François Mesléard

Salt tolerance in Phragmites australis populations from coastal Mediterranean marshes

Responses to salt in soil interstitial water were studied experimentally for nine populations of Phragmites australis (Cav.) Trin. ex Steud. from coastal Mediterranean marshes in France. The effects of exposure to salinity ranging from 0 to 25‰ were measured on germination of seeds collected in each site. Germination was up to 100% and was affected by salinity above 10‰. Variation in germination among populations was small in freshwater (ranging from 89 to 99% on average) but saline conditions increased the contrast (5–60% at 25‰). Growth of seedlings in shoot and biomass production were measured during a 2-month exposure to constant saline solutions, ranging from 0 to 20‰. The effects on the number and length of shoots from a 25-day temporary exposure at 25‰ followed by recovery in freshwater were measured. Growth decreased with increasing salinity (50% decrease at 7.5‰ when compared to freshwater) and 7–100% mortality depending on population, occurred at 15 and 20‰. Cumulative shoot length and biomass production varied significantly among populations in freshwater, but the difference was small in saline conditions, significant for biomass but not for length. The 25-day exposure at 25‰ stopped growth and plants recovered after flushing with freshwater. The stress effect was significant for shoot length but not for shoot number. Twenty-five days after the treatment ended, the numbers of shoots were not different between exposed seedlings and controls. The temporary stress suppressed the differences among populations but they were restored during subsequent growth in freshwater. None of the variation patterns observed during the experiment were related to the present condition of the site of origin.

Continuous basal sprouting from a lignotuber: Arbutus unedo L. and Erica arborea L., as woody Mediterranean examples

SummaryThe two dominant species of the Corsican mattoral,Arbutus unedo L. andErica arborea L., can produce abundant sprouts from the lignotuber not only immediately after fire but also more or less continuously in the absence of major disturbance. The lignotuber appears to be more important during the early stages of development; the result is an increase in the number of sprouts during the 25 years following the establishment of the individuals. Later the lignotuber seems to lose the ability to ensure the development of new basal sprouts. A hypothesis is that the presence of a lignotuber is related to the growth form.Arbutus unedo andErica arborea show behaviour intermediate between acrotony and basitony, as the shoots show acrotony, and continuous sprouting is characteristic of basitonic species. The fact that sprouting from the lignotuber is not necessarily a result of fire suggests that the relation between fire and vegetation in the Mediterranean region should be reconsidered.

Effects of insecticide applications on macroinvertebrate density and biomass in rice-fields in the Rhône-delta, France

The density of 23 macroinvertebrate species and the total macroinvertebrate biomass were compared between rice-fields treated with lindane and diazinon in June and alphamethine in August and untreated controls. The macroinvertebrates could be divided into four groups: (1) Taxa, in which the densities were lower in the insecticide treatment in July and August than in the non-insecticide treatment. (2) The Culicidae which occurred in the insecticide treatment in significantly lower density in July, but in significantly higher density in August. (3) Ischnura elegans (Vander L.) which was found in July after the lindane application in significantly higher numbers in the insecticide treatments, but in significantly lower numbers in the insecticide treatment in August after the application of the pyrethroid. In these three groups, we assumed that direct effects due to the insecticides toxicity were the reason for the differences in density. (4) The fourth group included three taxa in which the densities were significantly higher in the insecticide treatment in July and August than in the control. For this, indirect effects due to reduced biotic interactions may be responsible. The biomass was higher in the insecticide treatments in July, mainly because of a high increase in gastropod density, during the rest of the season it was similar between treatments and controls.

Plant community succession in a coastal wetland after abandonment of cultivation: the example of the Rhone delta

A synchronous study was carried out of plant succession in land abandoned after cultivation in the Camargue (southern France) in relation to the main biotic and abiotic environmental factors. Correlations between environmental factors and species abundance were established using Canonical correspondence analysis. The most strongly correlated variables were those of the water regime. Abandonment of cultivation does not always imply abandonment of management, but a change in land use. Water level management (flooding in winter, or in summer, irrigation) creates and maintains communities suitable for grazing. In the absence of water level management, the soils become saline and stable plant communities similar to those present under natural conditions quickly develop. The salt generally limits the installation of woody plants; only Phillyrea angustifolia can develop and then only when the water stable is quite deep. It appears that the forest stage can only be reached in the proximity of riverine gallery forest and irrigation canals where Populus alba is dominant.

Effects of cropping practices on the use of rice fields by waterbirds in the Camargue, France

The abundance of waterbirds was compared in 46 rice fields in relation with the age of the field and the planting practices in spring. Of the 29 species censused, Charadriiformes represented some 73%, Ciconiiformes 6%, and Anseriformes less than 15.5% of the total number of individuals, flamingos and moorhens being also observed. Insectivorous species were the main component of the waterbird community. Bird numbers decreased with increasing field age and were lower in dry-sown than in wet-sown fields. The present results suggest that intensive and/or repetitive soil management and pesticide use may decrease the food resources available to waterbirds.

