Atmospheric CO2 and O3 alter competition for soil nitrogen in developing forests

Plant growth responses to rising atmospheric CO₂ and O₃ vary among genotypes and between species, which could plausibly influence the strength of competitive interactions for soil N. Ascribable to the size‐symmetric nature of belowground competition, we reasoned that differential growth responses to CO₂ and O₃ should shift as juvenile individuals mature, thereby altering competitive hierarchies and forest composition. In a 12‐year‐long forest FACE experiment, we used tracer ¹⁵N and whole‐plant N content to assess belowground competitive interactions among five Populus tremuloides genotypes, between a single P. tremuloides genotype and Betula papryrifera, as well as between the same single P. tremuloides genotype and Acer saccharum. Under elevated CO₂, the amount of soil N and ¹⁵N obtained by the P. tremuloides genotype common to each community was contingent on the nature of belowground competition. When this genotype competed with its congeners, it obtained equivalent amounts of soil N and tracer ¹⁵N under ambient and elevated CO₂; however, its acquisition of soil N under elevated CO₂ increased by a significant margin when grown in competition with B. papyrifera (+30%) and A. saccharum (+60%). In contrast, elevated O₃ had no effect on soil N and ¹⁵N acquisition by the P. tremuloides genotype common in each community, regardless of competitive interactions. Under elevated CO₂, the rank order of N acquisition among P. tremuloides genotypes shifted over time, indicating that growth responses to CO₂ change during ontogeny; this was not the case under elevated O₃. In the aspen‐birch community, the competitive advantage elevated CO₂ initially conveyed on birch diminished over time, whereas maple was a poor competitor for soil N in all regards. The extent to which elevated CO₂ and O₃ will shape the genetic structure and composition of future forests is, in part, contingent on the time‐dependent effects of belowground competition on plant growth response.     

Improving grasslands: the influence of soil moisture and nitrogen fertilization on the establishment of seedlings

1. In order to investigate the factors influencing the establishment of seedlings in permanent grassland, the influence of soil moisture and nitrogen fertilization on competition between established plants of Lolium perenne and seedlings of Phleum pratense or Trifolium pratense was studied in two experiments under greenhouse conditions using the 'split-box'-technique.
2. There was no difference in the production of plant dry matter of P. pratense or T. pratense between 30% volumetric soil water content (−0·005 MPa) and 22% (−0·04 MPa), but 15% soil moisture (−0·33 MPa) reduced plant growth. L. perenne yields were linearly reduced by reduced soil moisture content.
3. Shoot competition from L. perenne reduced the plant dry matter yield of P. pratense and T. pratense more than did root competition in these experiments. When shoot competition was present, differences between moisture contents were not detected, indicating that light was probably the limiting resource under such conditions. No significant interaction between root competition and soil moisture was observed for plant weight.
4. Root competition was not prevented even though sufficient water and nitrogen were supplied. This indicated either that some other growth factor was limiting or the plants competed for resources at the root hair level even though sufficient resources were supplied at the pot or field scale. Therefore, in the situation of direct drilling of species during grassland renovation, it may be difficult to alleviate competition by adequate provision of water and nitrogen.     

Effect of neighbour presence and soil volume on the growth of Andropogon gerardii Vitman

Background: Theory predicts that plants can reduce their fitness in the presence of neighbours by allocating resources to root growth, in order to pre-empt resource capture. A number of studies that have tested this idea have done so by using experiments where neighbour presence is confounded with soil volume.

Aims : To avoid confounding effects of neighbour presence and soil volume we adjusted these variables independently from one another.

Methods: We grew Andropogon gerardii with and without neighbours, holding soil volume available to each plant constant, and compared plant performance with a treatment where both neighbour presence and soil volume were varied. We also grew plants with a quarter of the soil volume but four times the nutrient concentration to determine if changes in plant growth in response to soil volume are caused by access different levels of soil resources.

Results: We found no evidence that plants adjust root growth to the presence of neighbour roots alone. We did, however, find a significant reduction in plant growth when soil volume was reduced. The reduction was overcome by increasing nutrient concentrations in the growth media.

Conclusions: Our results suggest the effects of soil volume on plant growth are mainly due to changes in nutrient availability.     

