Evolution of vegetation landscapes during the Holocene in the central and downstream Loire basin (Western France)

Nine sites along the downstream and middle section of the Loire river at Cordemais (Loire Atlantique, France) situated in the estuary to Saint-Nicolas-de-Bourgueil (Indre et Loire, France) were investigated. Interdisciplinary studies combined palynology, geography, archaeology, sedimentology and history, thus enabling us to reconstruct the evolution and the long-term response of the Loire valley ecosystem to natural variations and anthropogenic pressure from the Preboreal to the present in an integrated manner. The Atlantic marine transgression (between 7000 and 5000 B.P.) caused the level of brackish water to increase at Oudon between 6740⁺²⁰⁵/_–200 and 5010⁺¹¹⁵/_–100 B.P. This phenomenon, which was the first of its kind to be detected near the central Loire region (approximately 80 km from the current mouth of the Loire river), was accompanied by the development of subhalophile vegetation (Chenopodiaceae) and the appearance of dinoflagellate cysts. A regressive phase occurred during the Subboreal, about 4500 B.P., and led to the erosion of most of the estuarian sediments and to the disappearance of plant species linked to salinity at Oudon: peat deposits built up at most other sites. Human activities had an early effect; moderate deforestation took place at Champtocé about 6600 B.P. as farming was already orientated towards rearing animals. However, possibly cultivated plants were present towards the middle and the end of the Neolithic period (wheat, rye, buckwheat, flax) at about 5600 B.P. and chestnut and walnut were probably exploited in the Loire valley region at about 4600 B.P. The Bronze Age seemed to mark a phase when societies settled down (planting of vineyards) and deforestation peaked from the Gallo-Roman period onwards. The textile industry (flax, hemp), in the context of crop rotation set up during the Iron Age, developed rapidly during the Middle Ages, whereas nowadays the rearing of animals is the dominant activity in the Loire Valley, following the introduction of maize into the region in 1950.     

草地沙化过程地上植被与土壤种子库变化特征

研究了草地沙漠化过程地上植被与土壤种子库的变化特征。获如下结论：(1)草地沙漠化过程地上植被与土壤种子库物种多样性的衰减模式不同,土壤种子库植物种数从潜在沙漠化阶段(固定沙地)到中度沙漠化阶段(半流动沙地)变化很小,而从中度沙漠化到严重沙漠化阶段(流动沙地)衰减速度明显加快;地上植被种数随着沙漠化程度增加而下降,其中从中度沙漠化到严重沙漠化发展阶段衰减幅度最大。(2)地上植被与土壤种子库密度随着沙漠化程度增加而下降,但下降速率因沙漠化发展阶段不同而异,从固定到半固定沙地是地上植被与土壤种子库密度下降最快的时期。(3)地上植被与土壤种子库共有种数随着沙漠化程度的增加而减少,从而导致了地上与土壤种子库群落组成的相异性增大。(4)4种退化沙地土壤种子库组成的相似性要高于地上植被,表明在沙漠化过程中土壤种子库群落组成的稳定性要高于地上植被。(5)地上植被密度与土壤种子库密度存在显著相关性,其关系可用二次曲线来描述。     

Longitudinal distribution of Trichoptera in the Loire River (France): simple ordination methods and community structure

The longitudinal distribution of Trichoptera at 37 sampling sites along 800 km of the Loire River (France) was analyzed in order to better understand large-scale changes in community structure occurring on a stream continuum. Different simple ordination methods — centered PCA and Correspondence Analysis — were performed to investigate these changes. Both analyses showed that a major change occurs in the community in the upper reaches of the Loire River between the Serre de la Fare and Vorey sampling sites with the appearance of Hydropsyche contubernalis and H. Exocellata. Even though other changes in community structure could exist, revealed either by Correspondence Analysis at the first sampling sites (crenal), or by PCA at sampling sites influenced by dams, this Trichoptera community appears to be stable enough in the latter part of the Loire River with the addition of a few species, Hydropsyche bulgaromanorum, Ecnomus tenellus and E. deceptor, rather than changes in species. The results are discussed in the light of different concepts in stream ecology, and the validity of such data analyses to describe some community patterns.     

