Environmental and management factors determining weed species composition and diversity in France

Multivariate analysis of data from approximately 700 arable fields from France was carried out to partition the respective importance of environmental factors versus management practices on weed species richness and composition. Overall, canonical correspondence analysis indicated that the major variations in species composition between fields were associated with human management factors; (1) the current crop type and (2) the preceding crop type. Three main weed communities were identified according to sowing season: winter, spring and summer-sown crops. The third most important gradient was associated with soil pH and soil texture to a lesser degree, resulting in highly contrasting weed communities on basic clay soils against those on acidic sandy soils. The influence of climate and geographical region was less pronounced and identified mainly through relationships with precipitation and longitude. Within individual crop types, the effect of other management practices became more prominent. Species richness is dependant on factors other than, or in addition to those influencing species composition, like those describing landscape organisation and/or tillage depth. Species richness (alpha-diversity) and community composition (beta-diversity) had, for example, contrasting relationship to altitude: 300-450 m altitude giving high species richness but low species turnover. The variations observed in this large scale data set help to identify the agricultural practices which have had the most significant impact on the loss of species diversity in arable fields in recent decades. (c) 2008 Elsevier B.V. All rights reserved.