Seasonal variation of growth and development of the roots of five second year conifer species in the nursery

Every other week over their second growing season, stem height, collar diameter, shoot and root dry masses, number of lateral roots and length of the tap root were measured on nursery grown seedlings ofAbies balsamea L. Mill.,Pinus banksiana Lamb.,Pinus resinosa Ait.,Picea mariana Mill. BSP andPicea glauca Moench Voss. Root elongation, branching and mycorrhizal development were also recorded.Given species showed distinct seasonal growth patterns. The rate and timing of maximum root growth (mg/dry weight/week) differed markedly between species.Except for the increase in height ofPinus banksiana, root and shoot growth were not negatively correlated.The results are discussed in relation to the performance of tree seedlings in the nursery.     

黄土高原土壤紧实度对蚕豆生长的影响

通过盆栽试验、连续 2年的田间小区试验和农户生产试验 ,研究了土壤紧实状况对蚕豆 (Viciafa ba)生长的影响 ,讨论了当地土壤容重较高的原因 ,并提出了改进措施 .结果表明 ,随着 0～ 7cm土层土壤容重的增加 ,蚕豆植株每株的茎与根干重降低 ,根腐病 (Fusariumspp .)引起的死亡率增加 ,种子产量减少 .田间试验条件下 ,与生长于容重为 1.5 5和 1.6 4 g·cm-3 小区内的植株相比 ,生长于容重 1.84 g·cm-3 小区内的植株每株茎与根干重可分别减少 2 7.9%和 30 .8% ,植株累计死亡率增加 2 1.0 %～ 4 8.7% ,种子产量每公顷减少 19.8% .在 8户蚕豆田中进行的多点生产试验表明 ,春季土壤容重与蚕豆幼苗的根与茎干重、秋季土壤容重与种子产量均呈显著负相关     

Root growth and water uptake in winter wheat under deficit irrigation

Root growth is critical for crops to use soil water under water-limited conditions. A field study was conducted to investigate the effect of available soil water on root and shoot growth, and root water uptake in winter wheat (Triticum aestivum L.) under deficit irrigation in a semi-arid environment. Treatments consisted of rainfed, deficit irrigation at different developmental stages, and adequate irrigation. The rainfed plots had the lowest shoot dry weight because available soil water decreased rapidly from booting to late grain filling. For the deficit-irrigation treatments, crops that received irrigation at jointing and booting had higher shoot dry weight than those that received irrigation at anthesis and middle grain filling. Rapid root growth occurred in both rainfed and irrigated crops from floral initiation to anthesis, and maximum rooting depth occurred by booting. Root length density and dry weight decreased after anthesis. From floral initiation to booting, root length density and growth rate were higher in rainfed than in irrigated crops. However, root length density and growth rate were lower in rainfed than in irrigated crops from booting to anthesis. As a result, the difference in root length density between rainfed and irrigated treatments was small during grain filling. The root growth and water use below 1.4 m were limited by a caliche (45% CaCO₃) layer at about 1.4 m profile. The mean water uptake rate decreased as available soil water decreased. During grain filling, root water uptake was higher from the irrigated crops than from the rainfed. Irrigation from jointing to anthesis increased seasonal evapotranspiration, grain yield, harvest index and water-use efficiency based on yield (WUE), but did not affect water-use efficiency based on aboveground biomass. There was no significant difference in WUE among irrigation treatments except one-irrigation at middle grain filling. Due to a relatively deep root system in rainfed crops, the higher grain yield and WUE in irrigated crops compared to rainfed crops was not a result of rooting depth or root length density, but increased harvest index, and higher water uptake rate during grain filling.     

