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.     

Biomass, morphology and nutrient contents of fine roots in four Norway spruce stands

Fine root systems may respond to soil chemical conditions, but contrasting results have been obtained from field studies in non-manipulated forests with distinct soil chemical properties. We investigated biomass, necromass, live/dead ratios, morphology and nutrient concentrations of fine roots (<2 mm) in four mature Norway spruce (Picea abies [L.] Karst.) stands of south-east Germany, encompassing variations in soil chemical properties and climate. All stands were established on acidic soils (pH (CaCl₂) range 2.8–3.8 in the humus layer), two of the four stands had molar ratios in soil solution below 1 and one of the four stands had received a liming treatment 22 years before the study. Soil cores down to 40 cm mineral soil depth were taken in autumn and separated into four fractions: humus layer, 0–10 cm, 10–20 cm and 20–40 cm. We found no indications of negative effects of N availability on fine root properties despite large variations in inorganic N seepage fluxes (4–34 kg N ha⁻¹ yr⁻¹), suggesting that the variation in N deposition between 17 and 26 kg N ha⁻¹ yr⁻¹ does not affect the fine root system of Norway spruce. Fine root biomass was largest in the humus layer and increased with the amount of organic matter stored in the humus layer, indicating that the vertical pattern of fine roots is largely affected by the thickness of this horizon. Only two stands showed significant differences in fine root biomass of the mineral soil which can be explained by differences in soil chemical conditions. The stand with the lowest total biomass had the lowest Ca/Al ratio of 0.1 in seepage, however, Al, Ca, Mg and K concentrations of fine roots were not different among the stands. The Ca/Al ratio in seepage might be a less reliable stress parameter because another stand also had Ca/Al ratios in seepage far below the critical value of 1.0 without any signs of fine root damages. Large differences in the live/dead ratio were positively correlated with the Mn concentration of live fine roots from the mineral soil. This relationship was attributed to faster decay of dead fine roots because Mn is known as an essential element of lignin degrading enzymes. It is questionable if the live/dead ratio can be used as a vitality parameter of fine roots since both longevity of fine roots and decay of root litter may affect this parameter. Morphological properties were different in the humus layer of one stand that was limed in 1983, indicating that a single lime dose of 3–4 Mg ha⁻¹ has a long-lasting effect on fine root architecture of Norway spruce. Almost no differences were found in morphological properties in the mineral soil among the stands, but vertical patterns were apparently different. Two stands with high base saturation in the subsoil showed a vertical decrease in specific root length and specific root tip density whereas the other two stands showed an opposite pattern or no effect. Our results suggest that proliferation of fine roots increased with decreasing base saturation in the subsoil of Norway spruce stands.     

Ectomycorrhizal root growth in scots pine stands in response to manipulation of litter and humus layers

 The effect on ectomycorrhizal root growth in a nitrogen-enriched planted stand of Scots pine (Pinus sylvestris L.) on podzolic sandy soil to manipulation of litter and humus layers (removal, doubling and control treatments) was examined, and compared to ectomycorrhizal root growth in an untreated naturally established Scots pine stand on nutrient-poor non-podzolic sandy soil. Half a year after manipulation of litter and humus layers in the planted stand, ingrowth-cores to a depth of 60 cm were installed in both stands. Scots pine roots were sampled four times during two growing seasons. Ectomycorrhizal roots were found at all sampled soil depths to 60 cm in all plots. Root growth and ectomycorrhizal development were greater in the naturally established stand than in all plots in the planted stand. Numbers of ectomycorrhizal root tips in the litter and humus removal plots were generally higher than in the control plots in the planted stand until May 1992. Doubling litter and humus did not significantly affect root length or the numbers of ectomycorrhizal root tips. The N _dissolved , NH₄ ⁺ and NO₃ ^– concentrations and the organic matter content in the upper 5 cm of the mineral soil in the planted stand on podzolic sandy soil were generally higher and the pH significantly lower than in the naturally established stand on non-podzolic sandy soil. Root growth and ectomycorrhizal development in the secondary stand may have been negatively affected by the chemical composition of the podzolic sandy soil. Accepted: 19 March 1997     

