Effect of a Grazing Intensity Gradient on Primary Production and Soil Nitrogen Mineralisation in a Humid Grassland of Western France

Large herbivores have a major influence on the structure and the functions of humid grasslands and especially on primary production. Earlier work on the study site showed that grazing intensity was spatially varied and created a diversity of vegetation patches in the grassland (Loucougaray, 2003). The first objective of this study was to determine whether the variation in grazing intensity led to variation of primary production within the three plant communities located at three topographic levels in the grassland. The second objective was to determine whether a relationship linked primary production variation and net soil nitrogen (N) mineralisation

Analyse de la dynamique végétale selon la nature et l'intensité du pâturage : exemple des marais communaux du marais poitevin

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Variability of nutrient and particulate matter fluxes between the sea and a polder after partial tidal

This paper aims to investigate the patterns of exchanges of nutrients and suspended sediments between the sea and a polder, after partial tidal restoration, and to assess if these are comparable to those observed in natural salt marshes. The study site, situated in the Bay of Veys, in Northwestern France, was embanked in the 1870s and accidentally reconnected to the sea in 1990. Water now flows in and out of the polder by a single communication point with the sea, which facilitated water sampling and flux calculation for dissolved and particulate elements. The study was carried out for two years, from May 2002 to April 2004. Results showed that for all the months studied the water flowing out of the polder had lower concentrations of nitrates and suspended sediments, which lead to a retention of these elements throughout the year. Nitrates uptakes in the polder were much higher in winter (up to 473.9 g N ha(-1) tide(-1)) than in summer where they were close to zero. The retention of suspended sediment could be over 80% of the import and was mainly composed of organic matter. Finally, the concentrations of dissolved organic carbon were higher in outflow than inflow water, but due to unbalanced water budgets this lead to low quantities imported in summer and higher amounts exported for all other seasons. No interpretable pattern was observed for ammonium. The nature of these fluxes, according to literature, is close to those observed in immature salt marshes, so as far as restoration is concerned, it has been shown that partial tidal restoration can allow the restitution of the salt marsh exchange functions that were studied. (c) 2005 Elsevier Ltd. All rights reserved

Numerical classification of the Carici caryophylleae-Genistetea lobelii in Corsica (France)

International audienceThe class ‘Carici caryophylleae-Genistetea lobelii Klein 1972’ corresponds to Cyrno-Sardian oromediterranean cushion-scrubs and related grasslands. In France, this class is only present in Corsica and the syntaxonomic scheme is debated among phytosociologists. This paper aims to highlight the main plant associations of the Carici caryophylleae-Genistetea lobelii Klein 1972 and to define the diagnostic species for each phytosociological unit. Diagnostic species are defined as species with a distinct concentration of occurrence or abundance in a particular vegetation unit. We compiled 519 vegetation plots and we applied the EuroVegChecklist expert system for the classes of European vegetation to only retain vegetation plots belonging to the Carici caryophylleae-Genistetea lobelii. We obtained a dataset with 189 vegetation plots, and we classified them using the Modified TWINSPAN classification. Our analyses identified six plant associations and three sub-associations that have already been described in the literature; and we were able to describe a new alliance corresponding to supramediterranean plant communities (Genistion salzmannii). For each of them, we identified diagnostic, constant, and dominant species and produced their distribution map. Formal definitions were then written for each phytosociological unit (from sub-association to class) and grouped in an expert system to classify the plant communities of the Carici caryophylleae-Genistetea lobelii

The response of Puccinellia maritima to burial: A key to understanding its role in salt‐marsh dynamics?

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After grazing exclusion, is there any modification of strategy for two guerrilla species : Elymus repens (L.) Gould and Agrostis stolonifera (L.)?

Elymus repens (L.) Gould and Agrostis stolonifera (L.), are competitive grasses with guerrilla strategy that invade grasslands with a low stocking rate. In this work, we tested the hypotheses that grazing exclusion facilitates vegetative development of rhizomes and stolons of these clonal grasses and that such change is a key mechanism for their abundance in set-aside grasslands. The competitive capacities of these two guerrilla species were characterised by samples in plant community (species richness and biomass) and on the level of individual species (morphometric measurements on stolons and rhizomes) during a growing season. Compared to grasslands where grazing was excluded for three years, species richness was higher in grazed site and the plant community structure differed. Indeed, with grazing exclusion, a shift from annual species with a diversified growth-form to perennial species with a tall tussock and graminoid growth-form was monitored. In ungrazed situation, Elymus repens and Agrostis stolonifera were the dominant grasses, and the standing biomass for the lowland community showed a significant increase compared to the grazed site. Vegetative development increased competitive capacities of these two guerrilla species and led by phenomenon of competitive exclusion to the disappearance of annuals species. With grazing cessation, Elymus repens was found to increase the size of aerial traits (shoot length and the number of leaves per shoot) and this may both be propitious for achieving dominance within plant communities and also maintaining its competitive local advantage. By contrast, Agrostis stolonifera showed an increase in a root trait, i.e. rhizome length, in the fenced site, which provide good ability for spatial propagation and then to explore adjacent patches. We concluded that Elymus repens presented a morphological capacity to change its colonising strategy from a guerrilla strategy to a phalanx strategy, by morphological variability of aerial organs, when it was submitted to competitive stress and environment modifications. Agrostis stolonifera showed a capacity to escape aerial competition resulting from grazing cessation, than to increase underground propagation capacity. The present study highlighted the capacities of Elymus repens to respond in an adaptative way to competitive pressure