Saxifrage bouc et veaux: une histoire d'amour

Version vulgarisée avec compléments de l'article en anglais (Vittoz et al., 2006). Situation et répartition de l'espèce au niveau européen, sociologie de l'espèce dans les différents marais jurassiens, hydrologie et microtopographie, enracinement, sol, rôle du bétail dans la conservation de l'espèce, génétique et quelques pistes pour sa protection

Inhibition of the apple scab pathogen Venturia inaequalis and the grapewine downy mildew pathogen Plasmopara viticola by extracts of green waste compost

Extracts of green waste compost have been shown to inhibit plant diseases. In this study, the factors influencing the mechanism of inhibition of apple scab (Venturia inaequalis) and grapevine downy mildew (Plasmopara viticola) were studied. Extracts were prepared from samples of 30 composts from commercial composting plants. Composts were extracted with 1:2 or 1:5 water for 2 or 7 days. Extracts were applied to seedling of apple and grapevine. The seedlings were artificially inoculated with V. inaequalis or P. viticola, respectively and incubated under controlled conditions. After inoculation, severity of diseases and lesion diameter were measured. The incubation time and the compost/water ratio did not influence the capacity of the extracts to protect the apple plants. All treatments with compost extracts reduced disease severity in both host pathogen systems, and there was no difference in efficacy between autoclaved, sterile filtrated (0. 2 µm) and untreated extracts. From this, we conclude that the inhibition by compost extracts is not linked to their microbial activity. Rinsing apple seedling leaves 1 and 48 hours after application of the compost extracts did not diminish the protective effect against V. inaequalis. On the other hand, the severity of P. viticola increased, when the seedlings were rinsed after the application of compost extracts, and was similar to the untreated control. Compost extracts enhanced in vitro germination of conidia of V. inaequalis and showed no fungicidal effect. Thus, inhibition apparently acts indirectly in this host-pathogen system. On the other hand, the activity of zoospores of P. viticola was inhibited by 70 % compared to the control. The salt content of the extracts and their effect on the zoospores were positively correlated. For this host-pathogen system, there is thus evidence for a direct inhibition by compost extracts. We conclude that the active principle against V. inaequalis and P. viticola must be a water soluble, heat-stable metabolite produced in the compost before its extraction. The mechanism of inhibition in both plant-pathogen systems is different

Soil Microbial Community Changes in Wooded Mountain Pastures due to Simulated Effects of Cattle Grazing

The effect of cattle activity on pastures can be subdivided into three categories of disturbances: herbage removal, dunging and trampling. The objective of this study was to assess separately or in combination the effect of these factors on the potential activities of soil microbial communities and to compare these effects with those of soil properties and plant composition or biomass. Controlled treatments simulating the three factors were applied in a fenced area including a light gradient (sunny and shady situation): (i) repeated mowing; (ii) trampling; (iii) fertilizing with a liquid mixture of dung and urine. In the third year of the experiment, community level physiological profiles (CLPP) (Biolog Ecoplates™) were measured for each plots. Furthermore soil chemical properties (pH, total organic carbon, total nitrogen and total phosphorus), plant species composition and plant biomass were also assessed. Despite differences in plant communities and soil properties, the metabolic potential of the microbial community in the sunny and in the shady situations were similar. Effects of treatments on microbial communities were more pronounced in the sunny than in the shady situation. In both cases, repeated mowing was the first factor retained for explaining functional variations. In contrast, fertilizing was not a significant factor. The vegetation explained a high proportion of variation of the microbial community descriptors in the sunny situation, while no significant variation appeared under shady condition. The three components of cattle activities influenced differently the soil microbial communities and this depended on the light conditions within the wooded pasture. Cattle activities may also change spatially at a fine scale and short-term and induce changes in the microbial community structure. Thus, the shifting mosaic that has been described for the vegetation of pastures may also apply for below-ground microbial communitie

Structure of microbial communities in Sphagnum peatlands and effect of atmospheric carbon dioxide enrichment

