Effect of arbuscular mycorrhiza on inter- and intraspecific competition of two grassland species

We were interested in the role of arbuscular mycorrhiza (AM) in the competition between plants of different sizes. A pot experiment of factorial design was established, in which AM root colonization and competition were used as treatments. Five-week-old Prunella vulgaris seedlings were chosen as target plants (i.e. plants whose response to competition was studied) and the following (13 replicates of each) were used as neighbours: (1) a large, 10-week-old P. vulgaris, (2) two P. vulgaris seedlings, and (3) a large, 10-week-old Fragaria vesca. In the experiment where small neighbours were grown together with small target plants, competition did not reduce target plant weight significantly, compared to the other two treatments. The competitive effects of large neighbours were significant, regardless of species (both older neighbours reduced the weights of target plants similarly), but there was a clear difference between intra- and interspecific competition when plants were mycorrhizal. In intraspecific competition with a large neighbour, the target plant shoot weight was reduced 24% when inoculated with AM. Thus, AM amplified rather than balanced intraspecific competition. In interspecific competition with old F. vesca, the shoot weights of target plants were 22% greater when inoculated with AM than when non-mycorrhizal. The results showed that, for given soil condition, AM might increase species diversity by increasing competitive intraspecific suppression and decreasing the interspecific suppression of small plants by larger neighbours.     

Interspecific competition enhances nitrogen fixation in an actinorhizal shrub

In forest understory restoration, the establishment of reintroduced species may be strongly linked to their ability to compete for belowground resources. In this study, we provide isotopic and morphological evidence for competition-induced increases in nitrogen fixation by Morella cerifera (L.) Small (wax myrtle) when planted with Pinus palustris Mill (longleaf pine). Compared to a competition-free treatment, we found no significant differences in tissue N concentrations for M. cerifera. However, ¹⁵N enrichment in leaves, stems and roots, as well as whole-plant values for nitrogen derived from fertilizer were significantly lower when the plants were subject to interspecific competition from P. palustris. Plants in the competition treatment also allocated a significantly greater percentage of belowground biomass to root nodules than those in the competition-free treatment (0.65 vs. 0.41%). This strongly suggests that M. cerifera is capable of upregulating nitrogen fixation in response to interspecific competition. This may help explain why M. cerifera outperformed non-nitrogen-fixing species reintroduced on the same site.     

Fine root responses to temporal nutrient heterogeneity and competition in seedlings of two tree species with different rooting strategies

There is little direct evidence for effects of soil heterogeneity and root plasticity on the competitive interactions among plants. In this study, we experimentally examined the impacts of temporal nutrient heterogeneity on root growth and interactions between two plant species with very different rooting strategies: Liquidambar styraciflua (sweet gum), which shows high root plasticity in response to soil nutrient heterogeneity, and Pinus taeda (loblolly pine), a species with less plastic roots. Seedlings of the two species were grown in sandboxes in inter‐ and intraspecific combinations. Nutrients were applied in a patch either in a stable (slow‐release) or in a variable (pulse) manner. Plant aboveground biomass, fine root mass, root allocation between nutrient patch and outside the patch, and root vertical distribution were measured. L. styraciflua grew more aboveground (40% and 27% in stable and variable nutrient treatment, respectively) and fine roots (41% and 8% in stable and variable nutrient treatment, respectively) when competing with P. taeda than when competing with a conspecific individual, but the growth of P. taeda was not changed by competition from L. styraciflua. Temporal variation in patch nutrient level had little effect on the species’ competitive interactions. The more flexible L. styraciflua changed its vertical distribution of fine roots in response to competition from P. taeda, growing more roots in deeper soil layers compared to its roots in conspecific competition, leading to niche differentiation between the species, while the fine root distribution of P. taeda remained unchanged across all treatments. Synthesis. L. styraciflua showed greater flexibility in root growth by changing its root vertical distribution and occupying space of not occupied by P. taeda. This flexibility gave L. styraciflua an advantage in interspecific competition.     

Effects of competition on root–shoot allocation in <Emphasis Type="Italic">Plantago lanceolata</Emphasis> L.: adaptive plasticity or ontogenetic drift?

