Phragmites australis root secreted phytotoxin undergoes photo-degradation to execute severe phytotoxicity

Our study organism, Phragmites australis (common reed), is a unique invader in that both native and introduced lineages are found coexisting in North America. This allows one to make direct assessments of physiological differences between these different subspecies and examine how this relates to invasiveness. Recent efforts to understand plant invasive behavior show that some invasive plants secrete a phytotoxin to ward-off encroachment by neighboring plants (allelopathy) and thus provide the invaders with a competitive edge in a given habitat. Here we show that a varying climatic factor like ultraviolet (UV) light leads to photo-degradation of secreted phytotoxin (gallic acid) in P. australis rhizosphere inducing higher mortality of susceptible seedlings. The photo-degraded product of gallic acid (hereafter GA), identified as mesoxalic acid (hereafter MOA), triggered a similar cell death cascade in susceptible seedlings as observed previously with GA. Further, we detected the biological concentrations of MOA in the natural stands of exotic and native P. australis. Our studies also show that the UV degradation of GA is facilitated at an alkaline pH, suggesting that the natural habitat of P. australis may facilitate the photo-degradation of GA. The study highlights the persistence of the photo-degraded phytotoxin in the P. australis''s rhizosphere and its inhibitory effects against the native plants.Key words: ultraviolet, gallic acid, mesoxalic acid, novel weapons, invasive species, Phragmites australis     

The Impacts of Above- and Belowground Plant Input on Soil Microbiota: Invasive <Emphasis Type="Italic">Spartina alterniflora</Emphasis> Versus Native <Emphasis Type="Italic">Phragmites australis</Emphasis>

Invasive plants affect soil food webs through various resource inputs including shoot litter, root litter and living root input. The net impact of invasive plants on soil biota has been recognized; however, the relative contributions of different resource input pathways have not been quantified. Through a 2 × 2 × 2 factorial field experiment, a pair of invasive and native plant species (Spartina alterniflora vs. Phragmites australis) was compared to determine the relative impacts of their living roots or shoots and root litter on soil microbial and nematode communities. Living root identity affected bacteria-to-fungi PLFA ratios, abundance of total nematodes, plant-feeding nematodes and omnivorous nematodes. Specifically, the plant-feeding nematodes were 627% less abundant when living roots of invasive S. alterniflora were present than those of native P. australis. Likewise, shoot and root biomass (within soil at 0–10 cm depth) of S. alterniflora was, respectively, 300 and 100% greater than those of P. australis. These findings support the enemy release hypothesis of plant invasion. Root litter identity affected other components of soil microbiota (that is, bacterial-feeding nematodes), which were 34% more abundant in the presence of root litter of P. australis than S. alterniflora. Overall, more variation associated with nematode community structure and function was explained by differences in living roots than root or shoot litter for this pair of plant species sharing a common habitat but contrasting invasion degrees. We conclude that belowground resource input is an important mechanism used by invasive plants to affect ecosystem structure and function.     

