Trade‐Offs in Seed Dispersal Strategies Across Riparian Trees: The How Matters as Much as the When

Riparian Salicaceae are prolific producers of short‐lived seeds that require very restrictive hydro‐geomorphic conditions for establishment. It is generally assumed that if floods are able to create nursery sites timed with seed dispersal, recruitment will occur. Other spatial and temporal seed dispersal patterns besides the dispersal period have historically received little attention. However, seed dispersal patterns can be highly variable between regions, species and over the years. In this paper, we report the seed dispersal patterns of three dominant riparian Salicaceae trees in Europe: Populus alba, P. nigra and Salix alba to suggest possible trade‐offs between seed dispersal patterns, germinability, longevity and establishment. Seed rain of the three species was monitored in 33 glue‐coated traps for three months yearly from 2006 to 2008 in an 8‐km stretch of the Middle Ebro River (N Spain), which has a pluvio‐nival regime. P. alba dispersed seeds earlier during a shorter time period and with a fewer number of seed release pulses compared with P. nigra, and especially with S. alba. With overlapping seed dispersal periods, the two latter species occupy similar landform units but rarely compete with P. alba, usually at higher elevations, as shown in a previous study in the same study area. The three species had very high germinability immediately after release (>90%), but longevity in S. alba was eight times shorter than that of its two Populus counterparts. We suggest that S. alba has compensated its lower seed quality with a more spaced seed release in several pulses of similar magnitude. With similar seed dispersal patterns and germinability but a higher longevity, P. nigra had a much higher density of individuals than S. alba in the recruitment zones of the study area. We hope that our results may inform river managers about how to optimize river flows to promote sexual regeneration of these species. Copyright © 2015 John Wiley & Sons, Ltd.     

Hydrologic thresholds for riparian forest conservation in a regulated large Mediterranean river

Most riparian trees are phreatophytic, water table‐dependent plants which broadly differ in their tolerance to drought and permanent flooding. In semi‐arid settings, as water is limiting, inundations may be regarded as inputs rather than stresses for the survival of phreatophytes. In this study, the mortality rates and abundances of Populus alba, P. nigra, Salix alba and local Tamarix spp. were examined in 43 plots with different hydrologic conditions distributed across the floodplain of a large semi‐arid and Mediterranean river, the Ebro River (Spain). The objectives were to determine hydrologic thresholds for the maintenance of declining populations of those species, while providing novel information on their phreatophytic nature, and to examine shifts in the species composition along hydrologic gradients. All species exhibited significant relationships between mortality rates and hydrologic variables (deepest water table—WT, flood duration—FD and flood frequency—FF). S. alba was found to be the species with lowest tolerance to drier conditions (hydrologic thresholds for maintaining a mortality rate <50%: WT > ?1.22 m; FD: out of observation range; FF > 5.4 events y^?1), followed by P. nigra (WT > ?2.18 m; FD > 11.1%; FF > 3.8 events y^?1), Tamarix spp. (WT > ?2.96 m; FD > 3.7%; FF > 2.5 events y^?1) and P. alba (WT > ?3.45 m; FD > 1.7%; FF > 2.0 events y^?1). Only a significant reduction in S. alba relative abundance was observed as conditions got drier. The results provided quantitative information useful to guide management plans for the protection of Mediterranean phreatophytic tree species from further degradation and suggested that eventual natural or regulation‐induced droughts and groundwater declines would accelerate the loss of all phreatophytic species, especially S. alba. Copyright © 2010 John Wiley & Sons, Ltd.     

Simulated recruitment of riparian trees and shrubs under natural and regulated flow regimes on the Wisconsin River,USA