Apical diameter and branching density affect lateral root elongation rates in banana

Thick roots elongate faster than thinner ones. However, within one species, the growth achieved by roots of a given diameter can be very variable, and root diameter can only be considered as a determinant of root potential elongation rate. As root elongation is highly correlated to carbon availability, it can be hypothesized that local competition for resources, expressed as the number of lateral roots per unit length (i.e. the branching density), modulates root elongation. Using novel methods in field conditions, we have estimated apical diameters, elongation rates and growth durations of nearly 3500 banana lateral roots, in a field experiment with high radiations and a shaded glasshouse experiment with low radiations. Apical diameters and branching densities were lower in the experiment with low radiation, but elongation rates were higher. In both experiments, mean elongation rates of first-order laterals and thick second-order laterals were negatively correlated with bearing root branching densities. It is hypothesized that, even though apical diameters were lower, low branching densities in the shaded glasshouse allowed enhanced lateral root elongation. In both experiments, second-order laterals elongated more slowly than first-order laterals of similar diameter. A specific effect of root order, independent of branching density and apical diameter, contributed to explain these slow second-order lateral elongation rates. Most lateral roots elongated between 9 and 21 days and growth duration was mainly correlated with root diameter.     

Vesicular-arbuscular mycorrhizae in a wastewater-irrigated oldfield ecosystem in Michigan

The incidence of vesicular-arbuscular mycorrhizae (VAM) in wastewater irrigated and non-irrigated oldfield soils in Michigan was studied. Soil and root samples were taken monthly from field plots on the second and third years of consecutive irrigation with municipal wastewater at rates of 0, 5 and 10 cm wk^–1. The oldfield ecosystem contained a high VAM fungal spore population density, but low species diversity. The most common VAM fungal species were Glomus mosseae and G. fasciculatum. Both spore density and root colonization were higher in irrigated than in non-irrigated plots. Irrigation effects were largest early in the growing season. In addition to increasing VAM incidence, wastewater irrigation shifted VAM fungal species composition. Irrigation favored G. mosseae over G. fasciculatum. Bioassays using either Sorghum vulgare or Daucus carota, an oldfield native species, indicated that the VAM systems were still functioning after the third year of consecutive wastewater irrigation. The data from experiments using nutrient solutions at wastewater concentrations suggest that the effects of wastewater irrigation on VAM are due to the effects of both water and nutrients. Since VAM are a very important component of the plant's water and nutrient uptake system and equally important in structuring plant communities under limiting growth conditions, it is suggested that the stimulatory effect of wastewater irrigation on VAM in an oldfield ecosystem enhances the ecosystem's ability to function as a living filter for wastewater clean up.Michigan Agricultural Experimental Station Journal Article No 13137.On sabbatical leave from ESAL, Lavras-MG, Brazil     

Roots,soil water and crop yield: tree crop interactions in a semi-arid agroforestry system in Kenya

Variations in soil water, crop yield and fine roots of 3–4 year-old Grevillea robusta Cunn. and Gliricidia sepium (Jacq.) Walp. growing in association with maize (Zea mays L.) were examined in semiarid Kenya during the long rains of 1996 and 1997. Even although tree roots penetrated more deeply than maize roots, maximum root length densities for both tree species and maize occurred in the top 200 mm of the soil profile where soil moisture was frequently recharged by rains. Populations of roots in plots containing trees were dominated by tree roots at the beginning of the growing season but because tree roots died and maize root length increased during the cropping season, amounts of tree and maize roots were similar at the end of the season. Thus, there was evidence of temporal separation of root activity between species, but there was no spatial separation of the rooting zones of the trees and crops within that part of the soil profile occupied by crop roots. Tree root length density declined with increasing distances from rows of trees and with depth in the soil profile. Although Grevillea trees were largest, plots containing G. sepium trees always contained more tree roots than plots containing G. robusta trees and Gliricidia was more competitive with maize than Grevillea. Overall, Gliricidia reduced crop yield by about 50% and Grevillea by about 40% relative to crop yield in control plots lacking trees and reductions of crop yield were greatest close to trees. There was less soil moisture in plots containing trees than in control plots. Such difference between control plots and plots containing trees were maximal at the end of the dry season and there was always less soil moisture close to trees than elsewhere in the plots. Plots containing Gliricidia trees contained less soil water than plots containing Grevillea trees.     

