清除优势草本植物对西双版纳飞机草-马唐群落中木本幼苗的影响

在农业弃耕地上通常会出现由草本植物占优势的演替早期群落。为研究优势草本植物对群落中木本幼苗及群落微环境的影响以及外来入侵植物和本地植物对木本幼苗的影响有何差异,在西双版纳地区选取由外来入侵植物飞机草和本地植物马唐共同占优势的演替早期群落,实施物种清除实验,观测样地中木本幼苗的高增长及死亡补充情况,同时对群落微环境进行观测。结果显示,清除优势种显著提高了木本幼苗的高增长,降低了其死亡率,并且使新增幼苗数量有所提高。清除优势种对0-50cm幼苗高增长和死亡率的影响均大于50-100(或200)cm幼苗。清除处理显著提高了样地的冠层下可见天空比例,但对土壤含水量和土壤养分的影响并不显著。飞机草和马唐均能形成浓密的冠层,通过对光的竞争抑制木本幼苗的生长,且这两者的抑制作用没有显著差异。该地区次生林中常见的先锋树种在研究样地内均有幼苗存在,但其生长却受到优势草本植物的强烈抑制。因此,对演替早期群落中草本植物的控制与管理应同时注意外来入侵物种与本地杂草。     

Endophyte species influence the biomass production of the native grass Achnatherum sibiricum (L.) Keng under high nitrogen availability

Research on the interaction of endophytes and native grasses normally takes infection status into account, but less often considers the species of endophyte involved in the interaction. Here, we examined the effect of endophyte infection, endophyte species, nitrogen availability, and plant maternal genotype on the performance of a wild grass, Achnatherum sibiricum. Six different Epichloë‐infected maternal lines of A. sibiricum were used in the study; three lines harbored Epichloë gansuensis (Eg), while three lines harbored Epichloë sibirica (Es). These endophytes are vertically transmitted, while Eg also occasionally produces stromata on host tillers. We experimentally removed the endophyte from some ramets of the six lines, with the infected (E+) and uninfected (E?) plants grown under varying levels of nitrogen availability. Eg hosts produced more aboveground biomass than Es hosts only under high nitrogen supply. Endophyte species did not show any influence on the maximum net photosynthetic rate (P_max), photosynthetic nitrogen use efficiency, or total phenolics of A. sibiricum under all nitrogen conditions. However, the plant maternal genotype did influence the P_max and shoot biomass of A. sibiricum. Our results show that endophyte species influenced the shoot biomass of A. sibiricum, and this effect was dependent on nitrogen supply. As with most coevolutionary interactions, A. sibiricum that harbored Eg and Es may show pronounced geographic variation in natural habitats with increased nitrogen deposition. In addition, stroma‐bearing endophyte (Eg) provides positive effects (e.g., higher biomass production) to A. sibiricum plants during the vegetative growth stage.     

Using consecutive prescribed fires to reduce shrub encroachment in grassland by increasing shrub mortality

Woody plant encroachment into open grasslands occurs worldwide and causes multiple ecological and management impacts. Prescribed fire could be used to conserve grassland habitat but often has limited efficacy because many woody plants resprout after fire and rapidly reestablish abundance. If fire‐induced mortality could be increased, prescribed fire would be a more effective management tool. In California's central coast, shrub encroachment, especially of Baccharis pilularis (coyote brush), is converting coastal prairie into shrub‐dominated communities, with a consequent loss of native herbaceous species and open grassland habitat. B. pilularis has not been successfully controlled with single prescribed fire events because the shrub resprouts and reestablishes cover within a few years. We investigated whether two consecutive annual burns would control B. pilularis by killing resprouting shrubs, without reducing native herbaceous species or encouraging invasive plants. As expected, resprouting did occur; however, 2 years after the second burn, B. pilularis cover on burned plots was only 41% of the cover on unburned plots. Mortality of B. pilularis more than doubled following the second burn, likely maintaining a reduction in B. pilularis cover for longer than a single burn would have. Three native coastal prairie perennial grasses did not appear to be adversely affected by the two burns, nor did the burns result in increased cover of invasive species. Managers wanting to restore coastal prairie following B. pilularis encroachment should consider two consecutive annual burns, especially if moderate fire intensity is achievable.     