Models that link ecological responses to hydrologic changes are important for assessing the effects of flow regulation on aquatic and riparian ecosystems. Based on the Recruitment Box Model, a graphical model used to prescribe environmental flows for cottonwood (Populus spp.) recruitment, we designed a simulation model to represent the influence of river flow dynamics on seedling recruitment of riparian pioneer woody plants. The model simulates the influence of temporal patterns of river stage on dispersal, germination, initial recruitment and over‐winter survival of first‐year seedlings of riparian pioneer shrubs and trees. We used the model to simulate seedling recruitment patterns for five species (Acer saccharinum, Betula nigra, Populus deltoides, Salix nigra and Salix exigua) on the Wisconsin River (Wisconsin, USA) under three flow scenarios: historic (1935–2002), simulated natural (1915–1975) and simulated regulated flows (1915–1975). Simulation results agreed well with field‐observed relative differences among years (1997–2000) in seedling densities for the five focal species. Simulated successful recruitment years were highly synchronous among species, but species differed in their sensitivity to flows at different times during the growing season, consistent with among‐species differences in seed dispersal timing. Comparison of simulated natural and regulated flows for 1915–1975 showed that flow regulation decreased monthly flow variability, increased late summer to winter baseflow and reduced the magnitude of spring peaks. Simulated recruitment and over‐winter survival of tree seedlings of all species was enhanced under the regulated flow scenario, likely due to increased summer baseflow and reductions in peak flood magnitude. Our analyses show the utility of extending the Recruitment Box Model to include multiple species of riparian shrubs and trees, and the effects of post‐colonization flows on their recruitment success. However, some key functional relationships between flow patterns and woody seedling demography (e.g. shear stress thresholds for seedling mortality) have not been adequately quantified and merit further study. Copyright © 2006 John Wiley & Sons, Ltd.     

A Twofold Strategy for Riparian Restoration: Combining a Functional Flow Regime and Direct Seeding to Re‐establish Cottonwoods

The transboundary St Mary River drains Glacier National Park, USA, and was progressively dammed and diverted over the 20th century to support agricultural irrigation in northern Montana and southern Alberta, Canada. Following reduced instream flows, the riparian cottonwoods collapsed, and by 2000, few parental trees remained to provide seeds for cottonwood replenishment. As a novel twofold restoration strategy we: (1) worked with the dam operators to deliver a functional flow regime, a regulated instream flow pattern intended to recover some ecological function and specifically seedling recruitment, and (2) delivered cottonwood seeds by direct spreading and by sticking cuttings with seed catkins to allow gradual seed dispersal. The combination of river regulation and seeding enabled cottonwood colonization, and around 1.5% of the applied seeds produced seedlings after the first summer, at sites without livestock or heavy recreational use. Around 15% of those seedlings survived through the fourth summer, with mortality due to drought stress and flood scour, and establishment and survival were higher for the prairie cottonwood, Populus deltoides, than the narrowleaf cottonwood, Populus angustifolia. This study confirmed that the lack of seed source trees limited cottonwood colonization and demonstrated that the twofold restoration strategy provides promise for severe situations where parental trees have been lost. However, this would require substantial effort, and it would be more efficient to provide survivable instream flow patterns that avoid cottonwood collapse. Copyright © 2015 John Wiley & Sons, Ltd.     

Physical engineering of an island‐braided river by two riparian tree species: Evidence from aerial images and airborne lidar

Following a review of the European distribution, habitat requirements and life history characteristics of two riparian tree species, Alnus incana (L.) Moench (grey alder, a member of the Betulaceae family) and Populus nigra L. (black poplar, a member of the Salicaceae family), we explore their changing spatial distribution and topographic position within a 7 km, island braided reach of the Tagliamento River, Italy, where Populus nigra dominates the woody vegetation cover. Combining field observations and information extracted from aerial images, airborne lidar data and river flow time series for the period 1986–2017, we investigate (a) the changing spatial distribution of all (P. nigra‐dominated) woody vegetation and of A. incana alone; (b) whether river bed topography can be associated with these changing spatial distributions and (c) we consider whether A. incana displays any particular characteristics in its spatial and topographic distribution that may indicate that it is complementing the physical engineering role of P. nigra. We show that A. incana predominantly grows in lines along channel, island and floodplain edges, bordering wooded areas dominated by P. nigra and that areas supporting A. incana are associated with the topographic development of the river bed. We conclude that A. incana appears to be acting as a complementary physical engineer to P. nigra, suggesting that similar complementary physical engineering of river beds may be achieved by species with different life history traits to influence landform development in other river environments.     