Fertilization in a mature Scots pine (Pinus sylvestris L.) stand—effects on fine roots

The effects of liquid and solid fertilizers on fine-root development were studied in a 130-year-old Scots pine (Pinus sylvestris L.) stand. Ingrowth cores,viz. initially root-free mesh bags with sieved mineral soil taken outside the plots and driven to a depth of 30 cm from the soil surface, were subsequently resampled and the amount of fine roots was estimated. The total accretion of both fine-root length and dry weight was comparatively high in the liquid fertilization plot. The most substantial net accretion, however, during the 1984 period was in the control plot. The results of the study is that the ramification pattern of the fine roots was strongly influenced by fertilization. The average number of root tips per unit length was 9.9 cm⁻¹ in the control plot cm⁻¹, compared with both the solid (A and B) and liquid fertilization plots (2.3, 3.2 and 3.3 cm⁻¹, respectively) due to a greater occurrence of mycorrhizal aggregates (‘ball mycorrhiza’). The effects of fertilization on the mineral nutrient concentrations in the fine roots were limited—the strongest effects were in the liquid-fertilized area. The observed increase in the concentration of most mineral nutrients in the latter experimental area, however, may be due to a change in the growth pattern of the fine roots rather than an effect of the fertilizers themselves.     

The effects of free-air CO2 enrichment and soil water availability on spatial and seasonal patterns of wheat root growth

Spring wheat [ Triticum aestivum (L). cv. Yecora Rojo] was grown from December 1992 to May 1993 under two atmospheric CO₂ concentrations, 550 μmol mol^–1 for high-CO₂ plots, and 370 μmol mol^–1 for control plots, using a Free-Air CO₂ Enrichment (FACE) apparatus. In addition to the two levels of atmospheric CO₂, there were ample and limiting levels of water supply through a subsurface trip irrigation system in a strip, split-plot design. In order to examine the temporal and spatial root distribution, root cores were extracted at six growth stages during the season at in-row and inter-row positions using a soil core device (86 mm ID, 1.0 m length). Such information would help determine whether and to what extent root morphology is changed by alteration of two important factors, atmospheric CO₂ and soil water, in this agricultural ecosystem. Wheat root growth increased under elevated CO₂ conditions during all observed developmental stages. A maximum of 37% increase in total root dry mass in the FACE vs. Control plots was observed during the period of stem elongation. Greater root growth rates were calculated due to CO₂ enhancement until anthesis. During the early vegetative growth, root dry mass of the inter-row space was significantly higher for FACE than for Control treatments suggesting that elevated CO₂ promoted the production of first-order lateral roots per main axis. Then, during the reproductive period of growth, more branching of lateral roots in the FACE treatment occurred due to water stress. Significant higher root dry mass was measured in the inter-row space of the FACE plots where soil water supply was limiting. These sequential responses in root growth and morphology to elevated CO₂ and reduced soil water supports the hypothesis that plants grown in a high-CO₂ environment may better compensate soil-water-stress conditions.     

Effects of irrigation on litterfall,fine root biomass and production in a semideciduous lowland forest in Panama

The effects of irrigation on fine root biomass, root production and litterfall were measured at the community level, in a semideciduous lowland forest in Panama. Biomass of roots less than 2 mm in dia. in the first 10 cm of the soil (measured with soil cores), was higher in irrigated (1.80 Mg ha^-1) than in non-irrigated plots (1.24 Mg ha^-1). During the dry season, productivity of roots (measured with ingrowth cylinders filled with root-free soil), was higher in irrigated (1.6 g m^-2 day^-1) than in control plots (0.3 g m^-2 day^-1). In control plots, root productivity was highly seasonal. Maximum root growth into the root-free soil, occurred during the transitions from dry to wet, and from wet to dry season, possibly as a response to water and/or nutrient pulses. Litterfall was not significantly different between irrigated (3.8 g m^-2 day^-1) and control plots (3.7 g m^-2 day^-1). The results of this study show that root-productivity is limited by the water supply during the dry season, and that water by itself, is not a limiting factor for community-level litter production. This revised version was published online in June 2006 with corrections to the Cover Date.     