Plant trait–digestibility relationships across management and climate gradients in permanent grasslands

1. Dry matter digestibility is a critical component of herbage nutritive value, a major service delivered by grasslands. The aim of this study was to test whether the dominance hypothesis applies to assess the impacts of environmental gradients and management regimes on thiscomponent of herbage nutritive value in permanent grasslands. 2. At the plant level, digestibility has been related to a number of functional traits, but whether this can be scaled up to the community level in species-rich grasslands and how such relationships are modulated by environmental conditions and management regimes remainunknown. Our primary objective was to test whether community-weighted means – species trait values weighted by the species abundance – of morphological, phenological and chemical traits could be used to explain variations in digestibility over a large range of climatic contexts,soil resource levels and management regimes. Our second objective was to explain variations in community digestibility within and among nine contrasting sites along large natural and man-induced environmental gradients.3. Over the whole data set, digestibility and most community-weighted means of traits responded to climatic factors and management regimes, but relations were not always significant when each site was considered separately. Community digestibility was significantly related to one or more plant traits within each site and to all of the measured traits when considering all the sites. Leaf dry matter content (LDMC) had the most consistent effects on digestibility, with a strikingly similar negative effect within each site. Potential evapotranspiration was negatively related to digestibility and contributed to explain a large part of the among-site variance. In addition, a low return interval of disturbance and a high disturbance intensity (biomass removal) were both associated with a high digestibility.4. Synthesis and applications. Disturbance regime, plant traits and local climate impacted dry matter digestibility roughly equally in grasslands. The effects of community composition on digestibility and its response to abiotic factors could be successfully captured by community weightedmeans of leaf dry matter content. This functional marker can be used to develop indicators and grassland management rules to support farmers in the refinement of their practices towards specific needs, such as target production outputs.

Competition between an introduced and an indigenous species: the case of Paspalum paspalodes (Michx) Schribner and Aeluropus littoralis (Gouan) in the Camargue (southern France).

Paspalum paspalodes, an introduced grass species, and Aeluropus littoralis, an indigenous species, develop abundantly in seasonally-flooded marshes in the Camargue (Rhône Delta, France). Although they occur together in many multispecies communities, neither species occurs when the other is dominat. The cultivation of cuttings of P. paspalodes and A. littoralis in a replacement series in a combination of five proportions (0/100, 25/75, 50/50, 75/25 and 100/0) and four salinities (0,2 4, and 6 g Cl- · 1-1) gave contrasting results for the two species: (1) strong asymmetrical competition in favour of P. paspalodes at 0 g Cl- · 1-1, (2) no significant effect of salinity on the mean above-ground and underground yields per plant for A. littoralis over the range tested, (3) a major decrease in the mean above-ground and belowground yields per plant for P. paspalodes with increasing salinity, (4) a reversal of the competitive balance between the species with increasing salinity. The cultivation of cuttings at high temperatures in a greenhouse in a combination of the same five proportions at two salinities (0 and 4 g Cl- · 1-1) refuted the hypothesis that the introduced species is better adapted to summer temperatures. Because it is not salt-tolerant, P. paspalodes cannot be considered as a potentially invasive species in the Camargue. Its abundance depends on newly created and artificially maintained habitats.

Impact of grazing on the species richness of plant communities in Mediterranean temporary pools (western Morocco)

The impact of grazing on the vegetation of Moroccan temporary pools has been studied at 2 scales: regional (inter-pools) and local (intra-pools). Half of the 16 forest pools studied is located in a reserve and ungrazed. The other half, located within public forest, is grazed. Vegetation relevés coupled to water-depths measurements were carried out in each pool. The results showed a significant effect of grazing on both scales of analysis. This effect was found in the species composition of the vegetation, which differed between the 2 types of pools, and in the lower species richness and abundance of plant species in the grazed pools. These differences are interpreted as resulting from the selection by herbivores and the differential tolerance of species to disturbance. These impacts are likely to expose certain species to local extinction by reducing their populations.

Alfalfa Leaf Curl Virus: an Aphid-Transmitted Geminivirus

ABSTRACT The family Geminiviridae comprises seven genera differentiated by genome organization, sequence similarity, and insect vector. Capulavirus, an eighth genus, has been proposed to accommodate two newly discovered highly divergent geminiviruses that presently have no known vector. Alfalfa leaf curl virus, identified here as a third capulavirus, is shown to be transmitted by Aphis craccivora. This is the first report of an aphid-transmitted geminivirus.

Geometagenomics illuminates the impact of agriculture on the distribution and prevalence of plant viruses at the ecosystem scale

Disease emergence events regularly result from human activities such as agriculture, which frequently brings large populations of genetically uniform hosts into contact with potential pathogens. Although viruses cause nearly 50% of emerging plant diseases, there is little systematic information about virus distribution across agro-ecological interfaces and large gaps in understanding of virus diversity in nature. Here we applied a novel landscape-scale geometagenomics approach to examine relationships between agricultural land use and distributions of plant-associated viruses in two Mediterranean-climate biodiversity hotspots (Western Cape region of South Africa and Rhône river delta region of France). In total, we analysed 1725 geo-referenced plant samples collected over two years from 4.5 × 4.5 km2 grids spanning farmlands and adjacent uncultivated vegetation. We found substantial virus prevalence (25.8–35.7%) in all ecosystems, but prevalence and identified family-level virus diversity were greatest in cultivated areas, with some virus families displaying strong agricultural associations. Our survey revealed 94 previously unknown virus species, primarily from uncultivated plants. This is the first effort to systematically evaluate plant-associated viromes across broad agro-ecological interfaces. Our findings indicate that agriculture substantially influences plant virus distributions and highlight the extent of current ignorance about the diversity and roles of viruses in nature.