Distribution of roots and root nodules and biomass allocation in young intensively managed grey alder stands on a peat bog

Development of below-ground biomass and biomass allocation were studied in two different stands of young grey alder stands growing on a peat bog. Both stands were given the same fertilization and irrigation treatment. The roots were investigated from 1) open plastic tubes enclosing the complete root systems in 1982, and 2) root cores 1984–86. Coarse roots (diameter>1 mm) were mainly found close to the trunk of the trees while fine roots (≤1 mm) were more evenly distributed in the stands. Root nodules were intermediate in distribution. The root systems were shallow, with more than 90% of the biomass in the uppermost 9–10 cm of the soil, probably because of low oxygen availability in the peat soil. The biomass allocation to the above-ground parts increased during the study period.     

Molecular analysis of natural root grafting in jack pine (Pinus banksiana) trees: how does genetic proximity influence anastomosis occurrence?

Natural root grafting has been observed in more than 150 tree species where up to 90 % of trees could be interconnected within a stand. Intraspecific root grafting was previously found in Pinus banksiana stands, ranging from 21 to 71 % of trees grafted with one another. It is not known why root grafting is frequent in some species and not in others, or why not all roots that cross form root grafts. We investigated genetic diversity of grafted and non-grafted trees to determine if there was a relationship between genetic distance and the probability of forming natural root grafts. Seven plots were hydraulically excavated in four natural forest stands and three plantations of P. banksiana in the western boreal forest of Quebec, Canada. At pairs scale, we studied the effect of geographic and genetic distances on root grafting occurrence. At stand level, we analysed the effect of tree density, soil type, stand type and mean pairwise relatedness on the mean number of grafts per tree and on the percentage of grafted trees per plot. At pairs scale, our analysis revealed that root grafting presence was influenced by spatial distance between trees and less importantly, by genetic distance between individuals. At stand level, root grafting frequency was correlated with stand type (greater in naturally regenerated stands), but not with genetic diversity between individuals. In conclusion, root grafting appears to be principally linked to tree proximity and slightly to genetic proximity between individuals.     

The clonal root system of balsam poplar in upland sites of Quebec and Alberta

Balsam poplar seeds are short‐lived and require moist seedbeds soon after they are released to germinate. In addition to sexual reproduction, balsam poplar stands can regenerate clonally by root suckering. The origin of stands will in turn affect their genetic structure and root system architecture, which are poorly understood for upland forest stands. Three stands were hydraulically excavated in Quebec (moist) and Alberta (dry) to determine the origin of trees and to characterize root systems with respect to presence of parental roots and root grafts connections. Clones were identified using single‐nucleotide polymorphism (SNPs), and all stems, roots and root grafts were aged using dendrochronology techniques. All 82 excavated trees were of sucker origin, and four of the six stands contained a single clone. Parental root connections were found between 22% and 25% of excavated trees, and 53% and 48% of trees were linked with a root graft between the same or different clones, in Alberta and Quebec, respectively. Mean distance between trees connected by parental root was significantly lower than the distance between unconnected trees (0.47 ± 0.25 m vs. 3.14 ± 0.15 m and 1.55 ± 0.27 m vs. 4.25 ± 0.13 m) in Alberta and in Quebec, respectively. The excavations also revealed many dead stumps with live roots, maintained through root connections with live trees. This research highlights that balsam poplar growing in upland stands is a clonal species that can maintain relatively high genotypic diversity, with frequent root connections between trees at maturity. Maintaining an extensive root system through root connections increases the chances of a clone surviving when the above ground tree is dead and may also enhance the resilience of balsam poplar stands after disturbance.     

The role of soil conditions in fine root ecomorphology in Norway spruce (Picea abies (L.) Karst.)