Seed banks and seed survival of submersion for an emerging plant invader in the Colorado River system,USA

Soil seed banks along shorelines and the ability of seeds to survive submersion in water are among potential factors that can influence seed supply, a frequent driver of riparian plant invasions. We investigated germinable soil seed banks and the ability of seeds to germinate after different durations of submersion in water for non-native ravennagrass (Saccharum ravennae), an emerging but poorly understood invader of riparian habitats in the American Southwest, including along the Lake Powell section of the Colorado River where we performed the study. We assessed soil seed bank composition in 27 ravennagrass-invaded plots using the emergence technique and evaluated effects on ravennagrass germination of submersion in river water for up to 15 months in a laboratory experiment. Ravennagrass was sparse in soil seed banks, detected in samples from only one of the 27 plots. Instead, soil seed banks were dominated by the non-native annual cheatgrass (Bromus tectorum) and a mixture of native early colonizers (e.g., Pseudognaphalium stramineum) and shrubs (e.g., Baccharis emoryi). Although ravennagrass seed germination was low (<9%) in the submersion experiment, some seeds retained germinability after 15 months of continuous submersion in water. Results suggest that while ravennagrass may have limited on-site, readily germinable soil seed banks, potential implications of its protracted seed viability in water for dispersal along waterways warrants further attention.     

Life‐History Plasticity of Riparian Annual Plants Adapted to Extreme Variations in Water Level: Mesocosm Experiments

The riparian zones of reservoirs associated with regulated rivers in China experience annual fluctuations in water level of up to 30 m that may vary in timing from year to year. Few plant species can tolerate such hydrological perturbation, but short‐lived riparian annuals might be evolutionarily pre‐adapted to such conditions. This study investigated plasticity of life history in four annual species: one typically associated with free‐flowing rivers (Panicum bisulcatum) and three that colonize reservoir margins (Cyperus michelianus, Fimbristylis miliacea and Eclipta prostrata). We found that all four species produced non‐dormant seeds that survived prolonged submergence; germination percentage was independent of the time of exposure by receding waters. Although growth was reduced as a result of shorter growing seasons, all four species completed their life cycles and produced seeds before winter. In addition, P. bisulcatum and C. michelianus allocated biomass to seed production, at the expense of roots and stems, in response to later establishment. All species responded to later establishment with a reduced vegetative growth period before seed production. C. michelianus, F. miliacea and E. prostrate could also delay the onset of flowering time by up to 2 months. P. bisulcatum, a plant that can flower only after exposure to short days, consequently had a fixed flowering time and could accommodate delayed establishment only with a progressively shorter period of vegetative growth. This lower flexibility might explain its absence from reservoir margins. The conceptual framework presented here offers a tool to predict the establishment of vegetation under hydrological disturbance in riparian environments and thereby provides insights into improved restoration practice. Copyright © 2014 John Wiley & Sons, Ltd.     

ECOLOGICAL RESTORATION UNDER PRESSURE FROM INVASIVE ANIMAL SPECIES: USE OF SALICACEAE CUTTINGS IN A RIVER BANK OVERRUN BY COYPU

Numerous studies have established the fundamental ecological role of riparian zones as well as the main threats they face, for example, by invasive species. Our study concerns a section of a gently sloping river in the western part of France where the coypu, an invasive aquatic rodent, damages streambanks and prevents the regeneration of natural vegetation. The aim was to assess the efficacy of different species used in bioengineering projects for streambank protection and for the restoration of woody habitats. A field experiment was established close to the river bank with cuttings of several woody species belonging to the Salicaceae family. The test consisted in comparing the following: (i) different species (six willows and one poplar); (ii) different cutting sizes; and (iii) the presence or absence of fencing (to protect against coypus). The experiment was conducted over three growing seasons (2009–2011). The measured response variables were the survivorship and the growth expressed firstly by the length and the number of shoots, and secondly by the height of plants. The results showed the following: (i) high growth and survivorship rates for all tested species, notwithstanding some significant differences between species; and (ii) positive effects of both the fencing and the planting of longer cuttings. After three growing seasons, Salix viminalis showed the highest growth in exclosures with a mean cumulative shoot length of 7.1 m, whereas outside exclosures S. purpurea had the highest growth with a mean cumulative shoot length of 3.9 m. Populus nigra and S. purpurea showed the lowest number of stems browsed by the coypu. The different methods were also compared for cost and time‐to‐establishment: the long cuttings of certain species afford a positive effect as significant as fencing (which is more expensive). Such analyses are useful when it is necessary to strike a balance between cost and efficiency, particularly in the context of invasiveness. Copyright © 2013 John Wiley & Sons, Ltd.     