We investigated how shoot and root allocation in plants responds to increasing levels of competitive stress at different levels of soil fertility. In addition, we analyzed whether different responses were due to adaptive plasticity or should be attributed to ontogenetic drift. Plantago lanceolata plants were grown during 18 weeks at five plant densities and four nutrient supply levels in pots in the greenhouse. Thereafter root and shoot biomass was measured. There were clear negative effects of increasing plant densities on plant weights revealing strong intraspecific competition. At the lower N-treatments, the proportional allocation to root mass increased with increasing competitive stress, indicating the important role of belowground competition. At the higher N-supply rate, the relationship between competitive stress and shoot to root ratio was neutral. These responses could not be attributed to ontogenetic drift, but could only be explained by assuming adaptive plasticity. It was concluded that at lower N-supplies belowground competition dominates and leads to increased allocation to roots, while at the higher N-supply competition for soil resources and light had balanced impacts on shoot and root allocation. An alternative hypothesis explaining the observed pattern is that light competition has far less pronounced impacts on root–shoot allocation than nutrient deprival.     

Species interactions at the level of fine roots in the field: influence of soil nutrient heterogeneity and plant size

Interference at the level of fine roots in the field was studied by detailed examination of fine root distribution in small soil patches. To capture roots as they occur in natural three-dimensional soil space, we used a freezing and slicing technique for microscale root mapping. The location of individual roots intersecting a sliced soil core surface was digitized and the identity of shrub and grass roots was established by a chemical technique. Soil patches were created midway between the shrub, Artemisia tridentata, and one of two tussock grasses, Pseudoroegneria spicata or Agropyron desertorum. Some soil patches were enriched with nutrients and others given only deionized water (control); in addition, patches were located between plants of different size combination (large shrubs with small tussock grasses and small shrubs with large tussock grasses). The abundance of shrub and grass roots sharing soil patches and the inter-root distances of individual fine roots were measured. Total average rooting density in patches varied among these different treatment combinations by only a factor of 2, but the proportion of shrub and grass roots in the patches varied sixfold. For the shrub, the species of grass roots sharing the patches had a pronounced influence on shrub root density; shrub roots were more abundant if the patch was shared with Pseudoroegneria roots than if shared with Agropyron roots. The relative size of plants whose roots shared the soil patches also influenced the proportion of shrub and grass roots; larger plants were able to place more roots in the patches than were the smaller plants. In the nutrient-enriched patches, these influences of grass species and size combination were amplified. At the millimeter- to centimeter-scale within patches, shrub and grass roots tended to segregate, i.e., avoid each other, based on nearest-neighbor distances. At this scale, there was no indication that the species-specific interactions were the result of resource competition, since there were no obvious patterns between the proportion of shrub and grass roots of the two species combinations with microsite nutrient concentrations. Other potential mechanisms are discussed. Interference at the fine-root level, and its species-specific character, is likely an influential component of competitive success, but one that is not easily assessed.     

The timing and degree of root proliferation in fertile-soil microsites for three cold-desert perennials

Summary Root proliferation in nutrient-rich soil patches is an important mechanism facilitating nutrient capture by plants. Although the phenomenon of root proliferation is well documented, the specific timing of this proliferation has not been investigated. We studied the timing and degree of root proliferation for three perennial species common to the Great Basin region of North America: a shrub, Artemisia tridentata, a native tussock grass, Agropyron spicatum, and an introduced tussock grass, Agropyron desertorum. One day after we applied nutrient solution to small soil patches, the mean relative growth rate of Agropyron desertorum roots in these soil patches was two to four times greater than for roots of the same plants in soil patches reated with distilled water. Most of the increased root growth came from thin, laterally branching roots within the patches. This rapid and striking root proliferation by Agropyron desertorum occurred in response to N-P-K enrichment as well as to P or N enrichment alone. A less competitive bunchgrass, Agrophyron spicatum, showed no tendency to proliferate roots in enriched soil patches during these two-week experiments. The shrub Artemisia tridentata proliferated roots within one day of initial solution injection in the N-enrichment experiment, but root proliferation of this species was more gradual and less consistent in the N-P-K and P-enrichment experiments, respectively. The ability of Agropyron desertorum to proliferate roots rapidly may partly explain both its general competitive success and its superior ability to exploit soil nutrients compared to Agropyron spicatum in Great Basin rangelands of North America.     