Bacteriocins and the assembly of natural Pseudomonas fluorescens populations

When competing for space and resources, bacteria produce toxins known as bacteriocins to gain an advantage over competitors. Recent studies in the laboratory have confirmed theoretical predictions that bacteriocin production can determine coexistence, by eradicating sensitive competitors or driving the emergence of resistant genotypes. However, there is currently limited evidence that bacteriocin‐mediated competition influences the coexistence and distribution of genotypes in natural environments, and what factors drive interactions towards inhibition remain unclear. Using natural soil populations of Pseudomonas fluorescens, we assessed the ability of the isolates to inhibit one another with respect to spatial proximity in the field, genetic similarity and niche overlap. The majority of isolates were found to produce bacteriocins; however, widespread resistance between coexisting isolates meant relatively few interactions resulted in inhibition. When inhibition did occur, it occurred more frequently between ecologically similar isolates. However, in contrast with results from other natural populations, we found no relationship between the frequency of inhibition and the genetic similarity of competitors. Our results suggest that bacteriocin production plays an important role in mediating competition over resources in natural settings and, by selecting for isolates resistant to local bacteriocin production, can influence the assembly of natural populations of P. fluorescens.     

Maternal provisioning is structured by species’ competitive neighborhoods

Differential maternal provisioning of offspring in response to environmental conditions has been argued as ‘the missing link’ in plant life histories. Although empirical evidence suggests that maternal provisioning responses to abiotic conditions are common, there is little understanding of how differences in maternal provisioning manifest in response to competition. Frequency manipulations are commonly employed in ecological studies to assess the strength of interspecific competition, relative to intraspecific competition, and we used frequency manipulations to test how competition in two soil moisture environments affects maternal provisioning of seed mass. Specifically, for 15 pairs comprised from 25 annual plant species that occur in California, we varied the relative frequencies of conspecific to heterospecific competitors from 90% (intraspecific competition) to 10% (interspecific competition). We found that conspecific frequency affected maternal provisioning (seed mass) in 12 of the 25 species (eight significantly (p < 0.05), four marginally significantly (p < 0.07)), and that these responses included both increased (five species) and decreased (six species) seed mass, as well as one species with opposing directions of response to conspecific frequency that depended on the soil moisture environment. Conspecific frequency also affected per capita fecundity (seed number) for 17 of the 25 species (15 significantly (p < 0.05), two marginally significantly (p < 0.09)), which generally decreased seed number as conspecific frequency increased. The direction and magnitude of frequency‐dependent seed mass depended on the identity of the competitor, even among species whose fecundity was not affected by competitor identity; the latter finding reveals competitive differences among species that would otherwise appear to be competitively equivalent. Our research demonstrates how species responses to different competitive environments manifest through maternal provisioning, and that these responses alter previous estimates of environmentally‐determined maternal provisioning and reproductive output; future study is needed to understand their combined effects on population and community dynamics.     

紫茎泽兰与非洲狗尾草单、混种群落土壤酶活性和土壤养分的比较

 植物对土壤有效养分的影响是植物竞争取胜的重要生态策略之一, 土壤酶活性对土壤有效养分的变化具有重要作用。该文研究了紫茎 泽兰(Ageratina adenophora)单种(A)、非洲狗尾草(Setaria sphacelata)单种(S)和两物种混种(A+S)群落4种重要的土壤酶活性和土壤养分的 变化及其规律, 并对土壤酶活性与土壤养分进行了相关分析。结果表明: 1)群落S土壤有效氮(NH₄⁺-N和NO₃^--N)含量高于群落A对应养分含量, 而与群落A＋S该养分含量没有显著差异; 群落S土壤有效磷和有效钾含量低于群落A对应指标, 而群落A＋S其含量最低。2)群落S、A+S和A土壤蛋 白酶和脲酶活性的高低次序分别为S>A＋S>A、A＋S>S>A; 群落A+S、S和A磷酸酶活性依次升高; 群落A+S、A和S蔗糖酶活性依次降低。3)在生长 过程中, 3类群落土壤NO₃^--N、有效磷和有效钾含量在生长初期(5月)最高, 随后逐渐降低; 土壤NH₄⁺-N含量呈现单峰趋势, 在生长后期(9月)最 高。3类群落土壤蛋白酶和蔗糖酶活性随生长时间推移而升高, 在生长末期(11月)最高; 土壤磷酸酶和脲酶酶活性在生长过程中呈现单峰趋势, 在生长旺盛期(7月)最高。3类群落间土壤酶活性与土壤肥力具有较一致的相关性。由此推断, 非洲狗尾草对土壤含氮化合物的活化能力比紫茎 泽兰强, 且在种间竞争中能够强烈抑制紫茎泽兰对土壤含磷、含钾化合物的活化能力是其竞争取胜的可能原因; 不同植物群落土壤酶活性的差 异是引起土壤有效养分变化的重要驱动机制之一。     