Native Plant and Microbial Contributions to a Negative Plant-Plant Interaction

A number of hypotheses have been suggested to explain why invasive exotic plants dramatically increase their abundance upon transport to a new range. The novel weapons hypothesis argues that phytotoxins secreted by roots of an exotic plant are more effective against naïve resident competitors in the range being invaded. The common reed Phragmites australis has a diverse population structure including invasive populations that are noxious weeds in North America. P. australis exudes the common phenolic gallic acid, which restricts the growth of native plants. However, the pathway for free gallic acid production in soils colonized by P. australis requires further elucidation. Here, we show that exotic, invasive P. australis contain elevated levels of polymeric gallotannin relative to native, noninvasive P. australis. We hypothesized that polymeric gallotannin can be attacked by tannase, an enzymatic activity produced by native plant and microbial community members, to release gallic acid in the rhizosphere and exacerbate the noxiousness of P. australis. Native plants and microbes were found to produce high levels of tannase while invasive P. australis produced very little tannase. These results suggest that both invasive and native species participate in signaling events that initiate the execution of allelopathy potentially linking native plant and microbial biochemistry to the invasive traits of an exotic species.Invasive weeds are a major source of agricultural costs due to reduced productivity and the labor expended for weed control. In addition, the extensive use of herbicides to control weed populations has undesirable environmental consequences. Therefore, understanding mechanisms that facilitate exotic plant dispersal and displacement of natives in new ranges is critical to predicting and controlling invasions and may yield insights into the ecological processes that govern homeostasis and perturbation in natural plant communities.Phragmites australis (Cav.) Trin ex. Steud. (common reed) has been present in the United States for at least 10,000 years as a major component of mixed tidal wetland plant communities (Saltonstall, 2002). However, over the past 200 years its distribution and abundance has expanded rapidly and it is now considered one of the most aggressive invasive species in marsh communities in North America. Chloroplast DNA analysis has shown that 13 native North American Phragmites haplotypes exist, while invasive populations possess a single chloroplast DNA haplotype (M) that is also widespread in Europe and Asia (Saltonstall, 2002). These data are supported by nuclear microsatellite DNA analysis (Saltonstall, 2003) and morphological differences that distinguish native, noninvasive from exotic, invasive Phragmites in North America (Saltonstall et al., 2004). When grown under the same conditions, exotic Phragmites has significantly higher aboveground and belowground biomass than native Phragmites (Vasquez et al., 2005; Saltonstall and Stevenson, 2007), and this pattern is typically observed under field conditions as well although exceptions exist (League et al., 2006; Meadows and Saltonstall, 2007). Unfortunately today, only remnant native P. australis populations remain along the Atlantic Coast of North America, indicating the near total displacement of native populations by exotic P. australis.Various hypotheses have been forwarded to explain the rapid invasion of P. australis, of which human activities, stress regimes, and hydrologic disturbances have received the greatest attention (Chambers et al., 1999). Compared to invasion in terrestrial ecosystems, invasiveness in marsh communities is less well documented and it is still not clear how environmental factors relate to the establishment of specific dominant marsh species. Although allelopathy has been superficially suggested as the main displacing mechanism in P. australis (Kaneta and Sugiyama, 1972; Drifmeyer and Zieman, 1979), there has been minimal success in characterizing the responsible allelochemical. Interestingly, three triterpenoids (β-amacin, taraxerol, and taraxerone) and a flavone (tricin) have been identified from aerial portions of P. australis (Kaneta and Sugiyama, 1972; Drifmeyer and Zieman, 1979). Regrettably, none of these identified chemicals were tested for possible allelopathic activity.Previously, we showed that a root exudate component of P. australis roots inhibits seedling growth, and that production of the exudates is higher in the invasive P. australis haplotype (Rudrappa et al., 2007). The active fraction of this exudate was found to be composed of gallic acid (3,4,5-trihydroxybenzoic acid). Gallic acid is toxic to a variety of weeds, crop plant species, and the model plant species Arabidopsis (Arabidopsis thaliana; Rudrappa et al., 2007; Rudrappa and Bais, 2008). Our published results also show the persistence of gallic acid in soil extracts from P. australis-invaded fields, which validates our in vitro results and strongly supports the idea that P. australis'' invasive behavior may partly be due to the exudation of gallic acid in the soil/marsh (Rudrappa et al., 2007). Our studies concur with the earlier established reports of phytotoxicity and persistence of gallic acid in soil (Weidenhamer and Romeo, 2004).Biochemically, the transition from simple galloylglucoses to complex gallotannins is marked by addition of further galloyl moieties to the pentagalloylglucose (Niemetz and Gross, 2005). It is now known that free gallic acid is released from complexed gallotannins by simple hydrolysis reactions, wherein a tannase activity breaks gallate ester to form free gallic acid, ellagic acid, and Glc (Mahoney and Molyneux, 2004). Treatment of fungal tannase from Aspergillus flavus results in hydrolysis of pellicle-localized gallotannin to form gallic acid, and ellagic acid as two phenolic components (Mahoney and Molyneux, 2004). As gallic acid is often complexed as gallotannins (Niemetz and Gross, 2005), we speculated that plant- or microbial-derived tannase may facilitate free gallic acid release in salt marsh soils.Aside from allelopathy, invasive plants may deleteriously affect interactions between rhizospheric microbial communities and native plant species (Klironomos, 2002; Wardle et al., 2004; Callaway et al., 2008) to promote their expansion in new ranges. One specific example is the disruption of interactions between native species and their arbuscular mycorhizae, upon which the native species rely for nutrient acquisition (Stinson et al., 2006). Another recent study suggests that the recruitment or establishment of an altered soil microbial community may negatively impact the ability of native species to survive in the same soils (Batten et al., 2008). Evidences suggest that soil biota have several effects on the success of invasive plants and the interactions are based in part on the biochemistry, i.e. novel biochemical weapons (Callaway and Ridenour, 2004). However, to our knowledge, no previous studies have directly tested whether P. australis or any other exotic plant may exploit the biochemical potential of native plant and microbial communities to release a phytotoxin (gallic acid) from a relatively benign precursor (gallotannin) in the rhizosphere. This report presents evidence that links native plant and microbial biochemistry to the invasive traits of an exotic species.     