The role of regional and local scale predictors for plant species richness in Mediterranean forests

Abstract

Both local and regional predictors play a role in determining plant community structure and composition. Climate, soil features as well as different local history and management affect forest understorey and tree species composition, but to date their specific role is relatively unknown. Few studies have addressed the importance of these predictors, especially in the Mediterranean area, where environmental conditions and human impacts have generated heterogeneous forest communities. In this study, the relationships between environmental variables and species richness of different groups of vascular plants (vascular species, woody species and open habitat species) and bryophytes were investigated in Tuscan forests. A total of 37 environmental variables were used by generalised linear model fitting in order to find parsimonious sub-sets of environmental factors (predictors) that are able to explain species diversity patterns at the local scale. Moreover, the role of regional and local variable groups on species richness of the considered plant groups was estimated by using the variance partitioning approach. We found that local variables, such as forest management and structure, explained more variance than regional variables for total species richness, open habitat species richness and bryophyte species richness. On the other hand, regional variables (such as elevation) played a central role for woody species richness.     

外来种入侵与物种多样性

在入侵生态学研究方面 ,物种多样性与生物入侵之间的关系已成为当前研究和争论的焦点。自Elton的经典假说提出以来 ,物种丰富度高的群落比物种贫乏的群落更能抵抗外来种入侵的观点得到广泛接受。一些理论模型和多样性处理实验支持了该假说。但现在越来越多的野外观测和实验研究开始对这一经典假说提出异议 ,甚至反对。同时 ,在入侵生态学广泛受到关注的今天 ,大量的实验研究也提出了一些新的观点。本文对Elton经典假说提出以来全球有关物种多样性与生物入侵关系的主要研究及其观点进行了评述 ,以期为我国有关研究工作的开展提供参考。     

不同类型人类活动干扰对河岸带外来植物群落的影响——以北京永定河为例

强烈的人类活动对自然生境的扰动促进了外来植物侵入河岸带,但不同类型的人类活动干扰对外来植物的影响有所不同。按照不同人类活动干扰类型设置了山峡段、平原段和城市段,通过比较河段间河岸带外来植物群落特征的差异,探讨不同人类活动干扰类型对河岸带外来植物群落的影响。结果表明:(1)永定河河岸带共有维管束植物27科72属101种,外来植物13科28属29种。外来种比例高达28.7%,其中82.4%的国外外来种来自美洲和亚洲。(2)不同河段间的外来植物群落的物种构成和优势度呈现出显著性差异。外来种比例由山峡段的20.9%,上升至平原段和城市段的30.2%和25.5%;而优势度由12.1%,分别上升至13.4%和17.5%。(3)不同河段间的外来植物群落生活型结构有显著性差异。多年生草本植物在山峡段比例最高,达到66.7%,在平原段和城市段较低,分别为46.2%和30.8%;而一年生草本植物在城市段比例最高,达到69.2%,在平原段和山峡段较低,分别为53.8%和33.3%。(4)平原段属于农业干扰类型,其河岸带外来植物主要以农业类杂草为主,如禾本科的假稻(Leersia japonica)和菊科的钻叶紫菀(Aster subulatu);城市段是城市干扰类型,外来植物中入侵种比例和优势度较高,典型入侵种为反枝苋(Amaranthus retroflexus)和牛筋草(Eleusine indica)等。北京永定河不同河段河岸带外来植物的种类构成特点,反映了河岸带外来植物受快速城市化、农业活动等不同人类活动干扰的影响呈逐渐扩大的趋势。     

Assessment of woody species encroachment in the grasslands of Nechisar National Park,Ethiopia

