Structural and Functional State of Thylakoids in a Halophyte Suaeda altissima before and after Disturbance of Salt–Water Balance by Extremely High Concentrations of NaCl

Halophyte plants Suaeda altissima L. were grown in water culture at different concentrations of NaCl in the medium, and their leaves were sampled to examine the ultrastructure of chloroplasts. In parallel tests, the functional state of chloroplasts was assessed from parameters of chlorophyll fluorescence. In addition the effects of NaCl on plant growth and on the contents of Na⁺, K⁺, and water in organs of S. altissima were investigated. At a wide range of external salt concentrations (0–750 mM NaCl), S. altissima plants retained the chloroplast ultrastructure and photosynthetic function in an intact condition. The impairment of thylakoid ultrastructure and the accompanying increase in nonphotochemical quenching of excited states of chlorophyll was only observed at an extremely high concentration of NaCl in the medium (1 M) that led to disruption of ionic homeostasis and lowered water content in tissues.     

Intraspecific diversity and dominant genotypes resist plant invasions

Numerous studies have asked whether communities with many species deter invasions more so than do species-poor communities or whether dominant species deter invasion by colonizing species. However, little is known about whether high intraspecific diversity can deter biological invasions or whether particular genotypes might deter invasions. In this study, we present experimental evidence that intraspecific diversity and particular genotypes of tall goldenrod, Solidago altissima , can act as a barrier to colonization by new species. We found that biomass of colonizing species was negatively correlated with genotypic diversity, and particular genotypes affected the richness, cover, and biomass of colonizing species. Stem density of S. altissima increased with genotypic diversity and varied among genotypes, suggesting that stem density is a key mechanism in limiting colonization dynamics in this system. Our results indicate that the loss of intraspecific diversity within a dominant plant species can increase susceptibility to plant invasions.     

Ecological application of biotic resistance to control the invasion of an invasive plant,Ageratina altissima

Biotic resistance is the ability of species in a community to limit the invasion of other species. However, biotic resistance is not widely used to control invasive plants. Experimental, functional, and modeling approaches were combined to investigate the processes of invasion by Ageratina altissima (white snakeroot), a model invasive species in South Korea. We hypothesized that (1) functional group identity would be a good predictor of biotic resistance to A. altissima, whereas a species identity effect would be redundant within a functional group, and (2) mixtures of species would be more resistant to invasion than monocultures. We classified 37 species of native plants into three functional groups based on seven functional traits. The classification of functional groups was based primarily on differences in life longevity and woodiness. A competition experiment was conducted based on an additive competition design with A. altissima and monocultures or mixtures of resident plants. As an indicator of biotic resistance, we calculated a relative competition index (RCI_avg) based on the average performance of A. altissima in a competition treatment compared with that of the control where only seeds of A. altissima were sown. To further explain the effect of diversity, we tested several diversity–interaction models. In monoculture treatments, RCI_avg of resident plants was significantly different among functional groups but not within each functional group. Fast‐growing annuals (FG1) had the highest RCI_avg, suggesting priority effects (niche pre‐emption). RCI_avg of resident plants was significantly greater in a mixture than in a monoculture. According to the diversity–interaction models, species interaction patterns in mixtures were best described by interactions between functional groups, which implied niche partitioning. Functional group identity and diversity of resident plant communities were good indicators of biotic resistance to invasion by introduced A. altissima, with the underlying mechanisms likely niche pre‐emption and niche partitioning. This method has most potential in assisted restoration contexts, where there is a desire to reintroduce natives or boost their population size due to some previous level of degradation.     

高榕的3类雌花形态及其繁殖特征

榕树(Ficus spp.)及其传粉榕小蜂(Agaonidae)之间经过长期的协同进化, 已形成了高度专一的互惠共生关系。在雌雄同株的榕树中, 同一个榕果内既繁殖种子, 又繁殖榕小蜂, 雌花资源在繁殖种子和榕小蜂之间是怎样分配的?该研究选择广泛分布于西双版纳地区的高榕(F. altissima)及其传粉榕小蜂系统来回答这个问题。高榕果内的雌花柱头形态变异较大, 有3种类型的柱头——球型、弯钩型和火炬型, 它们分别占到雌花总量的54.00%、36.93%和9.07%。3种类型柱头的雌花分别具有不同长度的花柱, 球型柱头雌花的平均花柱长度最短, 火炬型柱头次之, 弯钩型柱头雌花的平均花柱长度最长。在高榕果内, 有传粉者Eupristina altissima和欺骗者Eupristina sp.两类小蜂进入榕果内繁殖, 前者的产卵器长度比96.01%以上的雌花花柱长, 后者的产卵器也要长于85.73%的雌花花柱, 从产卵器长度和雌花花柱长度的匹配情况看, 它们应该可以利用绝大多数雌花产卵繁殖后代。然而, 繁殖榕小蜂的雌花主要是短花柱的雌花, 其中60.64%是球型柱头的雌花; 而繁殖种子的主要是花柱较长的弯钩型柱头和火炬型柱头的雌花。显然, 繁殖榕小蜂和种子的雌花不仅花柱长度有差异, 柱头也分化出了不同的形态, 变异的柱头形状也是调节榕树-榕小蜂繁殖平衡的手段之一。     