<Emphasis Type="Italic">Phragmites</Emphasis><Emphasis Type="Italic">australis</Emphasis> as a model organism for studying plant invasions

The cosmopolitan reed grass Phragmites australis (Poaceae) is an intensively studied species globally with a substantial focus in the last two decades on its invasive populations. Here we argue that P. australis meets the criteria to serve as a model organism for studying plant invasions. First, as a dominant species in globally important wetland habitats, it has generated significant pre-existing research, demonstrating a high potential for funding. Second, this plant is easy to grow and use in experiments. Third, it grows abundantly in a wide range of ecological systems and plant communities, allowing a broad range of research questions to be addressed. We formalize the designation of P. australis as a model organism for plant invasions in order to encourage and standardize collaborative research on multiple spatial scales that will help to integrate studies on the ecology and evolution of P. australis invasive populations, their response to global environmental change, and implications for biological security. Such an integrative framework can serve as guidance for studying invasive plant species at the population level and global spatial scale.     

Local and regional disturbances associated with the invasion of Chesapeake Bay marshes by the common reed <Emphasis Type="Italic">Phragmites australis</Emphasis>

The invasion of wetlands by Phragmites australis is a conservation concern across North America. We used the invasion of Chesapeake Bay wetlands by P. australis as a model system to examine the effects of regional and local stressors on plant invasions. We summarized digital maps of the distributions of P. australis and of potential stressors (especially human land use and shoreline armoring) at two spatial scales: for 72 subestuaries of the bay and their local watersheds and for thousands of 500 m shoreline segments. We developed statistical models that use the stressor variables to predict P. australis prevalence (% of shoreline occupied) in subestuaries and its presence or absence in 500 m segments of shoreline. The prevalence of agriculture was the strongest and most consistent predictor of P. australis presence and abundance in Chesapeake Bay, because P. australis can exploit the resulting elevated nutrient levels to enhance its establishment, growth, and seed production. Phragmites australis was also positively associated with riprapped shoreline, probably because it creates disturbances that provide colonization opportunities. The P. australis invasion was less severe in areas with greater forested land cover and natural shorelines. Surprisingly, invasion was low in highly developed watersheds and highest along shorelines with intermediate levels of residential land use, possibly indicating that highly disturbed systems are uninhabitable even to invasive species. Management strategies that reduce nutrient pollution, preserve natural shorelines, and limit nearshore disturbance of soils and vegetation may enhance the resilience of shorelines to invasion.     