Ecological survey was executed to assess woody species encroachment into the grassland plain of Nechisar National Park (NNP). Forty‐one woody species were recorded. Dichrostachys cinerea Wight & Arn., Acacia mellifera (Vahl) Benth., Acacia nilotica (L) Willd., Acacia senegal (L.) Willd., Acacia seyal Del. and Acacia tortilis (Forssk.) Hayne were among the major encroaching woody species. The majority of the woody species were found to be highly aggregated in their pattern of distribution, while only few species showed some degree of randomness. The mean woody species density was ca. 1995 woody plants ha^?1. Mean cover of woody, grass, unpalatable forbs and total herbaceous species were 31%, 58%, 68% and 121%, respectively. The woody species density and cover, unpalatable forbs and bare land cover were significantly higher in the highly grazed and fire‐suppressed part of the grassland plain. Pearson correlation coefficient matrix indicated that woody species cover and density were negatively correlated with total herbaceous and grass cover. The high woody, unpalatable forbs and bare land cover indicated the progressively increasing perennial grass species diversity deterioration in the grass plain of the Park. Decline in the grassland condition, unless reversed, will jeopardize the biological diversity as well as the aesthetic value of the NNP.     

Invasive alien plant control: The priority to save one of the most rapidly declining island-endemic plant species worldwide

Up to 6,800 plant species endemic to oceanic islands are highly threatened with extinction. Although habitat destruction and fragmentation have greatly contributed to this, it is generally recognised that invasive alien species currently pose the single most important threat to island plants. Most studies exploring the role of novel interspecific interactions in driving declines of island plants, focus on threats mediated by animals, be it direct (e.g. browsing, seed predation, mutualism disruption) or indirect (e.g. extinction of seed dispersal or pollination mutualists). Relatively few studies have investigated the specific role of plant-plant interactions, particularly in-situ. We studied a threatened island endemic plant in rapid decline to evaluate the short (1–2 years) and medium-term (about 1–2 decades) influence of invasive alien plants (IAPs) on individuals and a variety of proxies of plant fitness. We compared mortality of traceable individuals that were recorded 12–20 years previously between habitats that are invaded with IAPs and habitats where IAPs are absent, or have been removed decades ago. We also carried out an in-situ manipulative experiment using 14 randomly chosen plants from around which IAPs were removed, paired with controls, at two sites. Canopy cover change before and after IAPs’ removal was quantified along with above ground biomass of IAPs removed for use as potential explanatory variables of change in proxies of plant fitness. Ten branches were randomly selected per plant and branch dynamics, leaves’ sizes and reproductive structure production were monitored quarterly for two years. Over the medium term, plant mortality was recorded only in presence of IAPs (X² = 4.80, df = 1, p < 0.05). Over the short term, at the plant level, IAPs’ removal triggered overall weak to moderate improvements in the number of surviving and new branches as well as change in number of branches at one of the sites. At the leaf and branch levels, we found weak evidence for positive effects of IAPs removal on surviving leaves, flower buds produced and difference in leaf surface area per branch in one site. We therefore provide some experimental evidence of negative effects of alien plants on overall fitness of the threatened species in-situ presumably through competitive interactions. We posit that these effects were found to be weak to moderate due to the short experimental period over which they could develop (1–2 years). Overall, IAPs stand out as the most severe threat from among all documented threats to the species, for being the only one capable of causing mortality of adult plants. Results hence highlight island plants’ vulnerability to IAPs, and how their timely control would improve the survival and fitness of threatened plants, even at the scale of single individuals. Such a strategy could be more often employed. Our study stresses on prioritising IAPs’ control for rescuing long-lived threatened plants that grow in habitats invaded by alien plants (itself a very common situation on oceanic islands) before addressing other subtler, slower-acting threats, like disrupted pollination or seed dispersal mutualisms, florivory or seed predation.     

Towards a balanced view of pike in Ireland: a reply to Ensing

In our recent study of the population genetics of pike (Esox lucius) in Ireland (Pedreschi et al., 2014 , Journal of Biogeography, 41 , 548–560), we reported the existence of two main demographic units and showed that these may correspond to two independent and temporally staggered colonization events, the first of which may have been too old to be caused or assisted by human translocations. Ensing (2015, Journal of Biogeography, doi: 10.1111/jbi.12410 ) first used our genotypic data to explore alternative historical scenarios, then attempted to reconcile the ‘two‐wave’ colonization process of Ireland by pike with translocation activities by humans in Neolithic/Bronze age times. Here we illustrate why the evidence base for Ensing's reconstruction is weak and we outline a realistic strategy to better understand the role of pike in Irish freshwater ecosystems.     