The present study is an attempt to investigate the pattern of morphological variability of the short roots of Norway spruce (Picea abies (L.) Karst.) growing in different soils. Five root parameters – diameter, length and dry weight of the root tip, root density (dry weight per water-saturated volume) and specific root area (absorbing area of dry weight unit) were studied with respect to 11 soil characteristics using CANOCO RDA analysis. The investigation was conducted in seven study areas in Estonia differing in site quality class and soil type. Ten root samples per study area were collected randomly from the forest floor and from the 20 cm soil surface layer. Eleven soil parameters were included in the study: humus content, specific soil surface area, field capacity, soil bulk density, pH (KCl and H₂O dilution's), N and Ca concentrations, Ca/Al and C/N ratios, and the decomposition rate of fine roots (<2 mm dia.). Root morphological characteristics most strongly related to the measured soil characteristics in the different sites were specific root area, root density and diameter of the short roots, the means varying from 29 to 42 m² kg⁻¹, from 310 to 540 kg m⁻³ and from 0.26 to 0.32 mm, respectively; root density being most sensitive. The most favourable site and soil types resulting in fine roots with morphological characteristics for optimizing nutrient uptake (e.g. low short root density and high specific root area) were Umbric Luvisol (Oxalis), Dystric Gleysol (Oxalis) and Gleyic Luvisol (Hepatica). These soil types correspond to highly productive natural forest stands of Norway spruce in Estonia. All measured soil variables explained 28% of total variance of the root characteristics. The most important variables related to root morphology were the humus content, field capacity and specific soil surface area. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effect of lime and gypsum applications on a sessile oak ( Quercus petraea (M.) Liebl.) stand at La Croix-Scaille (French Ardennes) II. Fine root dynamics

Fine root distribution, quantities, dynamics and composition were studied in a sessile oak coppice stand in the French Ardennes on an acidic soil (< pH-H2O 4.5), one to five years after lime or gypsum applications. Fine root biomass and length increased and specific root length decreased after lime or gypsum treatments. The treatment responses were strongest four to five years after the applications, but the tendencies after one year were similar. The effects were pronounced in the top 15 cm but also at 30–45 cm four to five years after liming. The latter effect suggests an indirect positive feedback from the aerial parts of the trees into the deeper soil layers. Sequential sampling for two years revealed large differences in total fine root length between the years, and also indicated that fine root turnover was lower after liming or gypsum applications than in the control. This seemed to be related to a lower fine root mortality and higher longevity rather than to increased fine root production. The improved nutrient status of the fine roots corroborates this and coincides with improved foliar nutrition and tree growth. Moderate doses of lime and gypsum appeared effective in enhancing root system uptake function, resulting in increased above ground growth.     

Development processes and growth pattern of Pinus densiflora stands in central eastern Korea

Stand growth and developmental processes were investigated in Pinus densiflora Siebold et Zucc. stands of different ages in the central eastern region of Korea. Stands were inventoried and five trees per stand were sampled for stem analysis, age estimation, and growth analysis. More than 80% of sampled trees in a stand were established within 3–5 years, and most stands had a single cohort structure. The initial growth of pine seedlings was slow, but the height growth accelerated beyond 2–3 m height, 5–10 years after establishment. Linear growth was maintained until 10–12 m height, at which suppressed trees fell behind and might die out. The young stand was composed of pure pines, while few pine seedlings and saplings were found in the understory of older stands. The peak of diameter growth rate occurred around 5–15 years after tree establishment, implying that competition begins during that period. The pine stand development follows four stages: (1) the young stage when the growth rate increases and peaks; (2) the height competition stage when trees focus on height growth for light while maintaining a narrow DBH and height distribution; (3) the differentiation stage when suppressed trees die out, and the DBH distribution becomes wider; and (4) the mature stage when stands have a multi-canopy structure with a wide DBH and height distribution, while the understory is dominated by other tree species. The changes in growth rates and stand structure through forest development would be implemented to predict alterations of above-ground carbon sequestration rates.     