Biogeographic gradients in ecosystem processes of the invasive ecosystem engineer <Emphasis Type="Italic">Phragmites australis</Emphasis>

Latitudinal gradients in ecosystem patterns arise from complex interactions between biotic and abiotic forces operating at a range of spatial and temporal scales. Widespread invasive species, particularly invasive ecosystem engineers with large effects on their environment, may alter these gradients. We sampled 3–5 stands of the invasive common reed, Phragmites australis, in eight coastal wetlands ranging from Massachusetts (42°N) to South Carolina (32°N) to document geographic variation in P. australis primary production, associated plant and animal species diversity, and sediment carbon storage and to examine how local-, regional-, and large-scale environmental factors contribute to these patterns. Latitude best explained variation in P. australis density, but contrary to expectations, density increased with increasing latitude across our sites. Latitude also predicted macroinvertebrate species richness, which increased with latitude in a manner similar to P. australis density. In addition to latitude, P. australis leaf carbon:nitrogen ratios, distance to the open coast, and sediment oxygen levels were most important for explaining variation in P. australis production, as well as community (plant or animal species richness) and ecosystem (carbon storage) variables. The percent of developed land was positively associated with P. australis density, yet this variable had relatively low predictive power in our study. Our study provides an important biogeographic perspective for documenting and understanding variation in invasive P. australis that is fundamental both for managing the invasion and for understanding latitudinal gradients in ecosystem structure and function.     

Soil conditioning effects of Phragmites australis on native wetland plant seedling survival

Interactions between introduced plants and soils they colonize are central to invasive species success in many systems. Belowground biotic and abiotic changes can influence the success of introduced species as well as their native competitors. All plants alter soil properties after colonization but, in the case of many invasive plant species, it is unclear whether the strength and direction of these soil conditioning effects are due to plant traits, plant origin, or local population characteristics and site conditions in the invaded range. Phragmites australis in North America exists as a mix of populations of different evolutionary origin. Populations of endemic native Phragmites australis americanus are declining, while introduced European populations are important wetland invaders. We assessed soil conditioning effects of native and non‐native P. australis populations on early and late seedling survival of native and introduced wetland plants. We further used a soil biocide treatment to assess the role of soil fungi on seedling survival. Survival of seedlings in soils colonized by P. australis was either unaffected or negatively affected; no species showed improved survival in P. australis‐conditioned soils. Population of P. australis was a significant factor explaining the response of seedlings, but origin (native or non‐native) was not a significant factor. Synthesis: Our results highlight the importance of phylogenetic control when assessing impacts of invasive species to avoid conflating general plant traits with mechanisms of invasive success. Both native (noninvasive) and non‐native (invasive) P. australis populations reduced seedling survival of competing plant species. Because soil legacy effects of native and non‐native P. australis are similar, this study suggests that the close phylogenetic relationship between the two populations, and not the invasive status of introduced P. australis, is more relevant to their soil‐mediated impact on other plant species.     

Hurricane Activity and the Large-Scale Pattern of Spread of an Invasive Plant Species

Disturbances are a primary facilitator of the growth and spread of invasive species. However, the effects of large-scale disturbances, such as hurricanes and tropical storms, on the broad geographic patterns of invasive species growth and spread have not been investigated. We used historical aerial imagery to determine the growth rate of invasive Phragmites australis patches in wetlands along the Atlantic and Gulf Coasts of the United States. These were relatively undisturbed wetlands where P. australis had room for unrestricted growth. Over the past several decades, invasive P. australis stands expanded in size by 6–35% per year. Based on tropical storm and hurricane activity over that same time period, we found that the frequency of hurricane-force winds explained 81% of the variation in P. australis growth over this broad geographic range. The expansion of P. australis stands was strongly and positively correlated with hurricane frequency. In light of the many climatic models that predict an increase in the frequency and intensity of hurricanes over the next century, these results suggest a strong link between climate change and species invasion and a challenging future ahead for the management of invasive species.     

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。     

室内模拟波浪对2种典型滨海湿地挺水植物生物力学特征的影响

选取本地物种芦苇和入侵物种互花米草为研究对象,利用波浪装置对生长期的植物进行波浪处理后对其形态特征,根、茎、叶的拉力、弯曲特性进行研究,以探讨湿地植物对波浪作用的生物力学响应。结果表明,芦苇、互花米草经波浪处理后的茎直径均显著高于对照组（P=0.008,P=0.03）;互花米草总体呈现拉力或者弯曲载荷高于芦苇的现象（如茎秆）,但应力（根系）或静曲强度（茎秆）却要低于芦苇,即对应了互花米草比芦苇茎秆更粗壮的现象,但芦苇自身的强度会更强,其木质素和纤维素的结构成分显示出显著高于互花米草的现象（P<0.001,P<0.001）。总体来看,通过短时间的波浪模拟,植株在形态特征上发生了显著的变化,而植物自身的生物力学特性与形态、结构成分都息息相关。研究结果对于评价芦苇和互花米草在应对海浪影响的潜力、从植物生物力学角度探究滨海湿地的保护和修复都提供了一定的参考。     