耕作措施对冬小麦田杂草生物多样性及产量的影响

在连续5a秸秆全量还田的免耕、旋耕、耙耕、深松和常规耕作试验地中,设置了除草和不除草处理,研究了其对杂草总密度、优势杂草种类、生物多样性指数和冬小麦产量的影响,并分析了杂草与小麦间的竞争关系。结果表明,麦田杂草主要有7种,分别是麦蒿(Descuminia sophia (L.) Webb ex Prantl)、荠菜(Capsella bursa-pastoris (L.) Medik)、燕麦(Avena sativa L.)、田旋花(Convolvulus arvensis L.)、刺儿菜(Cirsium setosum (Willd.) Bieb.)、繁缕(Stellaria media (L.) Vill.)、麦家公(Lithospermum arvense L.)。在未除草条件下,免耕、深松的杂草总密度显著提高;而在除草条件下,杂草密度显著下降。免耕、深松、常规耕作在未除草条件下,优势杂草种类为麦蒿、荠菜,旋耕、耙耕条件下的优势杂草为麦蒿;而除草后各处理的优势杂草均只有麦蒿。耙耕、常规耕作措施在未除草条件下杂草群落具有较高的物种丰富度和均匀度。无论哪种耕作措施,除草能提高冬小麦产量,其中以深松耕作结合除草处理的小麦产量最高。在小麦抽穗期,未除草处理杂草株高接近或高于小麦株高,会造成杂草与小麦间的光竞争,对小麦的生长状况有显著影响,从而导致小麦产量降低。     

疏叶骆驼刺根系对土壤异质性和种间竞争的响应

近年来, 植物根系对土壤异质性的响应和植物根系之间的相互作用一直是研究的热点。过去的研究主要是针对一年生短命植物进行的, 而且多是在人工控制的温室条件下进行的。而对于多年生植物根系对养分异质性和竞争的综合作用研究很少。该文对塔里木盆地南缘多年生植物疏叶骆驼刺(Alhagi sparsifolia)根系生长对养分异质性和竞争条件的响应途径与适应策略进行了研究, 结果表明: (1)在无竞争的条件下, 疏叶骆驼刺根系优先向空间大的地方生长, 即使另一侧有养分斑块存在, 其根系也向着空间大的一侧生长; (2)在有竞争的条件下, 疏叶骆驼刺根系生长依然是优先占领空间大的一侧, 但是竞争者的存在抑制了疏叶骆驼刺的生长, 导致其枝叶生物量和根系生物量都明显减少(p < 0.01), 而养分斑块的存在促进了疏叶骆驼刺根系的生长; (3)疏叶骆驼刺根系的生长不仅需要养分, 也需要足够的空间, 空间比养分更重要; (4)有竞争者存在的时候, 两株植物的根系都先长向靠近竞争者一侧的空间, 即先占据“共有空间”。研究结果对理解植物根系觅食行为和植物对环境的适应策略有重要意义。     

Effects of Dominant Plant Species on Soils during Succession in Nutrient-poor Ecosystems

During the initial phases of succession on nutrient-poor, mineral substrates dead plant material accumulates rapidly in the soil. This accumulation of soil organic matter can result in a more than 10-fold increase in nitrogen mineralization within a few decades. These changes in soil features have important consequences for plant growth and the competition between plant species. During succession in heathlands an increase in nutrient mineralization leads to species with low maximum growth rates and low biomass loss rates being replaced by species with high potential growth rates and high biomass losses. The plant properties responsible for reduced biomass loss rates appear to result in the litter produced being poorly decomposable, whereas the litter from plants with high potential growth rates decomposes more easily. Model simulations suggest that such combinations of plant features greatly influence the increase in mineralization and the change in plant species composition during ecosystem development. Studies in the field and garden plot experiments confirmed this hypothesis.     

Effects of soil phosphorus on pollen production,pollen size,pollen phosphorus content,and the ability to sire seeds in Cucurbita pepo (Cucurbitaceae)

To determine the effects of soil phosphorus on pollen production, pollen grain size, phosphate concentration per pollen grain, and the siring ability of pollen, two cultivars of the common zucchini (Cucurbita pepo) were grown under two soil phosphorus conditions in an experimental garden. Overall, soil phosphorus availability had a significant effect on reproductive output through the female function and on traits affecting the male function of plants (staminate flower production, pollen production per flower, and pollen grain size). In addition, pollen produced by plants in the high phosphorus soils had a higher phosphate concentration than pollen produced by plants in the low phosphorus soils. A pollen mixture experiment revealed that pollen produced by plants in the high phosphorus treatment sired significantly more seeds than pollen produced by plants in the low phosphorus treatment. This study showed that growing conditions such as soil phosphorus can influence the size of a pollen grain and its chemical composition, which, in turn, can affect its ability to sire mature seeds.     