Root Morphology and Anchorage of Six Native Tree Species from a Tropical Montane Forest and an Elfin Forest in Ecuador

Root architecture of tree species was investigated at two different altitudes in tropical forests in Ecuador. Increasing altitude was accompanied by higher wind speeds and more shallow soils, while slope angles of both sites were comparable (20–50°). Three tree species typical for the montane forest at 1900 m (Graffenrieda emarginata (Ruiz & Pav.) Triana (Melastomataceae), Clethra revoluta (Ruiz & Pav.) Spreng. (Clethraceae), Vismia tomentosa Ruiz & Pav. (Clusiaceae)) and for the elfin forest at 3000 m (Weinmannia loxensis Harling (Cunoniaceae), Clusia spec. (Clusiacaea) Styrax foveolaria Perkins (Styraceae)) were examined. At 1900 m, 92% of the trees grew upright, in comparison to 52% at 3000 m. At 3000 m, 48% of the trees were inclined, lying or even partly uprooted. At this altitude, all trees with tap roots or with shoots connected by coarse rhizomes, 83% of the trees with stilt roots, and 50% of the trees in which stems or roots were supported by other trees grew upright, suggesting that these characteristics were relevant for tree stability. Root system morphology differed markedly between altitudes. In contrast to 1900 m, where 20% of structural roots originated in the deeper mineral soil, root origin at 3000 m was restricted to the forest floor. The mean ratio of root cross sectional area to tree height decreased significantly from 6.1 × 10⁻³ m² m⁻¹ at 1900 m to 3.2 × 10⁻³ m² m⁻¹ at 3000 m. The extent of root asymmetry increased significantly from 0.29 at 1900 m to 0.62 at 3000 m. This was accompanied by a significantly lower number of dominant roots at 3000 m (2.3 compared to 3.8 at 1900 m). In conclusion, native tree species growing in tropical montane and elfin forests show a variety of root traits that improve tree stability. Root system asymmetry is less important for tree stability where anchorage is provided by a deep and solid root–soil plate. When deep rooting is impeded, root traits improving the horizontal extension of the root–soil plate are more pronounced or occur more frequently. Furthermore, mutual mechanical support of roots and stems of neighboring trees seems to be an appropriate mechanism to provide anchorage in soils with low bulk density and in environments with high wind speeds.     

Root distributions in a Grevillea robusta-maize agroforestry system in semi-arid Kenya

Limited knowledge of root distributions in agroforestry systems has resulted in assumptions that various tree species are more suited to agroforestry than others, because they are presumed to have few superficial lateral roots. This assumption was tested for Grevillea robusta when grown with maize (Zea mays) in an agroforestry system in a semi-arid region of Kenya. At a site with a shallow soil, root lengths of both species between the soil surface and bedrock were quantified by soil coring, at intervals over four cropping seasons, in plots containing sole stands and mixtures of the trees and crop; the trees were 4–6 years old and they were severely pruned before the third season. Profiles of soil water content were measured using a neutron probe. Prior to pruning of the trees, recharge of soil water below the deepest maize roots did not occur, resulting in significant (P<0.05) suppression of maize root lengths and downward root growth. Maximum root length densities for both species occurred at the top of the soil profile, reaching 1.1–1.7 cm cm^-3 for G. robusta, but only 0.5 cm cm^-3 for maize grown with trees. Root populations in mixed plots were dominated by G. robusta at all times, all depths and all distances from trees and maize and, thus, there was no spatial separation of the rooting zones of the trees and crop. Competition between G. robusta and maize for soil water stored near the surface was unavoidable, although pruning reduced its impact; complementary use of water by the trees and crop would only have been possible if alternative sources of water were available. This revised version was published online in June 2006 with corrections to the Cover Date.     

The coarse‐root system of mature Populus tremuloides in declining stands in Alberta,Canada

Abstract. The coarse‐root dynamics of ramets of Populus tremuloides (aspen) were investigated with respect to persistence of the original root connections (roots of parent trees from which the ramets originated), the time of establishment of new roots at the base of the stem and the fate of the communal root system after death of individual trees. Parts of the root systems of three declining stands of aspen ramets were hydraulically excavated. From each stand, sections of all structural roots were collected at the base of live and dead trees and were analysed using dendrochronology techniques. Parent roots were identified in the root system of every tree. The trees initiated new structural roots shortly after suckering. Live roots were often connected to the stump of dead and decayed trees. Grafting was common, especially at or near the stumps. Death of trees along the parent roots over time did not seem to favour the entry of significant decay, nor promote breakage of the original root connections. Instead of becoming independent of the parent root system the ramets incorporated the parent roots into their own root systems, remaining interconnected.     