Fine root architecture, morphology, and biomass of different branch orders of two Chinese temperate tree species

We have limited understanding of architecture and morphology of fine root systems in large woody trees. This study investigated architecture, morphology, and biomass of different fine root branch orders of two temperate tree species from Northeastern China—Larix gmelinii Rupr and Fraxinus mandshurica Rupr —by sampling up to five fine root branch orders three times during the 2003 growing season from two soil depths (i.e., 0–10 and.10–20 cm). Branching ratio (R _b) differed with the level of branching: R _b values from the fifth to the second order of branching were approximately three in both species, but markedly higher for the first two orders of branching, reaching a value of 10.4 for L. gmelinii and 18.6 for F. mandshurica. Fine root diameter, length, SRL and root length density not only had systematic changes with root order, but also varied significantly with season and soil depth. Total biomass per order did not change systematically with branch order. Compared to the second, third and/or fourth order, the first order roots exhibited higher biomass throughout the growing season and soil depths, a pattern related to consistently higher R _b values for the first two orders of branching than the other levels of branching. Moreover, the differences in architecture and morphology across order, season, and soil depth between the two species were consistent with the morphological disparity between gymnosperms and angiosperms reported previously. The results of this study suggest that root architecture and morphology, especially those of the first order roots, should be important for understanding the complexity and multi-functionality of tree fine roots with respect to root nutrient and water uptake, and fine root dynamics in forest ecosystems.     

A large ephemeral release of nitrogen upon wetting of dry soil and corresponding root responses in the field

To assess changes in soil nutrients, root growth and mycrorrhizal infection in response to rain events, a water pulse was applied to a very dry soil. Wetting of a dry soil in the Great Basin of the Western United States led to a striking pulse of available soil nitrate in a field plot, but available phosphate was not affected. This is the first field demonstration of this phenomenon in the Great Basin as far as we are aware. This pulse was only apparent for a few days, probably due to microbial immobilization of the nitrogen. Root ammonium uptake capacity increased within one day of the water pulse, but new root growth was not apparent until 3 days after the water pulse. Thus, to capture this ephemeral release of nitrogen, enhanced uptake capacity of existing roots was probably more important than development of new roots. Mycorrhizal infection was not affected by the water pulse treatments. However, since the water pulse only affected nitrogen availability and mycorrhizae are generally most effective in facilitating acquisition of less mobile nutrients such as phosphate, mycorrhizae likely do not play an important role in taking advantage of this opportunity provided by the pulse of water.     

Effects of a 20-day-long dry period on cambial and apical meristem growth in Abies balsamea seedlings

There is general agreement that in many regions additional precipitation with climate change will not be able to balance the increased evaporation rate induced by higher air temperatures, causing periods of intense drought. Although seedlings of Abies balsamea growing in the boreal forest are known for their resistance to harsh environmental conditions, the impact of water stress on their growth still remains largely unexamined. The aim of this study was to investigate growth responses of this species during and after a dry period by monitoring cambial and apical meristem activity at short time scale. Meristem growth was studied from May to October 2005 on seedlings of A. balsamea submitted to a 20-day-long dry period in June–July. Lower rates of shoot lengthening were observed in non-irrigated seedlings only in the first part of the growing season. Irrigated and non-irrigated trees showed the same trend of xylem formation and timings of cell differentiation. Cell production during cambial activity was estimated at about one xylem cell per day thus achieving in 100 days 108 tracheids in the tree ring and a width of 2 mm, with thinner tree rings observed in non-irrigated plants. A reduction of up to 50% in lumen area and cell diameter was observed for the cells produced during the dry period. Response of A. balsamea seedlings to a 20-day-long dry period consisted of good resistance of the cambial meristems during and after water stress, high sensitivity and rapid recovery of cell sizes during water depletion and slow but effective recovery of shoot growth after treatment.     

Distribution and redistribution of extension growth along vertical and horizontal gravireacting maize roots

Horizontal primary roots of Zea mays L. were photographed during the course of their gravireaction and during a preceding growth period in the vertical orientation. The displacement, by root elongation, of marker particles on the root surface was recorded. The particle-displacement rates were used to estimate the distribution of elemental elongation rates along opposite sides of the growing root apex. In the temperature range 21–25°C there was a stimulation of local elongation rates along the upper side of a gravireacting root and a reduction (and sometimes a cessation) of elongation along the lower side. Elemental elongation rates have been related to the development of root curvature, and the magnitude of the differential growth between upper and lower sides required for a particular rate of bending has also been estimated. The results complement, and are compatible with, findings relating to the distribution of certain endogenous growth regulators believed to participate in the gravireaction.Abbreviation RELEL relative elemental rate of elongation     