种间竞争对香蒲与芦苇生长的影响

为了探究种间竞争对香蒲（Typha domingensis）与芦苇（Phragmites australis）生长的影响,利用根系分隔盆栽试验研究了3种不同分隔方式条件下香蒲与芦苇的种间竞争特性,包括塑料膜分隔（根系完全分隔,无相互作用,无物质交换）、尼龙网分隔（根系部分分隔,无相互作用,有物质交换）和不分隔（根系完全相互作用,有物质交换）,分析了香蒲与芦苇根系形态和地上部生长的变化,探究香蒲与芦苇种间竞争的差异。结果发现（1）在尼龙网分隔和不分隔处理中芦苇具有明显的劣势。与塑料膜分隔处理相比,芦苇的总生物量、植株密度在尼龙网分隔和不分隔处理中分别减少了39.14%、49.41%和82.08%、79.22%,总根长、总根表面积、总根体积分别减少了40.53%、44.84%、62.52%和85.7%、82.45%、89.67%,且均具有极显著差异（P<0.01）;根系分隔方式也影响芦苇的株高、茎粗和叶片数,表现为不分隔 > 塑料膜分隔 > 尼龙网分隔。（2）与塑料膜分隔处理相比,香蒲总生物量在尼龙网分隔和不分隔中虽有增加,但差异不显著,植株密度和株高在尼龙网分隔和不分隔处理中都有增加且具有显著差异（P<0.05）,其总根长、总根表面积、总根体积在尼龙网分隔中分别增加了57.93%、26.5%、8.04%,但在不分隔处理中分别减少了11.57%、14.92%、11.39%（P<0.05）,虽然根系的相互作用对香蒲根系的生长具有促进作用,但植物种间根系相互作用越强,对两者的生长越不利。（3）在不同的分隔方式中,芦苇与香蒲间也存在明显变化。在不分隔处理中,香蒲的生物量和植株密度是芦苇的1.7倍和6.74倍,与塑料膜分隔处理相比增加了6倍,表明芦苇与香蒲根系的完全相互作用,显著削减了芦苇的繁殖生长,增加了香蒲的根系分蘖。（4）通过种间相互作用强度（RII值）分析也表明,尼龙网分隔和不分隔处理下芦苇表现为抑制作用（RII值为负值）,香蒲表现为促进作用（RII值为正值）。香蒲与芦苇互作对芦苇具有抑制作用,说明种间相互作用是能改变植物的适应性和植物群落的繁殖,同时也表明植物根系不仅在吸收土壤中的水和养分中起着关键作用,在种间关系中也起着重要作用。因此利用种间竞争控制植物生长,可以为保护生物多样性和生态系统的功能提供有效的技术支撑。     

Fast seedling root growth leads to competitive superiority of invasive plants

The competitive superiority of invasive plants plays a key role in the process of plant invasions, enabling invasive plants to overcome the resistance of local plant communities. Fast aboveground growth and high densities lead to the competitive superiority of invasive species in the competition for light. However, little is understood of the role belowground root competition may play in invasion. We conducted an experiment to test the effect of root growth on the performance of an invasive shrub Cassia alata, a naturalized, non-invasive shrub Corchorus capsularis, and a native shrub Desmodium reticulatum. We compared seedling growth of the three species and their competitive ability in situ. The roots of the C. alata seedlings grew much faster than those of C. capsularis and D. reticulatum during the entire growth period although C. alata had shorter shoots than D. reticulatum. Furthermore, C. alata showed an apparent competition advantage compared to the other two species as evidenced by less biomass reduction in intraspecific competition and higher competitive effects in interspecific competition. Our study reveals that fast seedling root growth may be important in explaining the competitive advantages of invasive plants. Future studies should pay more attention to the belowground traits of invasive plants, the trade-off between shoot and root growth, and the role of root competition in affecting the population dynamics of invasive plants and the structures of invaded communities.     

