根的累积存活率逐渐增加,寿命延长;而不同季节出生的细根其寿命长短模式在树种间不一致,春季或夏季出生的细根寿命要长于秋冬季;(3)常绿树种(柳杉、石栎)的细根寿命要长于落叶树种(池杉、麻栎),同时,针叶树种(池杉、柳杉)的细根寿命要长于阔叶树种(麻栎、石栎)。在同一树种内,细根寿命受细根直径、根系分枝结构、土壤环境因子(土层)等因素显著影响,但在不同树种间,细根寿命可能更依赖于树木生长速率、碳分配模式等树木整体的功能性状差异。     

The effect of mechanical stimulation on root and shoot development of young containerised Quercus robur and Robinia pseudoacacia trees

Thigmomorphogenesis is a well-studied process in agricultural crops and coniferous trees. Nevertheless, the effects on both shoot and root characteristics for deciduous woody species received little attention so far. In this study, the objective was to understand the effect of aboveground flexing treatments on the development of structural, mechanical and physiological root and shoot characteristics for two deciduous tree species, Black locust (Robinia pseudoacacia L.) and English oak (Quercus robur L.). Flexing treatments were performed using an electromechanical device with a rotating arm touching and bending the plants at regular intervals. A wide range of stem, shoot as well as root system characteristics was measured. The different flexing treatments altered above- and belowground plant development for both species, with strongest effects on Quercus and most significant differences between the control and the unidirectional flexing treatment. Some responses are in accordance with previous findings, such as stem eccentricity and reduced shoot elongation under unidirectional flexing, but others are renewing, such as the lower stomatal density and larger epidermal cell surface for the Quercus plants under variable flexing direction. Despite some common responses, both species frequently differed in the way they were affected. Belowground, Quercus plants under unidirectional flexing invested relatively more in their first order root and deeper second order roots, whereas Robinia plants allocated relatively more to fine root biomass and horizontal shallow roots. Both strategies potentially increased pull-out as well as overturning resistance in their own way. The presented findings are valid for young trees grown in small containers. Based on practical know-how and shortcomings experienced in the course of this experiment, methodological recommendations are formulated. We finally stress the complex variability in growth responses, especially for root systems, observed in different studies and related to dissimilarity in species, soil conditions, plant history or type of mechanical perturbation.     

Growth and morphology of eucalypt seedling-roots, in relation to soil strength arising from compaction

Information on the response of root growth and morphology to soil strength is useful for testing suitability of existing and new tillage methods and/or for selecting plants suitable for a specific site with or without tillage. Although there is extensive published information on the root growth-soil strength relationships for annual agricultural plants, such information is scarce for woody, perennial tree species. The purpose of this study is to examine growth and morphology of the root systems of 17-day-old eucalypt seedlings with respect to variation in soil strength. Soil strength in this study was varied by compaction of a well-aggregated clay soil to bulk densities of 0.7–1.0 Mg m^-3 whilst maintaining adequate water availability and aeration for plant growth. Lengths and tip-diameters of primary and lateral roots were measured on the excavated root systems of seedlings.With increase in bulk density and also soil strength (expressed as penetrometer resistance), total length of primary and lateral roots decreased. There were 71 and 31% reduction in the lengths of primary and lateral roots respectively with an increase in penetrometer resistance from 0.4 to 4.2 MPa. This indicated primary roots to be more sensitive to high soil strength than the lateral roots. Average length of lateral roots and diameters of both primary and lateral root tips increased with an increase in soil strength as well. There was greater abundance of lateral roots (no. of lateral roots per unit length of primary root) and root hairs with increased soil strength. The observed root behaviour to variable soil strength is discussed in the context of compensatory growth of roots and overall growth of plants.