No evidence for cottonwood forest decline along a flow-augmented western U.S. river

In contrast to many other arid region rivers, streamflow in the South Platte River is heavily augmented by trans-basin water imports and irrigation return flows. Hydrological changes began in the 1880s, resulting in channel narrowing and the development of a continuous Populus-Salix forest by the mid-twentieth century. We assessed the composition, structure and regeneration status of the riparian forest and identified environmental variables affecting annual Populus deltoides tree growth. We sampled forest structure at four sites in 2015, and conducted dendroecological analysis at seven additional sites in 2019. The riparian forest was dominated by P. deltoides, which occurred at all sites, comprising 79% of total tree basal area and 62% of total tree density. Age structure data indicated ongoing though episodic recruitment of P. deltoides, at least over the past ~130 years. We tested 14 linear mixed effects models to describe the effect of climate and streamflow on individual tree growth (modeled as the log of BAI, n = 237 trees). The most parsimonious model selected with AICc explained 28.6% of BAI variability, and included hydrology and climate factors during the growing season (i.e., June–August streamflow, June–July PDSI), some aspects of off-season (i.e., previous November and March) streamflow, along with tree age and study site effects. The riparian forest developed in response to, and has been maintained by, current climate conditions and water management regimes. It may be negatively affected by future climate change and increased urban water demand in the basin.     

Tree colonization trends on a sediment bar after a major flood

Tree mortality and regeneration in riparian areas are greatly influenced by flooding. The elevational distribution of Salix spp. and Robinia pseudoacacia were investigated by observing densities and standing conditions before and after a major flood on a sediment bar in the middle reaches of the Arakawa River in Kumagaya, Japan. The subsequent tree recruitment was also examined. R. pseudoacacia was easily washed away with the eroded sediment, whereas Salix spp. was found to be more tolerant. Both species were able to survive even after collapsing, provided that their roots were fully or partially embedded in the sediment. Re‐shooting of collapsed trees, rather than emergence from saplings (either by seeds or vegetative fragments), was observed to be the major method of recruitment after the flood. Therefore, tree density before the flood was unchanged, unless the trees were subject to washing away. Salix spp. recruitment was mainly observed in low‐lying areas and R. pseudoacacia in elevated areas. Recruitment from saplings was species‐specific. Salix spp. at high elevations originated mainly from shoot fragments as they need high levels of moisture for seed germination and at higher elevations, sediment moisture content is very low. R. pseudoacacia, on other hand, originated mainly from roots and seeds. At a given elevation, past recruitment patterns indicated that the annual recruitment of trees increased with tree density up to a particular threshold of recruitment density. Further increases in tree density beyond that optimum value resulted in a decline in recruitment. Furthermore, threshold density was observed to increase along with elevation for R. pseudoacacia while declining with Salix spp. Copyright © 2010 John Wiley & Sons, Ltd.     

Secondary seed dispersal in hydro‐fluctuation belts and its influence on the soil seed bank

The completion of the Three Gorges project altered the water level fluctuations from “summer submersion and winter exposure” to “summer exposure and winter submersion.” The reverse seasonal flooding has dramatically changed both the time and duration of exposure and submergence of reservoir banks. Secondary seed dispersal by water is the primary method of dispersal in hydro‐fluctuation belts. This study conducted field sampling and germination experiments to evaluate the effects of reverse seasonal flooding on species composition, diversity, and density of the secondary seed dispersal system and the soil seed bank, and their changes with elevation, in the Xiangzi River. The results indicate that that the secondary seed dispersal system and soil seed bank were associated with 42 and 50 plant species, respectively, and their average seed densities were 1,876.27 and 7,322.18 seeds/m², respectively. The quantity of seeds from secondary seed dispersal accounted for 20.40% of the soil seed bank. The species life‐form composition of both consisted mainly of annual and perennial grasses. Among these grasses, the dominant species in the secondary seed dispersal system were Cynodon dactylon and Paspalum distichum, whereas the dominant species in the soil seed bank were Solanum nigrum and Rumex dentatus. Due to the effects of reverse seasonal water level fluctuations, the species composition and quantity of the soil seed bank and secondary seed dispersal system largely exhibited similar spatial distribution characteristics, but some differences were observed. The species quantity, species diversity index, and seed density in the soil seed bank were the highest in the middle section of the hydro‐fluctuation belt, followed by the top section and then the bottom section. Conversely, the species quantity, species diversity index, and seed density in the secondary seed dispersal system were highest at the top section, followed by the middle section and then the bottom section.     

