Succession secondaire et perte de diversité végétale après réduction du broutage dans un pâturage boisé des Alpes centrales suisses

Freléchoux F., Meisser M. and Gillet F. 2007. Secondary succession and loss in plant diversity following a grazing decrease in a wooded pasture of the central Swiss Alps. Bot. Helv. 117: 37 - 56. Reduced cattle grazing pressure in the Alps has caused the reforestation of many subalpine pastures during the last decades. To understand the dynamics of natural reforestation and to evaluate how this change affects plant species diversity, we described the vegetation of a wooded pasture in the central Swiss Alps (Sembrancher, Valais) using the integrated synusial method. Based on stratified vegetation relevés in 27 plots,we defined 11 community types at the synusial level (two tree-layer, five shrub-layer, and four herb-layer synusiae), and four community types at the phytocoenosis level (pasture, tall forbs and scrub, wooded pasture and forest). The spatial distribution of these four phytocoenoses suggests that they represent successional stages after abandonment, and that the pathway of vegetation succession depends on the aspect. We suppose that on northern oriented, cool and shady locations, abandoned pastures first develop towards tall-forb meadows and scrub with Alnus viridis, and then to a preforested stage with Picea abies and Larix decidua. In contrast, on western oriented, warm and sunny location, Larix decidua (mainly) and Picea abies directly colonize the abandoned pastures, but further succession finally leads to the same pre-forested stage as on northern slopes. Plant species richness was highest in open areas and decreased by 25% as tree cover increased from 6% to 65%. According to our successional model, plant species diversity is lost more rapidly on northern slopes (with species-poor green alder scrub) than on western slopes (with species-rich young larch forests), suggesting that northern slopes most urgently need an appropriate grazing managemen

Succession secondaire et perte de diversité végétale après réduction du broutage dans un pâturage boisé des Alpes centrales suisses

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Impact of drought on the functioning of grassland systems

A rainfall manipulation experiment was conducted over a two-year period on two semi-natural grassland sites, in order to determine the effects of drought on both forage production (dry matter and nutritive value) and plant-soil relationships. Water stress simulations were performed with the aid of rainout shelters. Among the nutritional parameters, lignocellulose content (ADF) and water-soluble carbohydrates (WSC) showed the greatest variation in drought conditions. Variations in ADF content were strongly linked to the drought-induced yield losses (less fibre in the small plants). By contrast, crude protein (CP) content remained fairly constant under drought conditions due to antagonistic processes: the detrimental effects on nitrogen nutrition were offset by slower plant growth (i.e. higher plant N concentration due to reduced shoot biomass). The phosphorus cycle (P cycle) was adversely affected by the water restrictions, with quite different responses depending on soil P content. A late stress (i.e. one occurring after the grass growth peak) had more-pronounced effects than an early one (occurring during the peak). This experiment allows the effects of water shortage to be placed in a broader context by showing that the variations in yield and quality caused by drought are of same order of magnitude as the natural variations that can be observed between different years

Drought-induced shifts in plants traits, yields and nutritive value under realistic grazing and mowing managements in a mountain grassland

Extreme drought events can dramatically impact grassland ecosystems, causing potential loss of forage production for livestock in temperate grasslands. However, knowledge of drought effects on forage production, nutritive value and plant community stability in the real context of farming management is scarce. For this purpose, the effect of a simulated summer drought was studied under two realistic management types on a semi-natural grassland in the Swiss Jura mountains. The first management ("grazing") consisted in six consecutive utilizations by animals over the growing season, representing a rotational grazing system as regionally conducted. The second management ("mowing") consisted of three harvests, corresponding to the usual mowing frequency of hay meadows. In both managements, drought caused minor short-term modifications of species composition and almost no persistent effects were observed. Besides, important short-term changes were observed in community weighted mean of several key functional traits, reflecting a strong decline in community growth during the drought followed by a partial recovery two months later. Forage yields, and to a lesser extent its nutritive value, thus declined during the drought period. Both were still affected in the following months, but had recovered in spring of the following year. Forage loss was twofold higher in the grazing management, but recovery as well as nutritive value was slightly improved in this management. The results suggest that rotational grazing can amplify negative drought impacts, compared to traditional mowing, and highlight the need to adapt such management in dry years. They also demonstrate that, even under a fairly intensive management, resilience of such mountain grasslands after one extreme drought event can be expected to be high, with no persistent changes in species and trait compositions. (C) 2015 Elsevier B.V. All rights reserved