高榕隐头果内寄生蜂种类及生态学特征初步观察

本研究采用定期、定点、定树采样调查和观察研究的方法,获得寄生于高榕隐头果内小蜂(Chalcidoidea)为6个科,14个属,25种。根据小蜂行为把高榕小蜂划分为2类,一类为传粉小蜂,另一类为非传粉小蜂,其中传粉小蜂有2种,主要的传粉小蜂只有Eupristina altissima 一种,另一种仅在部分季节少量出现。高榕隐头果果态变化大,直径2．5cm以上的大号果中仅寄生非传粉小蜂,这些小蜂对榕树—传粉者共生体系有着负面影响。本文初步研究了高榕隐头果中寄生的小蜂种类和生态特性,分析了部分寄生蜂对榕树—传粉者共生体系的影响。     

高榕小蜂群落结构及物种多样性的初步研究

在西双版纳地区采集干、湿两季5个样株的高榕(Ficus altissima)隐头果,单果收蜂,鉴定蜂种,共获高榕隐头果内小蜂标本18 000多号,隶属膜翅目5科9属14种。然后对样株小蜂群落结构、组成、性比及物种多样性进行对比研究。小蜂群落的结构和组成在干、湿两季有着明显的差异：湿季隐头果内的小蜂种类比干季丰富。另外,受人为干扰较少的样株隐头果内小蜂种类比受严重干扰的样株丰富。高榕的非传粉小蜂主要由造瘿者组成,寄生小蜂和寄居性小蜂仅占很少一部分;干、湿两季的小蜂群落多样性指数均很低,虽然在湿季群落内小蜂的丰富度指数较高,但不如干季小蜂分布得均匀;相同季节的小蜂群落均为中等相似,不同季节的2个榕树小蜂群落为中等不相似,有的树对达到极不相似水平;高榕隐头果内传粉小蜂的雄雌性比(♂：♀)较低,为0.14：1,而非传粉小蜂的雄雌性比相对较高,多在1：1左右。以上研究结果表明,西双版纳地区高榕隐头果内的小蜂受不同季节温湿度、人为干扰程度等因素的影响,保护并改善西双版纳地区的生态环境对丰富高榕隐头果小蜂物种多样性有重要意义。     

Experimental demography of rhizome populations of establishing clones of Solidago altissima

1 Solidago altissima forms large compact below-ground rhizome systems. The rhizome systems of 30 genets in a population that had colonized an old-field in 1984 were mapped yearly in situ from 1988 to 1992. Branching angles, lengths and survival of rhizomes, and production of daughter rhizomes, were calculated from digitized maps.
2 We hypothesized that genets of S. altissima would prevent intraclonal crowding by producing longer but fewer rhizomes as they increased in size. In addition, we tested the effects of environmental factors on rhizome dynamics by (i) mowing in late summer, (ii) cutting of rhizome connections, and (iii) removal of close neighbour plants.
3 Most rhizomes were initiated at an angle of 60°−65° relative to the parent rhizome. Subsequent growth tended towards the parent axis and towards more centrifugal growth. Rhizome angles were not influenced by any treatment.
4 In control genets, rhizome density increased 1.3-fold from March 1988 to March 1992. The number of newly produced rhizomes per genet decreased and their length increased from 1988 to 1992. Longer rhizomes had a higher survival probability than shorter rhizomes did. The number of newly produced rhizomes per parent rhizome was positively correlated with mean rhizome length per genet.
5 Mowing strongly decreased rhizome density from 1988 to 1992, mainly due to reduced production of new rhizomes.
6 Rhizome cutting led to a slight decline in the rhizome population and to shorter lengths of rhizomes in their second year compared with controls.
7 The removal of close neighbour plants led to increased rhizome densities. These rhizomes grew longer than controls in unmown plots and shorter in mown plots and had higher probabilities of surviving into their second year.
8 Yearly mowing could successfully prevent the further spread of S. altissima because it acts on rhizome and shoot dynamics as well as on seed output.     