Hydrodynamic behaviour of European black poplar (Populus nigra L.) under coppice management along Mediterranean river ecosystems

This study examined the hydrodynamic behaviour of European black poplar (Populus nigra L.) under coppice management in riparian areas with a multidisciplinary approach. An innovative procedure on the basis of the combination of plant allometric relations and hydrodynamic models was applied to assess drag forces and plant hydrodynamic bending as function of the basal diameter and module of elasticity, with a probabilistic approach. Cuttings of European black poplar from 2 close riverine environments of Southern Italy have been planted and subjected to the same coppice management strategy. The 2 different 3‐year‐old shoot poplar ensembles exhibited statistically similar morphological traits but stems with different module of elasticity. Drag forces were simulated with a model on the basis of the vegetative Cauchy number. Plant deformation under the hydrodynamic load was modelled as a base‐mounted cantilever beam. The differences in the observed elasticity were verified to be also significant from a hydrodynamic perspective. Diagrams were drawn to describe plant bending, drag forces, and basal momentum as function of basal diameter, accounting for the uncertainty in the module of elasticity. These results can be exploited for identifying objective hydrodynamic criteria to be adopted for coppice management of riparian woody vegetation in human controlled river ecosystems.     

Can the Regeneration of Vegetation from Riparian Seed Banks Support Biogeomorphic Succession and the Geomorphic Recovery of Degraded River Channels?

For rivers degraded by erosion and channel widening, the re‐establishment of riparian vegetation is essential. We assess the potential for riparian seed banks to facilitate natural channel contraction through the regeneration of plants involved in the biogeomorphic succession of three discrete geomorphic units of increasing age and height above the channel bed: bars, benches and floodplain. Standing vegetation upon each unit type was surveyed for four river reaches in the Hunter catchment of eastern Australia. Seed bank composition was determined using seedling emergence techniques on sediment sampled from the units. We compared species richness and composition, and longevity, growth form and seed dispersal mechanisms between the standing vegetation and seed bank species. The seed bank was similar across bars, benches and floodplain, containing mostly perennial pioneer herbs, sedges and rushes, dispersed by wind and hydrochory (water transport). While bar vegetation was similar to the seed bank, bench and floodplain vegetation included later successional species such as shrubs and trees, significantly more grasses and vines (benches: χ²_{5, N = 402} = 102.033, p < 0.001; floodplain: χ²_{5, N = 792} = 30.324, p < 0.001) and higher proportions of unassisted and animal‐dispersed seeds (benches: χ²_{5, N = 352} = 89.409, p < 0.001; floodplain: χ²_{5, N = 338} = 56.026, p < 0.001). The results suggest that seed banks may support early stages of biogeomorphic succession, via regeneration of pioneer plants. However, plants, such as shrubs and trees that are observed upon units of increasing age and height above the channel bed (i.e. benches and floodplain), are likely sourced from transient seeds produced by local vegetation, rather than seed banks. Copyright © 2014 John Wiley & Sons, Ltd.     

Composition of the seed bank in drawdown areas of Navigation Pool 8 of the Upper Mississippi River

In an effort to enhance aquatic plant production and habitat diversity on the Upper Mississippi River (UMR), resource managers considered water level reduction as a management tool to increase the area of emergent and submersed aquatic vegetation by natural seed germination. To quantify the availability of seed, we assessed the potential seed bank of selected areas of Navigation Pool 8 of the UMR from substrate samples collected in spring 2000. We tested these samples for viable seed content under four hydrologic conditions: dry, moist, shallow flooded and submerged. Forty‐seven species were identified in the seed bank, including 27 obligate wetland, 10 facultative wetland and 7 upland species. Dominant taxa within the seed bank included Sagittaria spp., Lindernia dubia, Zosterella dubia, Cyperus spp., Eragrostis spp. and Leersia oryzoides. Of the four hydrologic treatments, moist substrates had the greatest species diversity and were the most productive, yielding an average density of 1420 seedlings m^?2. Emergent and submersed aquatic species were widely distributed, each type occurring in more than 90% of the samples. Timing of seedling germination varied among species and has implications for scheduling drawdowns to promote establishment of desired species. Seed bank results were correlated with the vegetation response on substrates exposed during a reduction of water levels of Pool 8 during summer 2001. Experimentally determining the composition and viability of seed banks from drawdown areas provides information useful in predicting the types of vegetation that may develop on exposed substrates. Further, these findings provide resource managers a better understanding of the potential for achieving desired vegetation response through water level reductions. Published in 2008 by John Wiley & Sons, Ltd.     

