6533b853fe1ef96bd12acc32

RESEARCH PRODUCT

Variations spatiales et temporelles des communautés adventices des cultures annuelles en France

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Although arable weed species are particularly well adapted to habitats characterised by high levels of disturbance(ploughing, harvest), the degree and rate at which agro-ecosytems have been transformed in the last 50 years in Europe(use of chemicals, changes in crop rotations) has led to profound changes in the composition and diversity of arable weedcommunities. This thesis provides an update on the composition and diversity of weed communities that can be foundacross the main crop types and regions of France. It also addresses two objectives i) applying (community) ecologytheories for the analysis and interpretation of spatial and temporal variations in weed communities and ii) using the modelof arable weed community to test fundamental ecological theories. The present work is based on data sets collected atthree embedded spatial scales: the ‘Biovigilance Flore’ network covering the whole of France, the ‘Barralis-Chadoeuf’network covering the Côte-d’Or at 30 year intervals and a set of aggregated fields at the agricultural landscape scale.1. Weed community assembly rules. We show that arable weed communities are not random assemblages ofspecies but respond to i) crop type according to sowing date (winter crops versus spring crops), ii) edaphic and climaticconditions opposing flora of calcareous soils in dry regions on one hand and flora of acidic sandy soils in precipitation-richareas on the other hand, and iii) a latitudinal gradient opposing Mediterranean thermophilous species to eurosiberianspecies. The ‘crop type’ effect can be divided in two main types of constraints: ‘sowing date’ and ‘herbicides’, bothinducing trait convergence in communities. The other management practices (tillage regime) have less discriminatingpower. Overall, human direct influence on weed vegetation is of greater importance than abiotic environment.2. Changes in arable weed flora since the 1970s. The rate of species turnover among the most common speciesover the last 30 years ranged from 33% (winter wheat ) to 61% (maize) which was much higher than turnover ratesreported for secondary successions in undisturbed ecosystems. The ecological meaning of the observed changes wasinterpreted according to species biological and ecological attributes (fonctionnal traits, Ellenberg Indicator value, andspecialization index to crop type) which are used to classify weed species into functional groups. In sunflower crops, weshow that most of the species that have been increasing over the last 30 years belong to a single functional group whichgroups species exhibiting attributes that can defined as ‘sunflower mimicking’ traits: competitiveness (tall and nitrophilousspecies), low sensitivity to sunflower herbicides and short summer life cycle. A second case study focused on changes inthe composition of weed communities in Côte-d’Or. The classification of arable weeds according to their niche breadthshowed that the generalist/specialist ratio significantly increased while in parallel, oilseed rape specialist species increasedin response to the fivefold increase in the surface devoted to this crop.3. Biodiversity decline in agroecosystems. The diachronic study that we conducted over 158 fields in the Côte-d’Orshows that during the last 30 years, averaged species richness and species density per field declined by 44% and 67%,respectively. This trend has however not led to the homogenization of community composition. If specialist species ofparticular ecological conditions decreased, the importance of this process was low relative to the massive decline ofcommon species that were initially shared between many fields. Given the decrease of local (within-field) diversity and theincrease of regional (across-field) diversity, the number of species in the regional pool remained stable. The huge declineof arable weed diversity is not without consequences in terms of the functions provided by the weed community (decreasein functional diversity, massive loss of species that are important food resources for seed-eating birds or insectpollinators). The contribution of crop edges to species diversity at the landscape scale was evaluated and the potential roleof crop edges for the conservation of declining populations of weed species is discussed.Arable weed communities have proved to be a good model for testing fundamental ecological theories, especiallyi) the evolution of the specialist/generalist ratio within communities facing temporal heterogeneity in environmentalconditions and ii) the definition of assembly rules based on the interactions between environmental filters and speciestraits.