No evidence of spatial root system segregation and elevated fine root biomass in multi-species temperate broad-leaved forests

Differences in spatial rooting patterns among coexisting species have been recognized as an important mechanism for generating biodiversity effects on ecosystem functioning. However, it is not yet clear whether complementarity in root space exploration is a universal characteristic of multi-species woody communities. In a temperate broad-leaved forest with a mosaic of species-poor and species-rich stands, we tested two hypotheses related to putative below-ground ‘overyielding’ in more diverse forests, (1) that species mixture results in a partial spatial segregation of the fine root systems of different species, and (2) that stand fine root biomass increases with tree species diversity. We investigated 12 stands either with one, three, or five dominant tree species (4 replicate stands each) under similar soil and climate conditions for stand fine root biomass and spatial root segregation in vertical and horizontal direction in the soil. Fine roots of different tree species were identified using a morphological key based on differences in colour, periderm surface structure, and branching patterns. In species-poor and species-rich stands, and in all tree species present, fine root density (biomass per soil volume) decreased exponentially with soil depth at very similar rates. Stand fine root biomass in the densely rooted upper soil (0–40 cm depth) was not significantly different between stands with 1, 3 or 5 dominant tree species. We conclude that ‘below-ground overyielding’ in terms of higher fine root biomasses in species-rich stands as compared to monospecific ones does not occur in these broad-leaved forests which most likely results from a missing complementarity in vertical rooting patterns of the present tree species.     

Unusual Fine Root Distributions of Two Deciduous Tree Species in Southern France: What Consequences for Modelling of Tree Root Dynamics?

The spatial distribution of fine roots of two deciduous tree species was investigated in contrasting growing conditions in southern France. Hybrid walnut trees (Juglans regia×nigra cv. NG23) and hybrid poplars (Populus euramericana cv. I214) were both cultivated with or without annual winter intercrops for 10 years on deep alluvial soils. Soil samples for measuring the fine root distribution of both trees and crops were obtained by soil coring down to 3-m depth at several distances and orientations from the tree trunk. The distribution of live fine roots from walnut and poplar trees was patchy and sometimes unexpected. In the tree-only stands, fine root profiles followed the expected pattern, as fine root density decreased with increasing depth and distance from the tree trunk. However, many fine root profiles under intercropped trees were uniform with depth, and some inverse profiles were observed. These distributions may result from a high degree of plasticity of tree root systems to sense and adapt to fluctuating and heterogeneous soil conditions. The distortion of the tree root system was more pronounced for the walnut trees that only partially explored the soil volume: in the tree-only stand, the walnut rooting pattern was very superficial, but in the intercropped stand walnut trees developed a deep and dense fine root network below the crop rooting zone. The larger poplars explored the whole available soil volume, but the intercrop significantly displaced the root density from the topsoil to layers below 1 m depth. Most tree root growth models assume a decreasing fine root density with depth and distance from the tree stem. These models would not predict correctly tree–tree and tree–understorey competition for water and nutrients in 3D heterogeneous soil conditions that prevail under low-density tree stands. To account for the integrated response of tree root systems to such transient gradients in soils, we need a dynamic model that would allow for both genotypic plasticity and transient environmental local soil conditions.     