Climate and species affect fine root production with long-term fertilization in acidic tussock tundra near Toolik Lake,Alaska

Long-term fertilization of acidic tussock tundra has led to changes in plant species composition, increases in aboveground production and biomass and substantial losses of soil organic carbon (SOC). Root litter is an important input to SOC pools, although little is known about fine root demography in tussock tundra. In this study, we examined the response of fine root production and live standing fine root biomass to short- and long-term fertilization, as changes in fine root demography may contribute to observed declines in SOC. Live standing fine root biomass increased with long-term fertilization, while fine root production declined, reflecting replacement of the annual fine root system of Eriophorum vaginatum, with the long-lived fine roots of Betula nana. Fine root production increased in fertilized plots during an unusually warm growing season, but remained unchanged in control plots, consistent with observations that B. nana shows a positive response to climate warming. Calculations based on a few simple assumptions suggest changes in fine root demography with long-term fertilization and species replacement could account for between 20 and 39% of the observed declines in SOC stocks.     

Clean rain promotes fine root growth and soil respiration in a Norway spruce forest

Soil acidification and N saturation are considered to affect the decomposition of soil organic matter as well as growth and mortality of fine roots in many forest soils. Here we report from a field experiment where ‘clean rain’ has been applied to the soil for about 10 years under a roofed plot of a 71‐year‐old Norway spruce plantation at Solling, Central Germany. Reduced amounts of protons (?78%), sulphate (?53%), ammonium (?86%), and nitrate (?49%) were sprayed on the soil surface of the clean rain plot between 1992 and 2001. In an adjacent roofed control plot, throughfall was collected and immediately re‐sprinkled below the roof construction without any chemical manipulation. One year before the clean rain treatment started, live and dead fine root masses (≤2 mm) were determined from undisturbed soil cores down to 40 cm mineral soil depth. Total live fine root mass was significantly lower in the clean rain plot than in the control plot. After the first sampling, the soil holes were refilled with quartz sand and repeatedly sampled in June 1992, June 1996, and October 2001. There were no differences in live and dead fine root masses between the plots in 1992 and 1996. In 2001, both live and dead fine root masses of the clean rain plot were about twice as high as in the control plot, indicating that fine root growth recovered in the mineral soil following 10 years of clean rain treatment. Moreover, the clean rain treatment significantly reduced the total N concentrations of live fine roots and 1‐year‐old needles. Our results suggest that the reduced N input promoted fine root growth to compensate N deficiency. Reduced Al concentration in soil solution may have contributed to the recovery of fine root growth, however, the toxicity of Al species is largely unknown. Mean annual soil respiration rate was 24% higher in the period from 2000 to 2001, indicating that the clean rain treatment increased respiration of roots and heterotrophic microorganisms within the rhizosphere. Laboratory incubation of samples from the organic horizon and the top mineral soil revealed no differences between the plots in the decay rate of soil organic matter. Our results suggest that strong reductions in atmospheric N deposition from about 30 to 10 kg N ha^?1 yr^?1 and decreasing acid stress can have beneficial effects on growth of fine roots in the mineral soil within a decade. We conclude that biological recovery under reduced atmospheric loads can affect the nutrient and carbon budget of spruce soils in the long run.     