Plant regeneration from callus cultures of <Emphasis Type="Italic">Maclura tinctoria</Emphasis>, an endangered woody species

Summary Some native species produce seeds with a low frequency of germination accompanied with a period of dormancy. These features make it difficult to produce new phenotypes through sexual propagation. Maclura tinctoria has been considered an endangered species due to extensive use of its wood and low frequency of seed germination. The objective of the present study is to establish an in vitro propagation system for this species. Organogenic friable callus formation from nodal segments has been obtained using woody plant medium (WPM) supplemented with 10.74 μM 1-naphthaleneacetic acid (NAA)+4.43 μM 6-benzylaminopurine (BA). Results indicate that the highest frequency of shoot formation is observed when WPM supplemented with 4.03 μM NAA+4.43 BA is used. For root formation, the use of WPM medium (pH adjusted to 7.0) supplemented with 23.62 μM indole-3-butyric acid (IBA) and 4.7gl⁻¹ activated charcoal is recommended. For acelimatization, subjecting rooted plantlets to 70%, 50%, and 30% mesh screen, each successively for a period of 7 d, has resulted in 97% plantlet survival.     

Plant invasions differentially affected by diversity and dominant species in native‐ and exotic‐dominated grasslands

Plant invasions are an increasingly serious global concern, especially as the climate changes. Here, we explored how plant invasions differed between native‐ and novel exotic‐dominated grasslands with experimental addition of summer precipitation in Texas in 2009. Exotic species greened up earlier than natives by an average of 18 days. This was associated with a lower invasion rate early in the growing season compared to native communities. However, invasion rate did not differ significantly between native and exotic communities across all sampling times. The predictors of invasion rate differed between native and exotic communities, with invasion being negatively influenced by species richness in natives and by dominant species in exotics. Interestingly, plant invasions matched the bimodal pattern of precipitation in Temple, Texas, and did not respond to the pulse of precipitation during the summer. Our results suggest that we will need to take different approaches in understanding of invasion between native and exotic grasslands. Moreover, with anticipated increasing variability in precipitation under global climate change, plant invasions may be constrained in their response if the precipitation pulses fall outside the normal growing period of invaders.     

中条山中段木本植物群落植物种多样性分析

采用样方法取样,依据重要值指标运用双向指示种分析(TWINSPAN)法,将山西中条山中段植被群系分成16个群丛。应用丰富度指数（R₁、R₂）、Simpson指数（λ）,Shannon-Weiner指数(H′)和均匀度指数(E₁、E₅)研究了16个群丛的植物种多样性。结果表明：群系植物种多样性指数的大小受立地生境和人为活动的综合影响;森林群落的植物种多样性指数明显高于灌丛群落;群落植物种多样性在空间上的差异不仅决定于物种丰富度指数,更与各物种间的均匀度指数密切相关;进而影响不同层次的物种多样性对群落总体物种多样性的贡献。     

Impacts of a mineral lick‐centred land use system on woody vegetation cover in an East African Savannah

Evaluation of woody vegetation changes with distance from a salt crater was conducted in the semi‐arid rangelands of southern Ethiopia. Data on live woody plants were collected over three seasons at 0, 1, 4, 6, 9 and 12 km from the salt crater. The density and diversity of woody plants differed significantly (p < .01) along the distance gradient. Six woody plant families were identified of which Fabaceae and Burseraceae were the dominant families. Acacia drepanolobium, Acacia nilotica, Commiphora africana and Acacia mellifera were among the severely encroaching woody species. There were high proportions of seedlings and saplings recorded closer to the salt crater showing a vigorous recruitment by woody plants. Woody plant encroachment along the 12‐km transect ranged from a low to severe encroachment, which could be translated into poor rangeland condition. Changes in soil characteristics increased grazing pressure and sedentary settlement around the salt crater, and the breakdown of traditional institutions seems to be major contributing factors to these vegetation changes. We suggest that severely encroached areas could be improved through a combination of methods such as bush clearing, prescribed fire, browsing animals and proper grazing management.     