黄土丘陵半干旱区密植枣林随树龄变化的根系空间分布特征

该文研究了黄土丘陵半干旱区密植枣( Ziziphus jujuba ‘Lizao’)林群体根系随树龄变化的空间分布特征。对1年生、4年生、8年生和11年生4种树龄的密植枣林采用剖面法, 获得0-1 m土壤剖面上直径>3 mm、1-3 mm及<1 mm的根系数量和空间位置信息。利用方差分析, 评价了山地密植枣林林分根系随树龄变化的水平和垂直分布特征。结果表明: 3种直径的根系数量均随着树龄的增长而增加, 直径< 1 mm的根系增长速度最快; 随着土层加深, 根系数量递减, 1年生枣林的根系主要聚集在0-40 cm土层中, 4年生及以上树龄的根系主要分布在0-60 cm土层中; 0-1 m土层内, 1年生枣林(株距1.2 m)及4年以上树龄(株距2 m), 同树龄枣林中直径<1 mm的根系水平分布无差异; 同一土层中(0-20 cm, 20-40 cm, 40-60 cm), 无论树龄大小及离树干的水平位置如何, 不同直径根系的数量都无差异。研究表明: 在有水肥管理措施的条件下, 枣林根系垂直方向形成浅层型的适应模式; 在密植环境下, 枣林细根形成根网型的适应模式。     

Root deformation in plantations of container-grown Scots pine trees: effects on root growth,tree stability and stem straightness

Root system deformation was studied in 23 Scots pine (Pinus sylvestris L.) stands in central Sweden. The study comprised both plantations created with container-grown plants (Paperpot) and natural stands including young (7–9 year old) and older (19–24 year old) trees. Trees were measured with regards to distribution of roots, root deformation, stability, stem straightness and wood properties in stumps. Root distribution was most uniform for naturally regenerated trees. Older trees generally showed a better root distribution than young trees. The young planted trees displayed a high frequency of severely spiralled root systems, while only a few of the older trees had spiralled root systems. No severe root deformations were observed on naturally regenerated trees. Naturally regenerated trees were more stable than those which had been planted. Differences in bending moment, when trees were pulled to an angle of 10°, were considerable between young planted and naturally regenerated trees, but less pronounced for the older trees. Young planted trees had the highest frequency of severely crooked stem bases, while naturally regenerated trees had the straightest mode of growth. Tensile strength in peripheral wood samples of the stumps was substantially lower for planted than for naturally regenerated trees. Strain values to breakage of wood samples, taken from the root collar and the central- and peripheral part of the stump were lower for planted trees. The conclusions from this study are that root distribution, tree stability and stem straightness of planted Paperpot-grown trees will improve after a certain time and approach the state of naturally regenerated trees. As trees grow older, early established crooked stem bases will be compensated by radial growth and the tree will appear straighter. Inside the stem, however, problems may still remain with abnormal fibre direction and compression wood together with inferior root strength due to fibre disturbances as a result of spiralled roots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Distribution,length and weight of roots in young plantations ofEucalyptus grandis W. Hill ex Maiden irrigated with recycled water

Summary The root systems ofEucalyptus grandis W. Hill ex Maiden, irrigated with recycled municipal effluent at two sites in north-western Victoria, Australia, were studied by excavation and coring. Trees at Robinvale were four years-old and were irrigated using micro-sprays that covered only 70% of the ground surface area, whereas at Mildura, effuent was uniformly was uniformly applied to six years-old trees by flood and sprinkler irrigation. At Mildura where roots were excavated from a 2.80×2.80×1.20 m block of soil, a total root length of 1193 m.m⁻² and a total root weight of 3.1 kg m⁻² were estimated in the top metre. For roots >1 mm diameter, 77% of intercepts were at 0–30 cm, whereas only 50% were in the 50–100 cm soil horizon. At both sites where roots in the top 30 cm were studied by coring, the vertical distributions of root intercepts, length and weight were similar. Root length was greatest in the 0–10 cm soil horizon at both sites, and intercepts of roots <1 mm diameter comprised 73% and 81% of all roots at Mildura and Robinvale respectively. Roots <1 mm diameter contributed 85% of total length at both sites, but only 19% and 21% of total weight at Mildura and Robinvale respectively. The horizontal distribution of roots differed at the two sites. With uniform application of effuent at Mildura, root intercepts and length were concentrated in the centre of the irrigation bay, but at Robinvale, the concentration occurred closer to the tree row due mainly to the different method of irrigation. Root weight at both sites was highest within 50 cm of the tree row. Root densities of 0.11 to 0.57 cm cm⁻³ were estimated in the two plantations; these were similar to root densities measured inPinus radiata D. Don plantations up to 46 months old, but were considerably lower than those estimated for pastures. The implications of the results for the management of irrigated plantations of eucalypts are discussed.     