Responses of obligate versus facultative riparian shrubs following river damming

Riparian or streamside woodlands include obligate riparian trees and shrubs (obligates) that are restricted to streamside zones, and facultative riparian species that are abundant in, but not restricted to the riparian areas. Due to their distinctive life history requirements, it may be predicted that the ecological specialist obligates would be more vulnerable than the facultative generalists to impacts from river damming and flow regulation. We tested this along the Snake River through Hells Canyon, USA, where two native riparian shrubs dominate: the obligate sandbar willow (Salix exigua), and the facultative, netleaf hackberry (Celtis reticulata). We assessed riparian conditions over the past century by comparing ground‐level and aerial photographs taken after 1907 and in the 1950s in advance of three dams, versus recent conditions. These comparisons revealed three changes downstream from the dams: (1) the depletion of surface sands and sandbars and (2) reductions in sandbar willow versus (3) the proliferation of hackberry in dense bands above the typical high‐water line. The willow decline probably resulted from the depletion of sand following sediment trapping by the reservoirs, combined with changes in the seasonal water flow pattern. The increase in hackberry may have resulted from a beneficial ‘irrigation effect’ of daily water releases for power generation during the summer. The opposing responses reflect the plants' differing life histories and may partially resolve impacts of river regulation on alluvial sediments versus the instream flow pattern. We consider other riparian studies that suggest that obligates such as cottonwoods (Populus angustifolia, P. deltoides and P. fremontii) are highly vulnerable to river regulation, while facultative trees and shrubs such as trembling aspen (Populus tremuloides), wolf‐willow (Elaeagnus commutata) and velvet mesquite (Prosopis velutina) are more resilient. These results suggest that conservation of riparian woodlands should emphasize the ecological specialist obligates, while facultative species may be less vulnerable to river regulation. Copyright © 2009 John Wiley & Sons, Ltd.     

Effects of an enhanced flood on riparian plants of the River Murray,South Australia

In October 2000, the flow of the River Murray entering South Australia was increased from 32 000 to 42 050 ML day^?1 by release of water from an offstream reservoir, and a downstream weir was raised by 500 mm to impound the flood and enhance local floodplain inundation. The flood was maintained for about two weeks, although the duration of inundation was longer at low elevations on the floodplain. Vegetation at three sites was surveyed before and after the flood to examine the impact of inundation on the growth and germination of flood‐tolerant, flood‐dependent and flood‐intolerant species. Among 32 recorded species, Atriplex vesicaria (bladder saltbush, Chenopodiaceae), Sporobolus mitchellii (rats tail couch, Graminae) and Sarcocornia quinqueflora (samphire, Chenopodiaceae) accounted for nearly 82% of the total cover/abundance. Flood‐tolerant and flood‐dependent species (e.g. S. mitchellii) grew and germinated and flood‐intolerant species (e.g. A. vesicaria) senesced. No aquatic plants germinated or established, despite favourable conditions, suggesting an impoverished seed bank or grazing. Based on the growth but lack of germination of flood‐tolerant and flood‐dependent species, the value of small, occasional interventions in environmental flow management may be to maintain existing communities rather than restore degraded ones. Copyright © 2004 John Wiley & Sons, Ltd.     

淮北采煤塌陷区两个小型湖泊轮虫的群落结构特征

于2017年4月、7月、10月和2018年1月对淮北采煤塌陷区东湖和刘桥湖两个小型湖泊进行采样调查,探讨轮虫群落结构的季节变化及其与环境因子的关系。东湖共记录轮虫13科15属32种,刘桥湖共记录10科12属25种。东湖轮虫的年平均密度和生物量(2 722个/L和3.04 mg/L)明显高于刘桥湖(1 311个/L和1.30 mg/L);东湖主要优势种为刺盖异尾轮虫、暗小异尾轮虫、长肢多肢轮虫、长三肢轮虫、广布多肢轮虫和螺形龟甲轮虫,刘桥湖主要优势种为暗小异尾轮虫、长肢多肢轮虫、小链巨头轮虫、广布多肢轮虫、螺形龟甲轮虫和迈氏三肢轮虫;东湖和刘桥湖水体呈中度富营养状态;水温、总氮、总磷和叶绿素a质量浓度是影响两湖轮虫群落结构变化的重要因素,鲢、鳙鱼的捕食压力间接影响了两湖轮虫群落结构的变化。     