Epigenetic variation within <Emphasis Type="Italic">Phragmites australis</Emphasis> among lineages,genotypes, and ramets

Epigenetics is likely an important factor in morphological and physiological acclimation, phenotypic plasticity, and potentially ecological dynamics such as invasiveness. We propose that Phragmites australis is an ideal model species for studies of epigenetics as a factor in plant invasions and ecology due to natural clonal replication (controlling for genetic variation) and the co-occurrence of subspecies with distinct life history strategies such as differences in invasiveness. In earlier work, genotypes and constituent clonal ramets were identified using microsatellite markers. In this pilot study, we screened the same ramets for epigenetic variation with Methylation-Sensitive AFLPs (MS-AFLPs), a modified type of AFLP dependent on differentially methylation-sensitive restriction enzymes. We found a significant difference in epigenetic signatures between introduced and native subspecies, and found that introduced P. australis demonstrated more epigenetic variation than their native counterparts. In both subspecies we observed moderate variation between genotypes relative to the higher degree of epigenetic variation found within genotypes (among ramets), suggesting that epigenotype may be more closely aligned with microhabitat than within-subspecies genotype. Finally, we observed potential epigenetic variation by site. This is the first study to investigate natural variation in DNA methylation patterns of P. australis and establishes the baseline in our understanding of the ecological relevance of epigenetics in this species.     

Self-compatibility and plant invasiveness: Comparing species in native and invasive ranges

A longstanding hypothesis (“Baker's rule”) is that plant invasiveness is facilitated by floral self compatibility rather than self incompatibility. Extending this idea, invasive species whose individuals vary in degree of self compatibility within the native range might be self compatible in invading or weedy populations, due to natural selection on the mating system. We compared mating system between native and invasive ranges for two major world invasives, one annual (Echium plantagineum) and one perennial (Solanum elaeagnifolium). For an additional annual species (Centaurea solstitialis) we compared non-weedy and weedy populations in the native range. No species was strongly spontaneously self pollinating, but the degree of self compatibility after hand pollination varied dramatically. Both annuals were self incompatible in native or non-weedy populations but self compatible in invasive or weedy ones; the reverse was true for the perennial. Individuals within populations of all three species also varied in their degree of self compatibility, suggesting a basis for natural selection, and populations of the same species sharing a status (native/non-weedy, invading/weedy) varied in average self compatibility. These results support the hypothesis that differential selection of progeny during invasion can result in self-compatible populations derived from ancestrally self-incompatible ones, but that this process may be less important in perennial species, which experience multiple opportunities for sexual reproduction. Overall, however, mating system may not operate alone and its contributions to invasiveness may be conditional on other attributes of a species including physiology, morphology, and life history.     

Crustacean metamorphosis: an omics perspective

We quantified the independent impacts of flooding salinity, flooding depth, and flooding frequency on the native species, Phragmites australis and Scirpus mariqueter, and on the invasive species Spartina alterniflora in the Yangtze River Estuary, China. Total biomass of all three species decreased significantly with increasing salinity, but S. alterniflora was less severely affected than P. australis and S. mariqueter. Elevated flooding depth significantly decreased their live aboveground biomass of P. australis and S. mariqueter, while S. alterniflora still had high live aboveground biomass and total biomass even at the highest flooding depth. These findings indicated that S. alterniflora was more tolerant to experimental conditions than the two native species, and an unavoidable suggestion is the expansion of this non-native species in relation to the native counterparts in future scenarios of increased sea-level and saltwater intrusion. Even so, environmental stresses might lead to significant decreases in total biomass and live aboveground biomass of all three species, which would potentially weaken their ability to trap sediments and accumulate organic matter. However, the relatively high belowground-to-aboveground biomass ratio indicated phenotypic plasticity in response to stressful environmental conditions, which suggest that marsh species can adapt to sea-level rise and maintain marsh elevation.     