Sexual reproduction of two alien invasive Ludwigia (Onagraceae) on the middle Loire River,France

Ludwigia peploides (Kunth) Raven ssp. montevidensis (Spreng.) Raven and Ludwigia grandiflora (Michaux) Greuter and Burdet ssp. hexapetala (Hooker and Arn.) Nesom and Kartesz, are expanding their geographic range due to clonal reproduction; these taxa are considered the most important nuisance aquatic plants in French river habitats. During 1970s, these two species have colonized former channels and river banks of the middle Loire River, previously scarcely covered by aquatic vegetation. Now, the exotic Ludwigia build up dense and continuous stands with potentially high impact on biodiversity and sedimentation rates.     

Cloud forests of the Orinoco River Basin (Colombia): Variation in vegetation and soil macrofauna composition along the hydrometeorological gradient

Tropical montane cloud forests (TMCF) in the Orinoco River Basin are vulnerable to climate and regional land-use changes. These changes will force TMCF to migrate upwards, affecting biodiversity conservation and water flow regulation. Here, we evaluate how vegetation and soil macrofauna composition vary along the hydrometeorological gradient driven by an increase in fog incidence with elevation. Vegetation data were collected for all individuals with a diameter at breast height (DBH) > 5 cm in four vegetation plots (5 × 50 m; total: 0.1 ha) every 100 m in altitude between 1700 and 2200 m a.s.l. From each plot, we obtained three soil monoliths from the organic layer and three from the mineral horizon, and manually extracted their soil macrofauna. For these groups, we describe: (1) their compositional changes along the hydrometeorological gradient employing ordination analyses techniques and (2) the relation of the composition changes between vegetation and soil macrofauna communities using a symmetrical co-correspondence analysis. Our results show that the vegetation morphospecies composition and soil macrofauna-order composition vary significantly with the hydrometeorological gradient along elevation. The co-correspondence between vegetation and soil macrofauna reveals a shared breakpoint above the 2000 m a.s.l., where fog is more persistent. Furthermore, we identified eight indicator vegetation species and two soil macrofauna orders associated with specific elevations. These results suggest that under a climate-change-driven fog lift, the TMCF of the Orinoco River Basin will be displaced. Moreover, this study provides a baseline to monitor such displacement.     

Effects of seasonal flooding and grazing on the vegetation of former ricefields in the Rhône delta (Southern France)

Six management regimes were tested during 5 years in 18 abandoned ricefields in the Rh^one delta, France: two artificial floodings for 6 months (winter and summer flooding, 10 cm deep) and a control only flooded by rain, each flooding treatment either with or without grazing by cattle and horses. In the absence of artificial flooding and in presence of grazing by domestic herbivores (i.e., maintaining the initial management since the abandonment) no significant change in plant communities was recorded after 5 years. The vegetation was mainly composed of halophytes (Salicornia fruticosa and Inula crithmoides). The removal of grazing led to the dominance of a salt tolerant grass: Aeluropus littoralis. Flooding favoured the dominance of clonal plants and led to a decrease in the number of species. In the ungrazed fields, changes in plant communities were related to the height of species with Bolboschoenus maritimus and Phragmites australis becoming dominant. When grazing was combined with summer flooding, B. maritimus dominated the first two years of the experiment, but with a low cover, and was replaced in the 3rd year by Typha angustifolia. When grazing was combined with winter and early spring flooding the competitive exclusion of B. maritimus by Juncus gerardii slowed the establishment of the former. The management of former ricefields led to the establishment and dominance of emergent species common to Mediterranean wetlands. Although it is subordinate to the maintenance of artificial flooding, the project may be considered a restoration (or a rehabilitation) of seasonally flooded marshes as original functions existing before the land was put under cultivation are re-established.     

The role of soil mobility in controlling a vegetation gradient

Mature plants of species which characteristically occupy the mobile crests and upper slopes of the central Australian sandridges are more tolerant of sand burial or undercutting than those of the lower slopes and swales. Seedlings growing on the mobile crest areas are susceptible to sand burial, even during wet years. The young shoots of vegetatively spreading species are less susceptible than individual seedlings of the same size.     