Cladocera assemblages from reservoirs in Sri Lanka and their relationship to measured limnological variables

The potential of using surface‐sediment assemblages of Cladocera as bioindicators for reservoirs in Sri Lanka was assessed for their subfossil remains, along with contemporary physical and chemical measurements from each reservoir. The reservoirs span five climatic regions, from extremely arid environments to tropical montane forests, as well as three orders of magnitude changes in many physical and chemical variables. In total, although the remains of 39 Cladocera taxa from 21 genera were identified, only 31 taxa from 37 sites were present at sufficiently high abundances to assess their relation to measured environmental variables. Canonical correspondence analysis (CCA) identified surface area, maximum depth and chloride as the three most important measured environmental variables that could account for the variation in the cladoceran assemblages. Taxa such as Chydorus sphaericus, Alona aff. verrucosa and Leydigia acanthocercoides were more abundant in generally deeper, larger reservoirs, whereas Alonella excisa, Euryalona orientalis, Notoalona globulosa and Chydorus eurynotus were more abundant in shallow smaller reservoirs. Although there was a strong separation between climatic zones in terms of factors related to specific conductance, this factor only appears marginally important in separating cladoceran assemblages. Quantitative inference models developed to assess the strength of inferring environmental variables using partial least squares regression and calibration were all relatively weak, with jackknifed coefficient of determination values of 0.40, 0.28 and 0.27 for surface area, maximum depth and chloride, respectively. These results, in conjunction with large differences in eigenvalues between constrained and unconstrained ordinations, suggest that unmeasured environmental variables are also important in structuring cladoceran assemblages.     

The occurrence of vulnerable seagrass species Halophila beccarii Ascherson, 1871 from restored mangrove of Koduvally Estuary,south‐west coast of India

The ocean turf grass, Halophila beccarii Ascherson, 1871 was recorded for the first time in the intertidal region of the restored mangrove ecosystem of the Koduvally Estuary on the south‐west coast of India. It was found co‐existing in the shallow muddy area of the mangroves, dominated by mangrove trees of Rhizophora mucronata and Sonneratia alba, and macroalgae, Ulva sp., Enteromorpha sp. and Chaetomorpha sp. The meadow is sparsely scattered with shoots near the mangroves, with a mean density of 720 shoots/m². Details regarding the morphology and hydrographic conditions are described. The occurrence of H. beccarii in the Koduvally estuary is an encouraging sign of improved ecosystem health through mangrove restoration. The development of seagrass meadow in the estuary will improve the sediment stability and water quality. A further comprehensive study on the spatiotemporal variation of this species in the estuary is also recommended for conservation and management of this vulnerable seagrass species.     

DRIVERS OF RIPARIAN TREE INVASION ON A DESERT STREAM

Understanding mechanisms of exotic species' invasions is essential to managing riparian landscapes throughout the world. In the southwestern USA, the two most dominant invaders of riparian habitats are the exotic tree species tamarisk (Tamarix ramosissima, Tamarix chinensis, and their hybrids) and Russian olive (Elaeagnus angustifolia). These plants were introduced around 1900, and their success may be facilitated by river regulation, river channel changes, and precipitation patterns. We hypothesized that riparian invasion in Canyon de Chelly, Arizona, was initiated by a change point event such as plantings, dam construction, or channel incision and that establishment near a change point was tied to flood events. We aged tamarisk, Russian olive, and native cottonwood trees from study sites in Canyon de Chelly and used tree ring analysis to determine the year of establishment and the elevation of the germination point relative to the channel. We used Bayesian Poisson regression and information theoretics to identify change points and precipitation variables driving annual tree establishment. We found that most tamarisk and Russian olive trees established in the late 1980s, and most cottonwoods established in 1930‐1950 and 1980‐2000. Regression models indicated that change points occurred in 1983 for Russian olive and 1988 for tamarisk, and precipitation was important for establishment. Although plantings and river regulation probably played a role in tree invasion, our results suggest that these species required precipitation and stream channel change for widespread establishment in Canyon de Chelly. The factors driving riparian invasions may not be those often associated with degraded rivers, such as altered hydrographs and land management changes, thus requiring analyses of the full range of ecological and physical processes. Copyright © 2012 John Wiley & Sons, Ltd.