The effect of heat and smoke on the soil seed banks of heathlands on permanent freshwater swamps

In South Australia, Swamps of the Fleurieu Peninsula are critically endangered due to past vegetation clearance and changes in hydrology, but still contain a high diversity of threatened plant species. This vegetation community provides habitat for 82 threatened ground-stratum plant species, including the nationally critically endangered subshrub Hibbertia tenuis which is endemic to these swamps. With infrequent burning, native ferns and taller shrubs outcompete these species. We conducted glasshouse trials to determine the potential of fire to regenerate threatened and other swamp plant species. Soil samples from eight swamps were used in germination trials with half of each sample treated with heat plus smoke, and half left untreated. Samples were spread onto trays (n = 188), and seedling emergence was recorded for twelve months. Emergence was dominated by native species (2649 seedlings m⁻²) compared to exotics (675 seedlings m⁻²). In total, 48 native and 25 introduced species germinated, with 21% of germinated native species absent from the above-ground vegetation. The dominant native fern Gleichenia microphylla did not emerge from any soil samples, indicating that recovery likely lags behind that of ground-strata species, giving them time to establish and set seed before being outcompeted. Thirty-four rare or threatened species germinated, including five species absent from above-ground vegetation. Of all native species that emerged, only five were confined to heat plus smoke treatments, suggesting most species will regenerate without fire if overstorey competition is reduced. However, seedling emergence of native shrubs/subshrubs more than doubled with the fire treatment. Of particular importance, H. tenuis showed an 18-fold increase in germination when treated with heat plus smoke. This study supports the utility of ecological burns as a management tool to regenerate threatened plant species in long-undisturbed peaty heathlands on permanent swamps.     

低丘红壤复合农林系统光能竞争与生产力——以花生南酸枣间作为例

复合农林业 ,尤其是农林间作被设想为防治土壤退化 ,增进土壤肥力 ,提高农作物产量的策略[16～ 18] 。如何利用复合农林技术防治红壤退化 ,提高红壤生产力已成为红壤地区的研究热点[1～ 8] 。然而 ,生产者最关心的往往是农林间作中农作物的产量 ,在以往的农林间作技术研究中 ,有增产的报道[13] ,而更多的研究表明复合农林种植会造成农作物减产[3 ,4 ,6,10 ,12 ,15,19,2 0 ] 。农作物减产的原因可能是树木和农作物之间对光能、水分和养分等资源竞争作用所引起 ,更可能是其综合作用的结果[14 ] 。本文以前人推荐的花生 (Ａｒａｃｈｉｓｈｙｐ…     

Growth of seedlings of the invasives,Acacia saligna and Acacia cyclops in relation to soil phosphorus

Abstract Acacia saligna and Acacia cyclops are the dominant invasive alien plants of phosphorus-poor, sand-plain, lowland fynbos and the relatively phosphorus-rich strandveld vegetation of the southwestern Cape of South Africa, respectively, but their ranges overlap. Seedlings of the two species were grown in pots, in isolation and mixed, in up to seven treatments supplying a broad gradient in phosphorus (P) availability. Acacia saligna seedlings grew taller and had greater dry mass than those of A. cyclops at each level of P, but both peaked in response to the same relatively high soil P level and then tended to decline. Root : shoot ratios did not differ in response to P, but were greater in A. saligna. In mixture, A. saligna had a higher dry mass than A. cyclops at each level of P, but the relative differences between species were no greater than in isolation. Depth penetration of the soil by the tap root of A. saligna seedlings was over three times as rapid as that of A. cyclops over a 30 day period. The higher absolute growth rates of A. saligna were not related to seed size or seed nitrogen and P contents, as these were greater in A. cyclops. The contrasting distributions of the acacias do not appear to be a response to P availability per se, but possibly to the interaction of P with other factors such as moisture availability.     

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.     