基于FvCB模型的几种草本和木本植物光合生理生化特性

为研究不同生活型植物的光合能力及其叶片光合机构,采用直角双曲线修正模型和C3植物FvCB模型对7种木本植物和4种草本植物的CO₂响应曲线进行拟合,并对不同木本植物、不同草本植物和2种生活型植物的最大净光合速率(P_{n max})、Rubisco酶最大羧化速率(V_{c max})、最大电子传递速率(J_max)、光合暗呼吸速率(R_d)和叶肉阻力(r_m)等参数进行比较.结果表明: 7种木本植物P_{n max}大小顺序为乌桕、苎麻>润楠、海桐>青冈、苦槠、娜塔栎;乌桕、苎麻、润楠和海桐的V_{c max}显著大于青冈和苦槠;J_max大小顺序为乌桕>苎麻、海桐>娜塔栎、苦槠和青冈;润楠和苦槠的r_m显著大于乌桕、海桐和苎麻.商陆的P_{n max}显著大于藿香蓟和土牛膝;4种草本植物的V_{c max}无显著差异;商陆的J_max显著大于藿香蓟;龙葵和土牛膝的r_m显著大于藿香蓟;商陆的R_d显著大于藿香蓟和土牛膝.木本植物的P_{n max}、V_{c max}、J_max和r_m光合参数均显著大于草本植物,但二者的R_d无显著差异.不同物种之间以及2种生活型植物光合能力的差异主要是由叶片内部Rubisco酶羧化能力、电子传递能力和叶肉阻力等差异引起的.     

Slashing Phragmites (Phragmites australis) prior to planting does not promote native vegetation establishment

Phragmites or Common Reed (Phragmites australis) is a tall rhizomatous cosmopolitan grass. While native to Australia, it can be invasive in wetlands, forming dense monocultures and reducing their ecological integrity. We assessed the potential for the cutting of Phragmites reeds prior to planting to promote the establishment of indigenous shrubs that might ultimately outcompete Phragmites. We established ten 5 m × 5 m quadrats in an area dominated by Phragmites, brush‐cut the reeds to ground level in five of them and left five uncut as controls. Within each quadrat, we planted 20 plants (~40 cm tall) of each of five indigenous shrub species, unguarded (4 plants/m²). We surveyed the plants one month after planting and annually for the following four years for survival, height and browsing damage. Browsing damage to plants was common (>50%) and unaffected by cutting. After four years, overall plant survival rates were ~25% and mean plant heights for the five shrub species ranged between 120 and 174 cm. Cutting of Phragmites had no positive effect on plant survival or height. In fact, two Melaleuca species grew taller in the uncut quadrats. Cutting of Phragmites reed beds prior to planting is unlikely to promote the establishment of woody plantings. However, planting within established Phragmites with or without prior brush‐cutting is worthy of further trialling as a potential tool for reinstating native diversity at Phragmites‐dominated sites.     

Responses of a native plant species from invaded and uninvaded areas to allelopathic effects of an invader