Identification of an allelopathic compound from Ailanthus altissima (Simaroubaceae) and characterization of its herbicidal activity

Aqueous extracts of Ailanthus altissima bark and foliage were previously shown to be toxic to other plants. Using bioassay-directed fractionation, I isolated the phytotoxic compound from A. altissima root bark and identified it to be ailanthone, a quassinoid compound having molecular mass of 376. Ailanthone was highly phytotoxic, with concentrations of 0.7 ml/L causing 50% inhibition of radicle elongation in a standardized bioassay with garden cress (Lepidium sativum) seeds. Ailanthone exhibited potent pre- and postemergence herbicidal activity in greenhouse trials. Postcmergence activity was especially striking; even the lowest application rate (0.5 kg/ha) caused complete mortality of five of the seven plant species tested within 5 d of treatment. In contrast, the highest application rate (8 kg/ha) did not cause any detectable injury to A. altissima seedlings, indicating the presence of a protective mechanism in the producer species to prevent autotoxicity. Ailanthone was rapidly detoxified in field soil as a result of microbial activity. Applications of ailanthone equivalent to 0.5 and 4.0 kg/ha completely lost their phytotoxicity within ≤5 d when incubated in the presence of nonsterile soil. When incubated with sterile soil under identical conditions, however, ailanthone remained highly phytotoxic throughout the 21-d duration of the investigation. The high level of postemergence herbicidal activity in conjunction with its rapid biodegradation in soil suggest ailanthone may have potential for development as a natural-product herbicide.     

Herbicidal effects under field conditions of Ailanthus altissima bark extract,which contains ailanthone

Extracts of Ailanthus altissima stem bark were evaluated for herbicidal effects under field conditions in two outdoor trials. Previous investigations had shown A. altissima bark, extracted with methanol, yielded a strongly phytotoxic extract that contained ailanthone as one of the major herbicidal compounds. The first field trial investigated the level of activity and selectivity of the extract. A. altissima bark extract was sprayed post-emergence onto 17 species of weeds and crops at rates of 366, 177, 93, 47, 23, and 0 kg ha^–1. These application rates provided herbicidal activity equivalent to 4.5, 2.2, 1.1, 0.6, 0.3, and 0.0 kg of pure ailanthone per hectare, based on the results of a laboratory bioassay of extract and pure ailanthone. Strong herbicidal effects were observed within several days. Even the lowest rate caused mortality and injury in excess of 50% for nine of the 17 species, and a significant reduction in shoot biomass for 13 species. The second field trial tested the ability of bark extract to control weeds under field conditions with horticultural crops (bush bean, cauliflower, sweet corn, tomato). A. altissima bark extract was sprayed post-emergence at rates of 99, 50, 26, 13, and 0 kg ha^–1, providing herbicidal activity equivalent to 1.1, 0.6, 0.3, 0.14, and 0.0 kg of pure ailanthone per hectare. Extract treatment provided partial weed control (greatest reduction in weed biomass was 40%), but also caused serious crop injury. Bush bean was the only crop that showed a significant increase in shoot biomass and fruit yield, compared to the non-weeded control. None of the crops, regardless of application rate, showed a level of shoot biomass or fruit yield comparable to the hand-weeded control. The herbicidal effects of A. altissima bark extract declined within the first few weeks after application, supporting previous evidence that ailanthone is rapidly degraded under field conditions.     

Edge Effects on Species Composition and Exotic Species Abundance in the North Carolina Piedmont

Edges between forest and non-forest habitats often have significant effects on forest microclimate and resource availability, with corresponding effects on species composition and abundance. Exotic species are often increased in abundance near forest edges. This increase in abundance could be either because of the increase in resource availability near edges, or because of increased dispersal into forest edges. We measured species composition and a set of geospatial variables on transects at 66 edges in the North Carolina Piedmont in an attempt to distinguish between these two factors. Mantel tests show that species composition is significantly different in forest edges than in the forest interior, but that this effect only penetrates about 5 m into the forest. Indicator species analysis finds several species that are indicative of edge communities, including trumpet vine (Campsis radicans), two drought-tolerant oak species (Quercus stellata and Q. falcata), a serviceberry (Amelanchier arboreum), and a common exotic species, tree-of-heaven (Ailanthus altissima). Poisson regression techniques showed that in both the seedling and tree strata of the forest, exotic species increased in abundance on flat sites with a high potential seed source. Mapping predicted exotic species abundance onto the landscape. We find that large-scale variation in exotic species abundance is due mostly to variation in potential seed sources, while small-scale variation relates more to edaphic factors. Our results stress that both dispersal and environmental filters are important for determining exotic species abundance, but potentially the filters operate at different spatial scales.