亚热带典型树种根毛特征及其与共生真菌的关系

根毛和共生真菌增加了吸收面积,提高了植物获取磷等土壤资源的能力。由于野外原位观测根表微观结构较为困难,吸收细根、根毛、共生真菌如何相互作用并适应土壤资源供应,缺乏相应的数据和理论。该研究以受磷限制的亚热带森林为对象,选取了21种典型树种,定量了根毛存在情况、属性变异,分析了根毛形态特征与共生真菌侵染率、吸收细根功能属性之间的关系,探讨了根表结构对低磷土壤的响应和适应格局。结果表明:1)在亚热带森林根毛不是普遍存在的, 21个树种中仅发现7个树种存有根毛, 4个为丛枝菌根(AM)树种, 3个为外生菌根(ECM)树种。其中,马尾松(Pinus massoniana)根毛出现率最高,为86%;2)菌根类型是理解根-根毛-共生真菌关系的关键,AM树种根毛密度与共生真菌侵染率正相关,但ECM树种根毛直径与共生真菌侵染率负相关; 3) AM树种根毛长度和根毛直径、ECM树种根毛出现率与土壤有效磷含量呈负相关关系。该研究揭示了不同菌根类型树种根毛-共生真菌-根属性的格局及相互作用,为精细理解养分获取策略奠定了基础。     

Root system development of Larix gmelinii trees affected by micro-scale conditions of permafrost soils in central Siberia

Spatial distributions of root systems of Larix gmelinii (Rupr.) Rupr. trees were examined in two stands in central Siberia: an even-aged stand (ca. 100 yrs-old) and a mature, uneven-aged (240–280 yrs-old) stand. Five larch trees of different sizes were sampled by excavating coarse roots (diameter > 5 mm) in each stand. Dimensions and ages of all first-order lateral roots were measured. Micro-scale conditions of soil temperature and soil water suction (each 10 cm deep) were also examined in relation to earth hummock topography (mound vs. trough) and/or ground floor vegetation types (moss vs. lichens). All larch trees developed superficial root systems, consisting of the aborted short tap root (10–40 cm in soil depth) and some well-spread lateral roots (n= 4-13). The root network of each tree was asymmetric, and its rooting area reached about four times the crown projection area. Lateral roots generally expanded into the upper soil layers of the mounds where summer soil temperature was 1–6°C higher than inside nearby troughs. Chronological analysis indicated that lateral root expansion started successively from lower to upper parts of each aborted tap root, and some lateral roots occurred simultaneously at several decades after tree establishment. The process of root system development was likely to be primarily linked with post-fire dynamics of rhizosphere environment of the permafrost soils.     

Root suckering patterns in Populus euphratica (Euphrates poplar, Salicaceae)

To understand the spatial structure of monospecific Tugai forests (Xinjiang Province, China) growing as gallery woods nourished by ground water, root suckering in Populus euphratica was studied by a combination of morphological and molecular analyses. Seedlings grow a deep tap root and keep this as adult trees, whereas root suckers never develop a tap root but utilize the horizontally stretching root of their parent trees. The resulting reverse “T” root architecture distinguishes reliably even adult root suckers from generatively grown trees. Due to assimilate input from the root sucker, the distal root (pointing away from the parent tree) becomes thicker soon than its proximal root, which allows determination of the direction of vegetative growth. One stand including 279 young trees germinated from seeds and 267 root suckers was mapped completely, and selected suckers were assigned to parent trees by genotyping with microsatellite DNA. Root suckers develop up to 40 m away from parent trees on horizontal “spacer” roots, usually originating not deeper than 20 cm below surface. Trees begin with root suckering between 10 and 15 years, shortly before reaching flowering age. Cutting experiments indicated reduced survival of young root suckers disconnected from the parent tree. Without a tap root and with a rooting point close to the surface, declining ground water levels should lower the fitness of root suckers even more than that of generatively grown trees.