Little is known about the structure of microbial communities in Sphagnum peatlands, and the potential effects of the increasing atmospheric C02 concentration on these communities are not known. We analyzed the structure of microbial communities in five Sphagnum-dominated peatlands across Europe and their response to C02 enrichment using miniFACE systems. After three growing seasons, Sphagnum samples were analyzed for heterotrophic bacteria, cyanobacteria, microalgae, heterotrophic flagellates, ciliates, testate amoebae, fungi, nematodes, and rotifers. Heterotrophic organisms dominated the microbial communities and together represented 78% to 97% of the total microbial biomass. Testate amoebae dominated the protozoan biomass. A canonical correspondence analysis revealed a significant correlation between the microbial community data and four environmental variables (Na+, DOC, water table depth, and DIN), reflecting continentality, hydrology, and nitrogen deposition gradients. Carbon dioxide enrichment modified the structure of microbial communities, but total microbial biomass was unaffected. The biomass of heterotrophic bacteria increased by 48%, and the biomass of testate amoebae decreased by 13%. These results contrast with the absence of overall effect on methane production or on the vegetation, but are in line with an increased below-ground vascular plant biomass at the same sites. We interpret the increase in bacterial biomass as a response to a C02-induced enhancement of Sphagnum exudation. The causes for the decrease of testate amoebae are unclear but could indicate a top-down rather than a bottom-up control on their densit

Star Formation Histories in a Cluster Environment at z~0.84

We present a spectrophotometric analysis of galaxies belonging to the dynamically young, massive cluster RX J0152.7-1357 at z~0.84, aimed at understanding the effects of the cluster environment on the star formation history (SFH) of cluster galaxies and the assembly of the red-sequence (RS). We use VLT/FORS spectroscopy, ACS/WFC optical and NTT/SofI near-IR data to characterize SFHs as a function of color, luminosity, morphology, stellar mass, and local environment from a sample of 134 spectroscopic members. In order to increase the signal-to-noise, individual galaxy spectra are stacked according to these properties. Moreover, the D4000, Balmer, CN3883, Fe4383 and C4668 indices are also quantified. The SFH analysis shows that galaxies in the blue faint-end of the RS have on average younger stars (Delta t ~ 2 Gyr) than those in the red bright-end. We also found, for a given luminosity range, differences in age (Delta t ~ 0.5 - 1.3 Gyr) as a function of color, indicating that the intrinsic scatter of the RS may be due to age variations. Passive galaxies in the blue faint-end of the RS are preferentially located in the low density areas of the cluster, likely being objects entering the RS from the "blue cloud". It is likely that the quenching of the star formation of these RS galaxies is due to interaction with the intracluster medium. Furthermore, the SFH of galaxies in the RS as a function of stellar mass reveals signatures of "downsizing" in the overall cluster.Comment: 36 pages, 5 tables, 14 figures. Accepted for publication in The Astrophysical Journa

Situation de la vigne sauvage Vitis vinifera ssp. silvestris en Europe

Cette contribution dresse un bilan de la répartition actuelle de la vigne sauvage européenne - Vitis vinifera L. ssp. silvestris (Gmelin) Hegi - sur la base d'une revue bibliographique et d'observations originales effectuées lors de recherches sur son écologie dans les forêts alluviales et colluviales d'Europe. La situation de la vigne sauvage s'avère préoccupante dans la plupart des pays européens, même si elle est souvent mal connue, faute de données récentes fiables. La sous-espèce silvestris se distingue essentiellement des cépages cultivés par sa dioїcite. Les caractères morphologiques végétatifs sont très variables; en particulier, on montre que le dimorphisme sexuel est manifeste dans les populations d'Autriche, alors qu'il n'est pas apparent dans celles du Pays Basque.Occurrence of the wild vine Vitis vinifera ssp. silvestris in EuropeThis paper summarizes the occurrence of the European wild vine - Vitis vinifera L. ssp. silvestris (Gmelin) Hegi - from bibliography and observations made in the framework of research on its ecology in the alluvial and colluvial forests of Europe. The state of the wild vine is disquieting within most of the European countries; in some countries we noticed a lack of recent reliable data. The subspecies silvestris can principally be distinguished from cultivated grapevine by its dioicity. Morphological vegetative criteria are very variable; in particular, we demonstrate that sexual dimorphism occurs in Austrian populations, while it is not found in those of the Basque countries

Soil Phosphorus Uptake by Continuously Cropped Lupinus albus: A New Microcosm Design