南京市城区景观特征尺度动态

利用南京市1988、1998和2006年Landsat TM遥感影像,采用小波分析技术,对1988—2006年南京市中心城区的景观结构特征进行了研究.结果表明：应用小波分析技术分析景观样带数据时,取样宽度过小易导致检测结果的不稳定,而样带过宽则易导致较小尺度特征的丢失,应采用合适的样带宽度;基于TM影像（30 m空间分辨率）的南京市城区样带宽度以4个像元为最佳.1988、1998和2006年,南京市中心城区都存在4个相对稳定的特征尺度域,分别对应于功能小区、小型街区、中型街区和大型街区;研究期间,大型街区的特征尺度几乎没有变化,而其它3个尺度域的变化相对较大,但各尺度域的变化趋势一致;各尺度域内特征尺度之间的差异性均逐渐变小,表明期间中心城区的结构特征越来越稳定,不同部位的空间结构趋同,且各尺度域的变化有较强的同步性.     

植物地上竞争与地下竞争研究进展

植物地上光竞争和地下水分与营养的竞争过程十分复杂,并且与竞争机制密切相关。从地上和地下竞争理论、两种竞争过程的相互作用机制以及分离地上和地下竞争的研究方法和表达指标等几个方面综述和分析目前的研究状况,以期为国内研究者在本领域进一步展开实验提供理论依据和实验设计参考。总结了与地上、地下相对竞争强度有关的优化分配理论,地上、地下竞争随生产力梯度的变化,资源异质性对地上、地下竞争机制的影响。介绍了地上竞争和地下竞争相互作用类型以及目前提出的两种作用机制。对常用的3种分离地上、地下竞争的方法:盆分隔、间植分隔和目标植分隔法以及表达竞争强度和竞争重要性的指标和公式进行了归纳。提出未来的研究内容,认为在开展此类研究时,应考虑到实验植物的生理学特性、发育状况、立地生产力跨度范围以及竞争持续时间等方面因素,并认为发展地下竞争研究、深入探讨根系间相互作用的影响因子和过程是当前的研究热点。     

Significance of ecology in the development of biocontrol agents against soil‐borne plant pathogens

Approaches to the development of inoculant biocontrol agents (BCAs) of soil‐borne pathogens are discussed. Based on an analysis of the success of Peniophora (Phlebia) gigantea and Agrobacterium radiobacter it is argued that most subsequent attempts to develop inoculant BCAs have failed because the organisms were selected for in vitro antagonism but were ecologically unsuited to the environments where pathogens grow. The reported modes of action of BCAs are reviewed and in some cases reinterpreted. It is argued that antibiosis and some types of mycoparasitism have not been shown to be direct mechanisms of biocontrol in vivo; they might, instead, facilitate competition for substrates or sites as the primary mechanism of control. The ecology of BCAs in general is reviewed, with emphasis on interactions in microsites and the design of appropriate screening strategies. Specific examples are used to illustrate these points.     

‘Tales of Symphonia’: extinction dynamics in response to past climate change in Madagascan rainforests

Madagascar''s rainforests are among the most biodiverse in the world. Understanding the population dynamics of important species within these forests in response to past climatic variability provides valuable insight into current and future species composition. Here, we use a population-level approach to analyse palaeoecological records over the last 5300 years to understand how populations of Symphonia cf. verrucosa became locally extinct in some rainforest fragments along the southeast coast of Madagascar in response to rapid climate change, yet persisted in others. Our results indicate that regional (climate) variability contributed to synchronous decline of S. cf. verrucosa populations in these forests. Superimposed on regional fluctuations were local processes that could have contributed or mitigated extinction. Specifically, in the forest with low soil nutrients, population model predictions indicated that there was coexistence between S. cf. verrucosa and Erica spp., but in the nutrient-rich forest, interspecific effects between Symphonia and Erica spp. may have pushed Symphonia to extinction at the peak of climatic change. We also demonstrate that Symphonia is a good indicator of a threshold event, exhibiting erratic fluctuations prior to and long after the critical climatic point has passed.     

Nest density and distribution of subterranean ants in an evergreen broadleaf forest in Japan with special reference to Amblyopone silvestrii

The density and distributions of nests of the subterranean ants were studied by excavation of thirty 1‐m² quadrats in an evergreen broadleaf forest in central Japan. A total of 20 ant species were collected and the densities of Amblyopone silvestrii and Paratrechina flavipes were highest (both 0.83 nests/m²). The distribution patterns were analyzed by the Standardized Morisita Index (I_p) for the five species that had mean densities of more than 0.5 nests/m². Of them, A. silvestrii and Hypoponera sauteri showed random distributions, while nests of Paratrechina flavipes were significantly overdispersed and those of Carebara yamatonis and Myrmecina nipponica were weakly clumped. These distributional patterns were discussed in connection with the ecology of the ants. In particular, the social organization of A. silvestrii was investigated based on the distributional pattern and the composition of collected colonies.