The rotifer fauna of the River Loire (France), at the level of the nuclear power plants

In the heated affluents of nuclear power plants of the river Loire, rotifers are abundant. Cosmopolitan species are numerically dominant, but a tropical fauna is also present, among which the genus Brachionus, representing 20% of the total species, is best represented. From a comparison between water upstream and downstream of the power plants, it further appears that in downstream warmed-up waters, some species show an important development, but not in colder upstream waters.     

Seasonal dynamics of the zoobenthic communities in the mesohaline zone of the Loire estuary (France)

The application of correspondence factorial analysis to four types of sediment in the upper reaches of the Loire estuary enabled the significance of environmental factors in the spatial distribution, abundance and seasonal dynamics of the biotic communities to be determined. In coarse sand, the communities are stable but species are few in number and population densities are low. In muddy sediments (from muddy sand to fluid mud) population densities are high but undergo seasonal fluctuations due to changes in the hydrological regime (especially the dissolved oxygen regime). This is true for the composition of the biotic communities. The oligohaline phase, in which the fluvial element is predominant, is characterized by abundance maxima due to freshwater species. The mesohaline phase, in which the marine element is predominant, exerts a restrictive action on population densities; certain species are supplanted by others, their development being governed by the quality of the environment.     

Comparison of landscape diversity in forests of the upper Rhine and the middle Loire floodplains (France)

Traditional measures of biotic indices (, , ) from phytosociological relevés are used to compare diversity at three scale-levels in the upper Rhine valley (Alsace, northeastern region) and the middle Loire (Bourgogne, central region), France. Both study areas included a tributary. The Ill, which is the Rhine tributary in this sector, has no forest communities in common with the main river because of opposite hydrological and edaphic characteristics. This is not the case in the Loire valley, in which the fluvial characteristics are close to those of its tributary, the Allier.Nearly all forest communities (12/14) were shown to be species-rich, due to the role of natural disturbances. Species richness peaks in late-successional forests of the Rhine valley (for example, up to 50 woody species in Querco-Ulmetum). Sixty per cent of the forest-communities are comon to the two floodplains, which explains why 85% of the families, 65% of the genera and 55% of the species are in common. Compared to the Loire valley, the Rhine valleys shows higher biotic diversity. This was related first to its geographical situation, and second to higher landscape heterogeneity. Human disturbance through river management is responsible for changes in species composition as shown by floristic comparison of flooded and unflooded sites. Insularity of natural forests among man-made landscapes is involved in species diversity. In order to preserve the natural diversity of such rich landscapes, floodable areas should be recreated and the attitudes of foresters should evolve accordingly.     

Assessing the conservation value of waterbodies: the example of the Loire floodplain (France)

In recent decades, two of the main management tools used to stem biodiversity erosion have been biodiversity monitoring and the conservation of natural areas. However, socio-economic pressure means that it is not usually possible to preserve the entire landscape, and so the rational prioritisation of sites has become a crucial issue. In this context, and because floodplains are one of the most threatened ecosystems, we propose a statistical strategy for evaluating conservation value, and used it to prioritise 46 waterbodies in the Loire floodplain (France). We began by determining a synthetic conservation index of fish communities (Q) for each waterbody. This synthetic index includes a conservation status index, an origin index, a rarity index and a richness index. We divided the waterbodies into 6 clusters with distinct structures of the basic indices. One of these clusters, with high Q median value, indicated that 4 waterbodies are important for fish biodiversity conservation. Conversely, two clusters with low Q median values included 11 waterbodies where restoration is called for. The results picked out high connectivity levels and low abundance of aquatic vegetation as the two main environmental characteristics of waterbodies with high conservation value. In addition, assessing the biodiversity and conservation value of territories using our multi-index approach plus an a posteriori hierarchical classification methodology reveals two major interests: (i) a possible geographical extension and (ii) a multi-taxa adaptation.     