芦苇腐解土中酚酸类化感物质的水分响应特性研究

芦苇和虉草均具有较强的去污能力,常作为湿地植物配置于同一人工湿地进行污水处理。芦苇作为一种强化感植物对虉草具有较强的化感作用,在自然湿地和人工湿地中均会出现芦苇代替虉草的现象,且这一现象的发生与土壤含水量存在一定联系,此外,芦苇腐解土对虉草的化感抑制效应与腐解土中总酚酸的量密切相关。为了研究芦苇腐解土中主要酚酸类物质的水分响应特性,筛选出其中对水分响应较为明显的酚酸物质种类,该研究采用高效液相色谱法,通过芦苇枯落物腐解土的制备,对不同水分环境下芦苇腐解土中酚酸类物质进行了分离和鉴定。结果表明:芦苇腐解土中可分离出没食子酸、香豆酸、香草酸、丁香酸、对香豆酸、阿魏酸、水杨酸和苯甲酸等8种酚酸类物质,其中香豆酸、苯甲酸和阿魏酸等3种酚酸类物质含量较高。分离出的8种酚酸类物质的含量与腐解土的相对含水量均呈显著线性负相关关系,即随着腐解土相对含水量的上升,酚酸类物质的含量均呈现下降趋势,且各种酚酸类物质对水分的响应趋势均可用线性方程较好地拟合。其中,香豆酸、没食子酸和阿魏酸对芦苇腐解土的水分响应最为明显。因此,可将香豆酸、没食子酸和阿魏酸作为主要调控目标,通过调控湿地土壤中水分含量,削弱芦苇对虉草的化感抑制效应,从而维持人工湿地中虉草芦苇群落的长期稳定共存。     

潜流人工湿地中植物对氮磷净化影响效应的研究

采用潜流人工湿地系统,配制以NH⁺₄-N、NO^-₃-N和PO^3-₄-P为主要成分的模拟污水,通过间歇运行方式,考察了芦苇和小叶章的生长情况、生理生态学特性及其对污水中N、P净化效能的影响,并研究了植物对湿地系统pH变化、NO^-₃-N和NH⁺₄-N净化效率的影响。结果表明,当水力停留时间为7d时,小叶章和芦苇湿地对TN的去除率分别为65.1%和99.6%,去除负荷分别为1.66g · m^-3 · d^-1和2.53g · m^-3 · d^-1。小叶章和芦苇对去除TN的贡献率分别为14.7%、61.7%,对去除TP的贡献率分别为11.7%和12.9%;芦苇植株内N、P浓度分别为29.2mg/g和3.41mg/g。芦苇湿地的净化效能高于小叶章湿地。湿地系统中pH值先升高后降低的拐点可作为氨氧化反应结束的指示参数。     

The influence of an invasive plant on denitrification in an urban wetland

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Flooding events and rising water temperatures increase the significance of the reed pathogen <Emphasis Type="Italic">Pythium phragmitis</Emphasis> as a contributing factor in the decline of <Emphasis Type="Italic">Phragmites australis</Emphasis>

Pythium species are economically significant soilborne plant pathogens with worldwide distribution, causing seedling damping-off or root rot diseases. Pythium phragmitis is a newly described pathogen of common reed (Phragmites australis), widespread in the reed-belt of Lake Constance, Germany. It is highly aggressive towards reed leaves and seedlings, but obviously does not affect roots. In the context of ‘reed decline’ phenomena, P. phragmitis infection of reed inundated during flooding events may be of particular significance. We could show that flooding itself is not necessarily detrimental for reed plants. In the presence of the pathogen, however, most submerged leaves and plants were killed within several weeks. Clipped plants did not show regrowth in the Pythium infested treatments. Significant losses in assimilating leaf area of reeds could, thus, be the result of Pythium infection rather than of flooding alone. Therefore, we suggest that the combination of extended flooding and the presence of P. phragmitis might considerably contribute to ‘reed decline’ at Lake Constance. In parallel, we could show that pathogenicity and spread of this species are considerably favoured by rising temperatures. Since an increase in average water temperature has been found for Lake Constance, we propose that P. phragmitis could be an important factor in the dieback of reed stands likely to be promoted by predicted climate change phenomena.     