Invaders exert new selection pressures on the resident species, for example, through competition for resources or by using novel weapons. It has been shown that novel weapons aid invasion but it is unclear whether native species co‐occurring with invaders have adapted to tolerate these novel weapons. Those resident species which are able to adapt to new selective agents can co‐occur with an invader while others face a risk of local extinction. We ran a factorial common garden experiment to study whether a native plant species, Anthriscus sylvestris, has been able to evolve a greater tolerance to the allelochemicals exerted by the invader, Lupinus polyphyllus. Lupinus polyphyllus produces allelochemicals which potentially act as a novel, strong selective agent on A. sylvestris. We grew A. sylvestris seedlings collected from uninvaded (naïve) and invaded (experienced) sites growing alone and in competition with L. polyphyllus in pots filled with soil with and without activated carbon. Because activated carbon absorbs allelochemicals, its addition should improve especially naïve A. sylvestris performance in the presence of the invader. To distinguish the allelochemicals absorption and fertilizing effects of activated carbon, we grew plants also in a mixture of soil and fertilizer. A common garden experiment indicated that the performances of naïve and experienced A. sylvestris seedlings did not differ when grown with L. polyphyllus. The addition of activated carbon, which reduces interference by allelochemicals, did not induce differences in their performances although it had a positive effect on the aboveground biomass of A. sylvestris. Together, these results suggest that naïve and experienced A. sylvestris plants tolerated equally the invader L. polyphyllus and thus the tolerance has not occurred over the course of invasion.     

Competitive Interactions among First‐Year and Second‐Year Plants of the Invasive,Biennial Garlic Mustard (Alliaria petiolata) and Native Ground Layer Vegetation

We studied the effects of hand weeding of second‐year plants of the biennial garlic mustard (Alliaria petiolata) on first‐year plants (seedlings) and native ground layer vegetation. Garlic mustard is a Eurasian species that has invaded deciduous forest ground layers in eastern North America. Treatments consisted of a control and an early or late weeding of second‐year garlic mustard. The early treatment (early March) was applied before garlic mustard seeds had germinated and when most native species were dormant. The late treatment (mid‐May) occurred after plants had bolted, flowering was occurring, and most native species and new garlic mustard seedlings were actively growing. Pre‐treatment data were obtained in 2004 and treated and control plots were sampled in 2005, 2006, and 2007. No significant treatment effects were observed in 2004 or 2005. In 2006, mean cover of first‐year plants was higher in the early weeding treatment than in the late weeding treatment and control. In 2007, mean cover of first‐year garlic mustard was higher in the control than in either of the two weeding treatments. There were no significant treatment effects in any year on native vegetation cover, bare ground, or the five most abundant native species. Our data indicate that (1) late weeding of garlic mustard provided more effective control than early weeding because late weeding allows second‐year plants to compete with garlic mustard seedlings for a longer period of time and (2) competition between first‐ and second‐year plants is responsible for alternating dominance of first‐year and second‐year garlic mustard plants.     

Patterns of Plant Invasions: A Case Example in Native Species Hotspots and Rare Habitats

Land managers require landscape-scale information on where exotic plant species have successfully established, to better guide research, control, and restoration efforts. We evaluated the vulnerability of various habitats to invasion by exotic plant species in a 100,000 ha area in the southeast corner of Grand Staircase-Escalante National Monument, Utah. For the 97 0.1-ha plots in 11 vegetation types, exotic species richness (log₁₀) was strongly negatively correlated to the cover of cryptobiotic soil crusts (r = −0.47, P < 0.001), and positively correlated to native species richness (r = 0.22, P < 0.03), native species cover (r = 0.23, P < 0.05), and total nitrogen in the soil (r = 0.40, P < 0.001). Exotic species cover was strongly positively correlated to exotic species richness (r = 0.68, P < 0.001). Only 6 of 97 plots did not contain at least one exotic species. Exotic species richness was particularly high in locally rare, mesic vegetation types and nitrogen rich soils. Dry, upland plots (n = 51) had less than half of the exotic species richness and cover compared to plots (n = 45) in washes and lowland depressions that collect water intermittently. Plots dominated by trees had significantly greater native and exotic species richness compared to plots dominated by shrubs. For the 97 plots combined, 33% of the variance in exotic species richness could be explained by a positive relationship with total plant cover, and negative relationships with the cover of cryptobiotic crusts and bare ground. There are several reasons for concern: (1) Exotic plant species are invading hot spots of native plant diversity and rare/unique habitats. (2) The foliar cover of exotic species was greatest in habitats that had been invaded by several exotic species.(3) Continued disturbance of fragile cryptobiotic crusts by livestock, people, and vehicles may facilitate the further invasion of exotic plant species. This revised version was published online in July 2006 with corrections to the Cover Date.