Capture and allocation of nitrogen byQuercus douglasii seedlings in competition with annual and perennial grasses

Summary The spatial overlap of woody plant root systems and that of annual or perennial grasses promotes competition for soil-derived resources. In this study we examined competition for soil nitrogen between blue oak seedlings and either the annual grassBromus mollis or the perennial grassStipa pulchra under controlled outdoor conditions. Short-term nitrogen competition was quantified by injecting¹⁵N at 30 cm depth in a plane horizontal to oak seedling roots and that of their neighbors, and calculating¹⁵N uptake rates, pool sizes and¹⁵N allocation patterns 24 h after labelling. Simultaneously, integrative nitrogen competition was quantified by examining total nitrogen capture, total nitrogen pools and total nitrogen allocation.Stipa neighbors reduced inorganic soil nitrogen content to a greater extent than didBromus plants. Blue oak seedlings responded to lower soil nitrogen content by allocating lower amounts of nitrogen per unit of biomass producing higher root length densities and reducing the nitrogen content of root tissue. In addition, blue oak seedlings growing with the perennial grass exhibited greater rates of¹⁵N uptake, on a root mass basis, compensating for higher soil nitrogen competition inStipa neighborhoods. Our findings suggest that while oak seedlings have lower rates of nitrogen capture than herbaceous neighbors, oak seedlings exhibit significant changes in nitrogen allocation and nitrogen uptake rates which may offset the competitive effect annual or perennial grasses have on soil nitrogen content.     

植物根系养分捕获塑性与根竞争

为了更有效地从土壤中获取养分, 植物根系在长期的进化与适应中产生了一系列塑性反应, 以响应自然界中广泛存在的时空异质性。同时, 植物根系的养分吸收也要面对来自种内和种间的竞争。多种因素都会影响植物根竞争的结果, 包括养分条件、养分异质性的程度、根系塑性的表达等。竞争会改变植物根系的塑性反应, 比如影响植物根系的空间分布; 植物根系塑性程度差异也会影响竞争。已有研究发现根系具有高形态塑性和高生理塑性的植物在长期竞争过程中会占据优势。由于不同物种根系塑性的差异, 固定的对待竞争的反应模式在植物根系中可能并不存在, 其响应随竞争物种以及土壤环境因素的变化而变化。此外, 随着时间变化, 根系塑性的反应及其重要性也会随之改变。植物对竞争的反应可能与竞争个体之间的亲缘关系有关, 有研究表明亲缘关系近的植物可能倾向于减小彼此之间的竞争。根竞争对植物的生存非常重要, 但目前还没有研究综合考虑植物的各种塑性在根竞争中的作用。另外根竞争对群落结构的影响尚待深入的研究。     

垂直方向磷素竞争对杉木根系生长及生物量分配的影响

针对自然环境中有效磷养分主要分布于土壤表层而容易导致植物根系激烈竞争的问题,选择同一杉木(Cunninghamia lanceolata)无性系幼苗为研究对象,采用水平方向空间狭小而垂直方向空间大的室内盆栽模拟装置,以单株种植为对照,构建双株种植的竞争处理,通过设置3个供磷水平:不供磷处理(0 mg/kg KH_2PO_4)、低磷处理(6 mg/kg KH_2PO_4)和正常供磷处理(12 mg/kg KH_2PO_4),采用破坏性试验方式收获,分别在试验的前期(50 d)、中期(100 d)和后期(150 d)测定不同处理条件下杉木幼苗根系生物量与根系形态的变化,研究邻株杉木根系在垂直方向上对有限磷素资源的竞争策略。结果表明:竞争处理和供磷水平对杉木幼苗根系长度、平均直径等形态指标的影响存在交互作用(P0.05),对杉木幼苗生物量分配、比根长等指标的影响均不存在明显的交互作用(P0.05)。竞争处理中杉木根系形态增量均明显高于非竞争处理的单株幼苗,且随着胁迫时间的增加,根系形态增量均呈现显著的上升趋势,其中在胁迫中期和后期的增量明显高于前期,且邻株竞争处理明显提高了杉木的比根长,提升了根系觅磷的能力;随着供磷水平的提高,根表面积和根体积增量大体上呈现先上升后下降的趋势。与非竞争处理相比,竞争条件下杉木地上部生物积累量差异不明显,而根系生物量、根冠比均低于非竞争处理的单株幼苗。     