Effects of mycorrhizae on growth and demography of tallgrass prairie forbs

The effects of arbuscular mycorrhizal (AM) symbiosis on ramet and genet densities, vegetative growth rates, and flowering of three forb species were studied in native tallgrass prairie in northeastern Kansas. Mycorrhizal activity was experimentally suppressed for six growing seasons on replicate plots in an annually burned and an infrequently burned watershed with the fungicide benomyl. Benomyl reduced mycorrhizal root colonization to an average of 4.2%, approximately a two-thirds reduction relative to controls (13.7% colonization). Mycorrhizae influenced the population structure of these forbs. Although mycorrhizal suppression had no long-term effect on genet densities and no effect on ramet survivorship throughout the growing season, the number of ramets per individual was significantly increased such that ramet densities of all three species were approximately doubled in response to long-term mycorrhizal suppression. Effects of mycorrhizae on ramet growth and reproduction varied among species. Ramet growth rates, biomass, and flowering of Salvia azurea were greater in plots with active mycorrhizal symbiosis, whereas mycorrhizae reduced ramet growth rates and biomass of Artemesia ludoviciana. Aster sericeus ramet growth rates and biomass were unaffected by the fungicide applications, but its flowering was reduced.The pattern of responses of these three species to mycorrhizae differed considerably between the two sites of contrasting fire regime, indicating that the interaction of fire-induced shifts in resource availability and mycorrhizal symbiosis together modulates plant responses and the intensity and patterns of interspecific competition between and among tallgrass prairie grass and forb species. Further, the results indicate that effects of mycorrhizae on community structure are a result of interspecific differences in the balance between direct positive effects of the symbiosis on host plant performance and indirect negative effects mediated through altered competitive interactions.     

Fibrous carrot root responses to irrigation and compaction of sandy and organic soils

Field experiments were performed in Southern Finland on fine sand and organic soil in 1990 and 1991 to study carrot roots. Fall ploughed land was loosened by rotary harrowing to a depth of 20 cm or compacted under moist conditions to a depth of 25–30 cm by three passes of adjacent wheel tracks with a tractor weighing 3 Mg, in April were contiguously applied across the plot before seed bed preparation. Sprinkler irrigation (30 mm) was applied to fine sand when moisture in the 0–15 cm range of soil depth was 50% of plant-available water capacity. For root sampling, polyvinyl chloride (PVC) cylinders (30 × 60 cm) were installed in the rows of experimental plots after sowing, and removed at harvest. Six carrot plants were grown in each of in these soil colums in situ in the field.Fine root length and width were quantified by image analysis. Root length density (RLD) per plant was 0.2–1.0 cm cm^-3 in the 0–30 cm range. The fibrous root system of one carrot had total root lengths of 130–150 m in loose fine sand and 180–200 m in compacted fine sand. More roots were observed in irrigated than non-irrigated soils. In the 0–50 cm range of organic soil, 230–250 m of root length were removed from loosened organic soils and 240–300 m from compacted soils. Specific root surface area (surface area divided by dry root weight) of a carrot fibrous root system averaged 1500–2000 cm² g^-1. Root length to weight ratios of 250–350 m g^-1 effectively compare with the ratios of other species.Fibrous root growth was stimulated by soil compaction or irrigation to a depth of 30 cm, in both the fine sand and organic soils, suggesting better soil water supply in compacted than in loosened soils. Soil compaction increased root diameters more in fine sand than it did in organic soil. Most of the root length in loosened soils (fine sand 90%, organic soil 80%) and compacted soils (fine sand 80%, organic soil 75%) was composed of roots with diameters of approximately 0.15 mm. With respect to dry weight, length, surface area and volume of the fibrous root system, all the measurements gave significant resposes to irrigation and soil compaction. Total root volumes in the 0–50 cm of soil were 4.3 cm³ and 9.8 cm³ in loosened fine sand and organic soils, respectively, and 6.7 cm³ and 13.4 cm³ in compacted sand and organic soils, respectively. In fine sand, irrigation increased the volume from 4.8 to 6.3 cm³.     

Effect of soil water content on the gravitropic behavior of nodal roots in maize

The direction of root growth is an important factor that determines the spatial distribution of roots in the soil. The influence of soil water content on the direction of growth of maize nodal roots was studied both in the field and in the greenhouse. In the field experiment, the one plot was regularly irrigated (I-plot) and the other non-irrigated (N-plot). In the greenhouse experiment, three water treatments were conducted on plants grown in pots: continuously wet (CW), early drying (ED), and late drying (LD). The direction of root growth was quantified by the angle from the vertical, measured at 1 cm intervals for 10 cm from the first five internodes. Nodal roots grew more vertically in the N-plot and ED treatment than those in the I-plot and CW treatment. This was due to the decrease of the initial angle and/or the liminal angle. It is therefore thought that two events regulate the growth direction of nodal roots under dry soil conditions: gravitropic bending at root emergence from the stem and the later establishment of the angle of growth. Nodal roots appearing after rewatering in the ED treatment grew in a similar direction as those in the CW treatment. It follows from this that the water content of the surrounding soil has a direct effect on the direction of growth. Nodal roots that emerged in rapidly drying soil in the LD treatment ceased growing after showing negative gravitropism. The possible mechanisms determining the growth direction of nodal roots in drier soils are discussed.     