When grown in soils with sparingly available phosphorus (P), white lupin (Lupinus albus L.) forms special root structures, called cluster roots, which secrete large amounts of organic acids and concomitantly acidify the rhizosphere. Many studies dealing with the understanding of this P acquisition strategy have been performed in short time experiments either in hydroponic cultures or in small microcosm designs with sand or sand:soil mixtures. In the present study, we applied an experimental design which came nearer to the natural field conditions: we performed a one-year experiment on large microcosms containing 7kg of soil and allowing separation of rhizosphere soil and bulk soil. We planted six successive generations of lupins and analysed P uptake, organic P desorption, phosphatase activities and organic acid concentrations in different soil samples along a spatio-temporal gradient. We compared the rhizosphere soil samples of cluster (RSC) and non-cluster roots (RSNC) as well as the bulk soil (BS) samples. A total shoot biomass of 55.69 ± 1.51g(d.w.)y−1 was produced and P uptake reached 220.59 ± 5.99mgy−1. More P was desorbed from RSC than from RSNC or BS (P < 0.05). RSC and RSNC showed a higher activity of acid and alkaline phosphatases than BS samples and a higher acid phosphatase activity was observed in RSC than in RSNC throughout the one-year experiment. Fumarate was the most abundant organic acid in all rhizosphere soil samples. Citrate was only present in detectable amounts in RSC while malate and fumarate were recovered from both RSC and RSNC. Almost no organic acids could be detected in the BS samples. Our results demonstrated that over a one-year cultivation period in the absence of an external P supply, white lupin was able to acquire phosphate from the soil and that the processes leading to this P uptake took place preferentially in the rhizosphere of cluster root

The Red Sequence of High-Redshift Clusters: a Comparison with Cosmological Galaxy Formation Models

We compare the results from a semi-analytic model of galaxy formation with spectro-photometric observations of distant galaxy clusters observed in the range 0.8< z< 1.3. We investigate the properties of their red sequence (RS) galaxies and compare them with those of the field at the same redshift. In our model we find that i) a well-defined, narrow RS is obtained already by z= 1.2; this is found to be more populated than the field RS, analogously to what observed and predicted at z=0; ii) the predicted U-V rest-frame colors and scatter of the cluster RS at z=1.2 have average values of 1 and 0.15 respectively, with a cluster-to-cluster variance of 0.2 and 0.06, respectively. The scatter of the RS of cluster galaxies is around 5 times smaller than the corresponding field value; iii) when the RS galaxies are considered, the mass growth histories of field and cluster galaxies at z=1.2 are similar, with 90 % of the stellar mass of RS galaxies at z=1.2 already formed at cosmic times t=2.5 Gyr, and 50 % at t=1 Gyr; v) the predicted distribution of stellar ages of RS galaxies at z=1.2 peaks at 3.7 Gyr for both cluster and field populations; however, for the latter the distribution is significantly skewed toward lower ages. When compared with observations, the above findings show an overall consistency, although the average value 0.07 of the observed cluster RS scatter (U-V colors) at z=1.2 is smaller than the corresponding model central value. We discuss the physical origin and the significance of the above results in the framework of cosmological galaxy formation.Comment: 14 pages, accepted for publication in ApJ. Updated one referenc

A z~3 radio galaxy and its protocluster: evidence for a superstructure?

We present spectroscopic follow-up observations of Lyman Break Galaxies (LBGs) selected in the field surrounding the radio galaxy MRC0316-257 at z~3.13 (0316). Robust spectroscopic redshifts are determined for 20 out of 24 objects. Three of the spectroscopically confirmed galaxies have 3.12<z<3.13 indicating that these objects reside in a protocluster structure previously found around the radio galaxy. An additional 5 objects are found 1600 km/s blue-shifted with respect to the main protocluster structure. This is in addition to three [OIII] emitters found at this redshift in a previous study. This is further evidence that a structure exists directly in front of the 0316 protocluster. We estimate that the foreground structure is responsible for half of the surface overdensity of LBGs found in the field as a whole. The foreground structure is associated with a strong surface density peak 1.4 Mpc to the North-West of the radio galaxy and a 2D Kolmogorov-Smirnov test indicates that the spatial distributions of the 0316 and foreground galaxies differ at the 3 sigma level. In addition, we compare the properties of protocluster, foreground structure and field galaxies, but we find no significant differences. In terms of the nature of the two structures, a merger scenario is a possible option. Simple merger dynamics indicates that the observed relative velocity of 1600 km/s can be reproduced if the two structures have masses of ~5x10^14 Msun and have starting separations of around 2.5 to 3 Mpc. It is also possible that the foreground structure is unrelated to the 0316 protocluster in which case the two structures will not interact before z=0.Comment: 13 pages, 6 figures, accepted for publication in MNRA