黄顶菊对入侵地群落动态及植物生长生理特征的影响

为明确黄顶菊对入侵地植物群落和土著植物生理生长的影响机制,采用同质园试验对入侵和非入侵土壤的植物群落开展了整个生育期动态监测,并分析了黄顶菊入侵对狗尾草、羽叶鬼针草、灰绿藜、地肤4种土著植物生长和生理特征的影响规律。结果表明:黄顶菊入侵土壤植物群落多样性指数低于非入侵地,且有季节性差异,随生育期的推进差异逐渐减小;黄顶菊对本地植物的生长指标有显著影响(P0.05),随时间变化显著,但存在物种差异;4种植物的净光合速率(Pn)、气孔导度(Cd)、蒸腾速率(Tr)在非入侵土壤生长显著高于入侵地土壤(P0.05);而4种植物在入侵土壤生长的比叶面积(SLA)、比根长(SRL)、比根面积(SRA)显著高于本地土壤(P0.05)。综上,黄顶菊入侵抑制了本地植物的光合效率,减少了生物量的积累,导致本地植物群落的生物多样性水平降低,但表现出季节差异;不同物种对黄顶菊入侵胁迫的响应表现种间特异性,为理解入侵种对群落结构影响和实现入侵生境恢复提供了理论依据。     

Testing for allelopathic effects in plant competition: does activated carbon disrupt plant symbioses?

Activated carbon (AC) is widely used in ecological studies to elucidate the role of allelopathic substances in interspecific plant competition. However, by adsorbing chemical signalling compounds AC may also have negative effects on plants with symbiosis partners such as arbuscular mycorrhizal fungi and rhizobia. Here we test whether addition of AC has detrimental effects on the mycorrhizal root colonization of the native forb Plantago lanceolata and the exotic legume Lupinus polyphyllus, the nodulation of L. polyphyllus, and the nutrient uptake and growth of the plants growing in intra- and interspecific competition. Allelopathic effects probably occurred in the germination and seedling establishment phase when P. lanceolata suffered from the presence of L. polyphyllus. However, this negative effect of L. polyphyllus on P. lanceolata was not ameliorated by AC addition. AC negatively affected L. polyphyllus root biomass in week 4, and root and shoot biomass of P. lanceolata in week 9 of the experiment; both effects were independent of the presence and absence of the competing plant species. Mycorrhizal root colonization of both plant species was reduced in the presence of AC, although the effect tended to be stronger for L. polyphyllus. No significant effect of AC on the nodulation of L. polyphyllus was detected. P. lanceolata was the superior competitor and led to reduced biomasses of L. polyphyllus in interspecific competition. We conclude that AC can reduce the mycorrhization and performance of plants which may lead to changes in interspecific competition without the involvement of allelopathy. Contrary to former studies the AC used in our study did not enhance the nutrient availability for the plants, but reduced plant growth and mycorrhization. We suggest that the nutrient properties of the used AC are of crucial importance for the direction and the mechanisms of the effects and should always be reported.     

Effects of plant species and soil history on root morphology,arbuscular mycorrhizal colonization of roots,and biomass in four tallgrass prairie species

Although dominant C₄ grasses in tallgrass prairie are highly mycotrophic, for many non-dominant species neither extent of mycorrhizal colonization nor root morphology effects on plant–soil feedback interactions are known. In a laboratory study conducted from November 2013 to February 2014 at Governors State University (University Park, IL), we grew plants of a dominant C₄ grass (Andropogon gerardii) and three non-dominant forbs (Symphyotrichum laeve var. laeve, Symphyotrichum novae-angliae and Parthenium integrifolium) individually in soil collected in the field beneath a conspecific, collected beneath another study species, or in sterilized soil. The study addressed the following questions: (1) Is extent of mycorrhizal colonization of roots related to root structure? (2) How does soil history interact with plant root traits to influence plant–soil feedback? (3) How might plant–soil feedback patterns influence competitive interactions among study species? We found that proportion arbuscular mycorrhizal fungi (AMF) colonization decreased with increasing specific root length. Soil history had a stronger influence than plant species on total biomass of plants, with all species having highest total biomass when grown in Andropogon soil. Consequently, net, or heterospecific, feedback did not differ among pairwise species combinations, and was not different from 0. While these results suggest that no study species should have a competitive advantage in the field, Andropogon might still have an advantage through mechanisms such as competition for light. Future work in the field and including less mycotrophic species is needed to better understand AMF effects on competitive interactions.