色季拉山急尖长苞冷杉叶片及细根性状随海拔的变异特征

叶片和细根是植物对环境变化响应的主要功能器官,在气候变化趋势下,研究其随环境梯度的变化,对理解植物对环境的适应策略具有重要意义。本文是在色季拉山急尖长苞冷杉分布区,按海拔梯度(3800、3900、4000、4100、4200、4300、4400m)选择7个典型样地,在样地内对其叶片基本特征、叶绿素含量、比叶重和细根特征(0—60cm)等参数进行了测量。结果表明:(1)随海拔梯度升高,急尖长苞冷杉叶片叶面积减小、叶片厚度增加、叶绿素含量逐渐降低、比叶重显著增大。3900m处急尖长苞冷杉的叶片形态特征表现最好,叶片长度、宽度和面积均显著优于其他海拔,海拔4200m叶片厚度达到最大值,叶片面积、叶绿素含量随海拔升高呈下降趋势,但在4200m处出现第二峰值。(2)随海拔梯度增加,细根各性状与海拔表现出非线性关系,其中细根生物量和细根体积在3900m和4200m处出现两次峰值,3900m处细根生物量达到最大值,4200m处细根体积达到最大值,4400m处细根生物量和细根体积均显著少于其他海拔。细根根长密度在海拔3900、4200、4300m较高,三者之间差异不显著,但显著高于其余海拔,4400m海拔细根根长密度最小。细根表面积在3900m海拔处达到最大值,显著高于其他海拔,4200和4300m次之,3800、4000和4400m海拔下细根表面积相对较小。4400m处细根比根长达到最大值。各海拔上细根均主要分布在0—20cm土层。随土层厚度增加,各海拔细根生物量和根系体积在0—60cm土层范围内均逐渐减小;细根根长密度、表面积在20—40cm和40—60cm显著提高;同一海拔细根比根长随土层深度增加呈明显的增加趋势。各海拔40—60cm土层细根比根长显著高于20—40和0—20cm土层。(3)综合叶片及细根特征,海拔3900m为急尖长苞冷杉的最适宜生长区域;随海拔升高,环境因子逐渐恶劣,环境胁迫加剧,急尖长苞冷杉最终形成在4400m处为其分布上限的海拔梯度格局。     

Competition within the root system of rubber seedlings (Hevea brasiliensis) studied by root pruning and blockage

The effects of taproot blocking and pruning on the developmentof the early secondary roots (ESR) of rubber seedlings werestudied in root observation boxes under controlled conditions.During shoot flush, both the mean elongation rate and mean apicaldiameter of the ESR decreased regardless of treatments. Thereafter,the elongation rate of the ESR increased greatly when the taprootwas blocked, slightly for the control and scarcely for the prunedsystems in which fast growing regenerated roots developed. Thedifferences between treatments were related to the proportionof ESR which ceased growing. Following shoot arrest, the apicaldiameter of ESR increased greatly for blocked seedlings andto a lesser extent for pruned seedlings. Branching density ofthe ESR and elongation of tertiary roots were also higher forseedlings without a growing taproot. The dynamics of ESR response was not consistent with activeinhibition of their development by the growing taproot. Moreover,this response was dependent on concurrent development of shootand regenerating roots, hence competition processes were morelikely to be determining. In such an hypothesis, root elongationcan be limited by assimilate availability, but also by eachroot's maximum growth rate in non-limiting conditions, i.e.growth potential. Since the latter is related to apical diameter,a significant acceleration of elongation required a parallelincrease in apical diameter and this may explain the relativeinertia of ESR to taproot alteration. Conversely, regeneratingroots could have a high growth potential because they were initiatedin a favourable context, thus their development competed stronglywith elongation of ESR. Key words: Hevea brasiliensis, root system, development, growth potential, root diameter, competition