Prioritization of fish assemblages with a view to conservation and restoration on a large scale European basin,the Loire (France)

The hierarchical organization of important sites for the conservation or the restoration of fish communities is a great challenge for managers, especially because of financial or time constraints. In this perspective, we developed a methodology, which is easy to implement in different locations. Based on the fish assemblage characteristics of the Loire basin (France), we created a synthetic conservation value index including the rarity, the conservation status and the species origin. The relationship between this new synthetic index and the Fish-Based Index allowed us to establish a classification protocol of the sites along the Loire including fish assemblages to be restored or conserved. Sites presenting disturbed fish assemblages, a low rarity index, few threatened species, and a high proportion of non-native species were considered as important for the restoration of fish biodiversity. These sites were found mainly in areas where the assemblages are typical of the bream zone, e.g. with a higher number of eurytopic and limnophilic species. On the contrary, important sites for conservation were defined as having an important conservation potential (high RI, a lot of threatened species, and few non-natives fish species) and an undisturbed fish assemblage similar to the expected community if habitats are undisturbed. Important sites for conservation were found in the Loire basin’s medium reaches which host assemblages typical for the grayling and the barbell zones, e.g. with a higher number of rheophilic species. The synthetic conservation value index could be adapted and completed with other criteria according to management priorities and capacities.     

The influence of seasonal salinity and turbidity maximum variations on the nursery function of the Loire estuary (France)

In the Loire estuary which is a macrotidal estuary, the river flow and tides induce large seasonal variations in salinity gradients and in location and density of the turbidity maximum. These characteristics influence colonization patterns developed by juveniles of euryhaline fish populations (structure, distribution, density) and the dynamics of benthic communities which constitute their food resources. In some hydrological conditions, the upstream limit of the nursery function may be controlled by the turbidity maximum which induces anoxic conditions: reduction in densities or mass mortalities may be observed.     

Persistent soil seed bank and standing vegetation at a high alpine site in the central Chilean Andes

The persistent soil seed bank (viable seeds >1 year) and standing vegetation were investigated in the upper alpine belt (3250 m) in the Andes of central Chile, 33° S. Nine species (eight in standing vegetation) were found in a total persistent seed bank of 899 seeds m⁻². Seven additional species were represented by physically intact, non-viable seeds. Over 90% of the persistent seed bank was concentrated in Montiopsis sericea (Portulacaceae), Pozoa coriacea (Umbelliferae), Phacelia secunda (Hydrophyllaceae) and Oxalis compacta (Oxalidaceae). Examination of the seed/cover ratio revealed different propensities for persistent seed bank formation among species, and annuals formed persistent seed banks more frequently than perennial species. Abundance in the standing vegetation had predictive value for abundance in the persistent seed bank only when non-persistent seed bank species in the standing vegetation were discarded from the analysis. At the local scale, species diversity in the persistent seed bank and standing vegetation were correlated, but compositional similarity was low. Secondary down-slope dispersal promoted by frost heaving in combination with runoff, and life-form correlates are discussed as possible factors accounting for poor correspondence between the persistent seed bank and the standing vegetation at a local scale. The high Andean seed bank is similar to or larger than that reported for two Arctic tundra sites, but smaller than for a northern hemisphere subalpine site. If seed bank size is considered in relation to plant cover, the Andean seed bank greatly exceeds that of one Arctic site. Our study constitutes the first demonstration of a sizable persistent seed bank at an alpine site in the South American Andes and in southern hemisphere temperate mountains in general. Received: 26 May 1998 / Accepted: 21 November 1998     

Factors influencing the spatio-temporal distribution of benthic Microcystis aeruginosa colonies (Cyanobacteria) in the hypertrophic Grangent Reservoir (Loire, France)

The spatio-temporal distribution of benthic colonies of Microcystis aeruginosa in Grangent Reservoir (France) in 2000 was not homogeneous and appeared to be controlled by many external factors: lake depth, station morphometry, substratum and hydraulic regime (lacustrine or fluvial). A most important concentration of benthic colonies was found at deep sites with fine sediment or at sites where the sediment was rich in organic matter. In spite of a stable water level and a minimum flow during summer, the number of benthic colonies showed great variation in the lacustrine downstream part of the reservoir. These variations may be explained by the dynamics of planktonic cyanobacteria.     