Hydrokenopyrochlore, (0,#)2Nb2O6·H2O, a new species of the pyrochlore supergroup from the sahatany pegmatite field, Antananarivo province, Madagascar

The new mineral species hydrokenopyrochlore, ideallyA(0,#)2BNb2XO6·YH2O, has been discovered in the Antandrokomby pegmatite, Sahatany Pegmatite Field, Antananarivo Province, Madagascar. It occurs as tan to beige subhedral crystals, up to 1 mm in size, with a resinous luster, associated with quartz, tourmaline, orthoclase, Li-bearing mica, hübnerite, stibiotantalite, and an undetermined heftetjernite-like mineral. Electron-microprobe analysis of hydrokenopyrochlore yielded (in wt%): WO3 8.14, Sb2O5(tot) 14.33, Nb2O5 44.09, Ta2O5 13.97, SiO2 0.51, SnO2 0.21, CaO 0.86, MnO 0.04, Na2O 1.79, Cs2O 14.47, H2Ocalc 2.23, total 100.64. On the basis of 2 B-site cations per formula unit and by recalculating the Sb2O3:Sb2O5 ratio in agreement with structural data, the chemical formula of hydrokenopyrochlore is (1.32Sb3þ0.35Na0.26Ca0.07)S2(Nb1.47Ta0.28W0.16Sb5þ0.05Si0.04)S2.00O6[(H2O)0.55 Cs0.45]. The main diffraction lines, corresponding to multiple hkl indices, are [d in A (visually estimated relative intensity) hkl]: 3.136 (s) 311, 3.006 (s) 222, 2.010 (ms) 333/511, 1.846 (s) 440, and 1.588 (ms) 622. The crystal structure study returned a cubic unit cell, space group Fd3m, with a = 10.4887(8) A, V = 1153.9(3) A3, Z = 8. The crystal structure has been solved and refined to R1 = 0.056 on the basis of 105 unique reflections with Fo > 4s(Fo) and 14 refined parameters. The crystal structure is formed by a framework of BO6 octahedra, as is typical of the members of the pyrochlore supergroup. The B site has a mixed (Nb, Ta, W) occupancy. The A site is mainly vacant, whereas the Y site is occupied by both H2O groups and Cs. The high Cs content of hydrokenopyrochlore seems to be indicative of the central miarolitic and most evolved portions of the pegmatitic dikes in the Sahatany Pegmatite Field

Nutrient limitations induced by drought affect forage N and P differently in two permanent grasslands

Drought events can strongly affect ecosystem functioning by modifying relationship between plants, microbes and soil chemistry, with consequent impacts on nutrient cycling. However, the potential impacts of a soil moisture reduction on the nitrogen (N) and phosphorus (P) cycling in grasslands remain poorly understood, especially in regard to. forage production.To fill this knowledge gap, a drought experiment was carried out using rainout shelters in two permanent grasslands, characterized by similar vegetation communities but contrasted soil nutrient limitations. Drought treatments were applied during two months, either when plant growth was highest (Early-season drought) or after the peak of biomass production (Late-season drought). Dry matter production, forage N status (NNI) and P content as well as N and P contents in microbial biomass and soil were determined.Both early and late-season drought significantly reduced soil moisture during the vegetation growth period. Forage yield was also reduced by drought, but only when it occurred late in the season. Using a structural equation model, we showed that soil moisture reduction had a direct effect on forage N status, suggesting that water shortage induced lower transpiration and water fluxes. Soil moisture reduction also affected forage P by reducing the availability of soil P. However, other mechanisms played a larger role and were site-specific. At the more fertile site, reduction in soil moisture directly impaired forage P, suggesting that water stress mainly resulted in lower diffusion rates to roots, while at the less fertile site, an indirect reduction of forage P through a pathway implying microbes (decrease in microbial P) was detected.Our results suggest that the two grasslands suffered mainly from water shortage per se, but also from drought induced nutrient deficiency (mainly P), which amplified yield losses and further decreased forage quality. Overall, our findings emphasize the need for further research on the plant-soil-microbe system functioning, in order to secure a sustainable and resilient forage production in the context of climate change