What happens in Vegas,better stay in Vegas: <Emphasis Type="Italic">Phragmites australis</Emphasis> hybrids in the Las Vegas Wash

While hybridization between Native and Introduced Phragmites australis has not been documented across much of North America, it poses an ongoing threat to Native P. australis across its range. This is especially true for native populations in the biologically rich, but sparsely distributed wetlands of the southwest United States, which are among the most imperiled systems in North America. We identified multiple Hybrid P. australis stands in the Las Vegas Wash watershed, NV, a key regional link to the Colorado River basin. Rapid urbanization in this watershed has caused striking changes in water and nutrient inputs and the distribution of wetland habitats has also changed, with urban wetlands expanding but an overall reduction in wetland habitats regionally. Native P. australis has likely been present in the Wash wetland community in low abundance for thousands of years, but today Hybrid and Native plants dominate the shoreline along much of the Wash. In contrast, Introduced P. australis is rare, suggesting that opportunities for novel hybridization events remain uncommon. Hybrid crosses derived from both the native and introduced maternal lineages are widespread, although the conditions that precluded their establishment are unknown and we did not find evidence for backcrossing. Spread of Hybrid plants is likely associated with flooding events as well as restoration activities, including revegetation efforts and construction for erosion control, that have redistributed sediments containing P. australis rhizomes. Downstream escape of Hybrid plants to Lake Mead and wetlands throughout the lower Colorado River basin is of management concern as these Hybrids appear vigorous and could spread rapidly.     

Cuscuta australis restrains three exotic invasive plants and benefits native species

In this study we conducted field investigations to examine the effects of native Cuscuta australis on three exotic invasive plants (i.e. Ipomoea cairica, Mikania micrantha, and Wedelia trilobata) and on the invaded native communities. The results showed that C. australis produced high infection rates on the exotic invasive hosts but low ones on the native species. Furthermore, the results showed that C. australis exhibited vigorous growth and high reproduction when it grew on M. micrantha and W. trilobata, indicating that these exotic invasive plants are more rewarding hosts than are native plants for C. australis. C. australis infection was positively related to the growth traits (e.g. biomass, cover, and total leaf area) and nutrient contents (e.g. N, P, and K) of the exotic invasive plants. The infections of C. australis significantly decreased the growth and nutrient contents of exotic invasive hosts, and the host?Cparasite interactions benefited the native species with increased species richness and biodiversity, facilitating the recovery of invaded native communities. This study provides a model for a native agent to both resist exotic invasive plants and benefit other native species. Furthermore, it indicates that certain native agents in invaded regions can be an effective and environmentally benign alternative to traditional biological control.     

闽江河口湿地植物枯落物立枯和倒伏分解主要元素动态

采用分解袋法,对闽江河口湿地2种挺水植物——芦苇(Phragmites australis)和互花米草(Spartina alterniflora)花和叶枯落物的立枯和倒伏分解过程及C、N、P元素动态进行研究。结果表明:(1)立枯分解是2种湿地盐沼植物重要的分解阶段,干物质损失率在13.26%—31.89%之间。多项式模型能较好描述2种植物花和叶的枯落物分解残留率动态。(2)立枯分解阶段,芦苇花和叶的C含量主要为波动下降,互花米草较为稳定;倒伏阶段后期,2种植物都以升高为主。立枯分解阶段2种植物枯落物N含量略有下降,而倒伏阶段逐渐上升。分解过程中枯落物P含量的波动较大。(3)2种植物花和叶C、N的NAI值在分解过程中<100%。芦苇的花和叶中P的NAI值在立枯和倒伏分解阶段都经历了明显下降和升高的过程,而互花米草在立枯阶段变化不大,倒伏阶段下降较为明显。(4)与芦苇相比,互花米草的花和叶枯落物C库较高,N库较低,P库差异不大。