     

Interactions between functionally diverse fungal mutualists inconsistently affect plant performance and competition

Plants form mutualistic relationship with a variety of belowground fungal species. Such a mutualistic relationship can enhance plant growth and resistance to pathogens. Yet, we know little about how interactions between functionally diverse groups of fungal mutualists affect plant performance and competition. We experimentally determined the effects of interaction between two functional groups of belowground fungi that form mutualistic relationship with plants, arbuscular mycorrhizal (AM) fungi and Trichoderma, on interspecific competition between pairs of closely related plant species from four different genera. We hypothesized that the combination of two functionally diverse belowground fungal species would allow plants and fungi to partition their symbiotic relationships and relax plant–plant competition. Our results show that: 1) the AM fungal species consistently outcompeted the Trichoderma species independent of plant combinations; 2) the fungal species generally had limited effects on competitive interactions between plants; 3) however, the combination of fungal species relaxed interspecific competition in one of the four instances of plant–plant competition, despite the general competitive superiority of AM fungi over Trichoderma. We highlight that the competitive outcome between functionally diverse fungal species may show high consistency across a broad range of host plants and their combinations. However, despite this consistent competitive hierarchy, the consequences of their interaction for plant performance and competition can strongly vary among plant communities.     

Trade-offs between arbuscular mycorrhizal fungal competitive ability and host growth promotion in Plantago lanceolata

In this study we tested for trade-offs between the benefit arbuscular mycorrhizal (AM) fungi provide for hosts and their competitive ability in host roots, and whether this potential trade-off shifts in the presence of a plant stress (herbivory). We used three species of AM fungi previously determined to vary in host growth promotion and spore production in association with host plants. We found that these AM fungal species competed for root space, and the best competitor, Scutellospora calospora, was the worst mutualist. In addition, the worst competitor, Glomus white, was the best mutualist. Competition proved to have stronger effects on fungal infection patterns than herbivory, and competitive dominance was not altered by herbivory. We found a similar pattern in a previous test of competition among AM fungi, and we discuss the implications of these results for the persistence of the mutualism and feedbacks between AM fungi and their plant hosts.     

不同水分条件下混种密度对芦苇和反枝苋种间竞争的影响

与本地植物的种间竞争是影响外来植物能否成功入侵的关键因素之一,该研究通过受控模拟试验研究了本地植物芦苇(Phragmites australis)和外来入侵植物反枝苋(Amaranthus retroflexus)在淹水和干旱两种水分条件下混种密度(6∶2、4∶4和2∶6)对其种间竞争的影响。结果表明:(1)芦苇和反枝苋的相对产量与相对产量总和均小于1,即两种植物存在种间竞争。(2)种间竞争使芦苇和反枝苋的生长均受到了不同程度的抑制,表现在两者的株高和生物量均随着竞争者密度的增加而降低。(3)植株地上部分和地下部分的氮浓度表现出与株高和生物量相同的趋势,且在不同水分条件下存在差异。(4)芦苇和反枝苋分别在淹水和干旱环境下具有较强竞争力,但在各自较高混种密度下亦具有较强竞争力。可见,芦苇和反枝苋的种间竞争受到了水分和混种密度的影响。因此,在有反枝苋分布的湿地中,植物生长初期可通过增加土壤水分和/或增加芦苇等本地植物的种群密度以降低反枝苋的种群密度来限制其竞争能力,防止反枝苋在湿地中生长建群和扩散入侵。     

Using coloured roots to study root interaction and competition in intercropped legumes and non-legumes