Inventory of wild rodents and lagomorphs as natural hosts of Fasciola hepatica on a farm located in a humid area in Loire Atlantique (France)

With the objective of studying the role of wild fauna in the epidemiology of fasciolosis disease, a definitive wild-host inventory was carried out in a french farm where infected domestic hosts (cows) cohabit with wild potential ones. Liver flukes, faecal eggs and antibodies were looked for in lagomorphs (Oryctolagus cuniculus) and rodents (Myocastor coypus, Ondatra zybethicus, Rattus norvegicus, Arvicola sapidus and micromammal species) trapped in the study area. Presence of Fasciola hepatica was detected in two species: O. cuniculus and M. coypus. Infection rates were respectively 34% (42/124) and 55% (106/193). Liver flukes were found in 78 M. coypus (n = 192) and 11 O. cuniculus (n = 35). No other species was infected by F. hepatica. The number of animals shedding fluke eggs was higher in M. coypus (49 out of 127 sampled; 38.6%) than in O. cuniculus (two out of 17 sampled; 11.7%). The results indicate that M. coypus may play a role in the maintenance and the dissemination of F. hepatica in various environments and open a discussion on the role of other natural wild hosts.     

Spatial relationships between the standing vegetation and the soil seed bank in a fire-prone encroached dehesa in Central Spain

Postfire vegetation regeneration in many fire-prone ecosystems is soil seed bank dependent. Although vegetation and seed bank may be spatially structured, the role of prefire vegetation patterns and fire in determining postfire vegetation patterns is poorly known. Here, we investigated the spatial patterning of species abundance and richness in the vegetation and seed bank of a Mediterranean encroached dehesa in Central Spain. The seed bank was studied with and without a heat shock simulating a spatially homogeneous fire. Semivariograms and cross-semivariograms showed that species richness in the vegetation was aggregated in patches, mainly of herbs, with highest values corresponding to high herb cover and low tree cover. Species richness in the seed bank was also structured in patches, but the spatial pattern was weak. Seedling density of germinates in the seed bank also showed weak spatial pattern. Heating increased overall germination and species richness, and the intensity of the spatial pattern of species richness, particularly of herbaceous species. However, seed bank density patterns disappeared after heat shock because of increased germination of shrubs without spatial pattern. Our results document that the spatial structure of plant richness in the vegetation may persist after fire due to the spatial patterns of herbaceous species in the seed bank, and that postfire species richness patterns can arise independently of fire intensity patterns. However, the spatial structure of the vegetation after fire can be altered by the feedback between shrub encroachment and an eventual fire because of the ubiquitous germination of shrubs.     

Pattern development of the vegetation during colonization of a burnt heathland in Brittany (France)

Plant Ecology - The main part of the heathlands in the Paimpont area (Morbihan, and Ille-et-Vilaine, France) was burnt during spring and summer 1976. The pattern development of the vegetation was...     

喀斯特峰丛洼地植被演替对土壤微生物生物量碳、氮及酶活性的影响

采用经典统计分析与通径分析,研究了桂西北喀斯特峰丛洼地4种植被演替阶段(草丛、灌木林、次生林、原生林)表层(0—15 cm)土壤微生物生物量和土壤酶活性的变化特征,探讨了其与土壤理化性质之间的关系。结果表明:土壤微生物生物量和土壤酶活性随植被正向演替的变化规律并不完全一致。土壤微生物生物量碳、微生物生物量氮和碱性磷酸酶活性整体表现为随植被正向演替而增加。而土壤蔗糖酶活性表现为:次生林草丛≈灌木林原生林,脲酶活性表现为:草丛≈次生林≈灌木林原生林。通径分析结果表明,土壤微生物生物量的直接影响因素和主要影响因素为土壤有机碳;蔗糖酶活性的直接影响因素为土壤有机碳和土壤微生物生物量碳,而从总效应来看,各因素对蔗糖酶活性的影响均较小;脲酶和碱性磷酸酶活性的直接影响因素和主要影响因素均为全氮,但全氮对脲酶活性表现为强烈的负效应,而对碱性磷酸酶活性表现为强烈的正效应。此外,土壤微生物生物量碳、氮及蔗糖酶、脲酶和碱性磷酸酶活性的剩余通径系数均较大,说明存在其它未被考虑因素对其具有影响。     

Effects of foliage litter of a pioneer shrub (Artemisia halodendron) on germination from the soil seedbank in a semi-arid sandy grassland in China

Journal of Plant Research - Vegetation recovery during succession is an important process for ecological restoration of the soil, especially in degraded sandy land. However, the driving mechanisms,...