Aims Root interactions between neighbour plants represent a fundamental aspect of the competitive dynamics in pure stand and mixed cropping systems. The comprehension of such phenomena places big methodological challenges, and still needs clarification. The objectives of this work were (i) to test if a species with coloured roots can be used to examine the interaction in a legume-non-legume intercropping system; (ii) to verify the importance of initial root growth on the successive root development of mixture component plants; (iii) to test if the root interaction in the shallow layers has consequences for deep root growth and (iv) to compare the effect of intraspecific and interspecific competition on root development and biomass growth.Methods A detailed study on root growth and interaction was carried out using rhizotron tubes where two legume species were grown in pure stands or were intercropped with red beet, a variety of Beta vulgaris L. with clear red roots. Within the rhizotrons, the three species were grown either without competitors, with two plants of the same species to measure intraspecific competition or with one legume and one red beet plant to study interspecific competition. The use of mixtures where one component has clearly coloured roots, together with several scalar measurements of root depth and proliferation, allowed the measurement of the root system of each species when grown in the mixtures.Important findings The use of rhizotron tubes coupled with species with coloured roots represented a valuable method to study the belowground interaction in mixed cropping systems. The initial root growth was a very important feature for the subsequent dominance of a species and it was not related to seed dimension. Initial root growth was also important because the root interactions in the shallower soil layers were found to influence the root growth in deeper soil. The root system of the red beet showed much faster and deeper growth than that of the legumes, and made red beet the dominant component in the mixtures while the legume root system was confined to the shallower soil layer. Intraspecific competition was well tolerated by the legumes, but it was limiting for the highly competitive red beet. The outcome of root interaction between neighbour plants was confirmed to be species-specific as it changed according to the intensity of the competitive effect/response of each species of the mixture: both legumes were slightly affected by the intraspecific and highly affected by interspecific competition while red beet was more affected by intraspecific competition but strongly dominant when intercropped with legumes.     

核桃-小麦复合系统中细根生长动态及竞争策略

以核桃(Juglans regia)-小麦(Triticum aestivum)间作复合系统为研究对象,用微根窗和根钻相结合的方法采样,研究复合系统中核桃和小麦细根年内年际的生长动态和竞争适应策略,为农林复合系统的经营管理和竞争模型的建立提供理论依据和技术支持。结果表明,间作核桃和小麦根系均在上半年有一个大的生长高峰(5月和4月),在下半年有一个小的生长高峰(9月和11月),二者的竞争主要发生在上半年的大生长高峰期。在各年份各土层,间作核桃的根长密度均低于单作核桃,且在从第7年开始存在显著差异。在0—20 cm土层间作小麦根长密度在第3—7年间获得迅速提高,从第7年开始显著高于单作小麦,但在20 cm以下土层则相反。间作使核桃和小麦细根生态位实现了分离,11年的观察期内间作核桃比单作核桃细根的垂直分布中心下移了6.59 cm,间作小麦比单作小麦的上移了8.59 cm。在根系竞争策略方面,小麦根系是通过短期内的快速生长,迅速占据土壤空间获得竞争优势;而核桃根系是通过根系的逐年积累,逐步占据土壤空间从而获得竞争优势。可以干扰核桃根系积累过程的"竞争-干扰-再平衡"农林复合经营管理策略可以让复合系统中核桃和小麦保持各自竞争优势的情况下实现共存。在根系形态方面,自身细根直径较小者小麦在剧烈竞争区域以增加细根直径减小比根长来适应竞争,而自身细根直径较大者核桃则相反。     

Effects of hydrological regimes on competitive interactions of Schoenoplectus fluviatilis and two co-occurring wetland plants

We investigated the effects of different hydrological regimes (wet [flooded at a constant water depth for duration of the study], cycle [reflooded at eight weeks following natural drying], and wet–dry [initially flooded and allowed to naturally dry for duration of the study]) on the competitive ability of Schoenoplectus fluviatilis (Torr.) M. T. Strong with an annual, native wetland plant (Polygonum pensylvanicum L.) and a perennial, wetland plant (Schoenoplectus tabernaemontani [C. C. Gmel.] Palla). To assess competitive response of the plants, we used a greenhouse target-neighbor study with neighbor plants planted at varying densities (0 [control], 1, 10, and 15 plants pot⁻¹). Our results suggest that S. fluviatilis is competitively superior to S. tabernaemontani and P. pensylvanicum. S. tabernaemontani and P. pensylvanicum biomass declined by 90% and 75% in presence of S. fluviatilis, respectively. However, the competitive ability of S. fluviatilis was generally not enhanced by flooding regime. The competitive coefficients of S. fluviatilis were similar among the three hydrological regimes under intraspecific competition and interspecific competition with S. tabernaemontani, but for interspecific competition with P. pensylvanicum, the competitive coefficient for S. fluviatilis was higher for the cycle treatment compared to the wet–dry and dry treatments. Interestingly, S. tabernaemontani was a strong competitor against S. fluviatilis in the wet and cycle treatments, indicating that maintaining longer hydroperiods could be used as a management tool to encourage growth of S. tabernaemontani and reduce encroachment of S. fluviatilis.