The comparison of arable weed communities in Mediterranean (i.e. in Catalonia) and temperate (i.e. in Lower Saxony) agroecosystems revealed very similar patterns in additive diversity components as affected by farming practices, despite substantial differences in species composition across these contrasting regions.
α-, β- and γ-diversity in contrasting regions
Mediterranean climatic conditions, with irregular, stormy rainfall and dry, hot summers, have selected a particular arable weed flora. This flora differs from those of humid and temperate climates (Guillerm & Maillet 1982; Nekola & White 1999; Fried et al. 2008). Nevertheless, the α-, β- and γ-diversity were two to three times higher in organic farms compared to conventional ones despite the marked differences in the species composition of the arable weed communities in the two study regions. The inputs of herbicides and mineral fertilizers and the less-diverse rotational schemes in conventionally managed arable fields have often been shown to reduce weed species richness (Benton et al. 2003), whereas chemical-free management and more complex crop rotations in organic fields can favour species-rich weed communities (Hole et al. 2005; Romero et al. 2008), as shown in our study for broad-leaves, legumes and grasses.
A pool of broad-leaves occurred at high frequencies in both farming systems. These species included ruderal and generalist, widespread arable weeds, such as Fumaria officinalis L. (common fumitory), Galium aparine L. (goosegrass), Myosotis arvensis (L.) Hill (field forget-me-not), Papaver rhoeas L. (corn poppy), Stellaria media (L.) Vill. (common chickweed), Veronica hederifolia L. (ivy-leaved speedwell) and Viola tricolor L. (European wild pansy); perennials, such as Cirsium arvense (L.) Scop. (creeping thistle); or ruderal species with long-persistent seedbanks, such as Polygonum aviculare L. (prostrate knotweed). These generally tolerant species are less strongly affected by intensive management practices and are well known to become increasingly dominant, thereby causing agricultural problems (Tiley 2010; Torra et al. 2010). In contrast, most of the species that only occurred on organic farms were characterized by a low percentage of cover and low frequencies of occurrence, i.e. they were relatively rare. For example, Bifora testiculata (L.) Roth (European bishop), Coronilla scorpioides (L.) Koch (yellow crown vetch), Ranunculus arvensis L. (corn buttercup) and Kickxia spuria (L.) Dumort. (roundleaf fluellen) could only be found on organic farms in Catalonia, whereas Ranunculus repens L. (creeping buttercup), Mentha arvensis L. (wild mint) and Anagallis arvensis L. (scarlet pimpernel) could only be found on organic farms in Lower Saxony. Apparently, these species are sensitive to intensive agricultural practices. Diverse arable weed communities may therefore experience a marked homogenization under highly intensive farming practices (McKinney & Lockwood 1999; Albrecht 2003). The higher α-diversity of broad-leaves encountered in Catalonia could also be related to the relatively low intensity of historical agricultural production compared with the situation in Lower Saxony (Suárez et al. 2003).
The low contribution of legumes, especially on conventionally managed farms, was a common pattern in both regions. In our survey, the number and frequency of legumes substantially increased on organic farms [e.g. Medicago lupulina L. (black medick), Medicago polymorpha L. (bur clover), Vicia cracca L. (bird vetch) and Vicia hirsuta (L.) Gray (hairy vetch)]. However, this pattern was dominated primarily by the presence of cultivated species, such as Medicago sativa L. (alfalfa), Vicia ervilia (L.) Willd. (blister vetch), Vicia faba L. (broadbean), and Trifolium sp. (clover). According to van Elsen (2000), the cessation of mineral nitrogen fertilization may restore the presence of legumes in organic farming. However, the occurrence of legumes in organic fields may not only result from the abandonment of mineral fertilizers but could also be influenced by other farming practices, such as more complex rotations compared to conventional fields or the re-use of crop seeds, which allow the maintenance of weed diversity (Armengot et al. 2011).
The number of grass species was relatively low compared to total species richness in both regions, but grasses were among the most frequently recorded species. This result is consistent with the findings of Daehler (1998) that species of the plant family Poaceae were over-represented among both serious and widespread weeds. Moreover, in cereal crops, the long-term application of herbicides can favour genotypes that are tolerant and resistant to herbicide applications (Heap 1997), such as Apera spica-venti (L.) P. Beauv. (loose silkybent) and Lolium rigidum Gaudin (wimmera ryegrass) (Krzakowa & Adamczewski 2007; Preston et al. 2009), which often cause crop yield losses (Melander et al. 2008; Cirujeda & Taberner 2009). The higher diversity of grasses encountered in Lower Saxony was due to cultivated species [Avena sativa L. (common oat)] or species frequently used in pastures [e.g. Lolium multiflorum Lam. (Italian ryegrass), Lolium perenne L. (perennial ryegrass) and Alopecurus pratensis L. (meadow foxtail)]. This result could be related to the higher percentage of pastures in Lower Saxony compared to Catalonia (14.54 ± 1.33% vs 0.42 ± 0.00%, respectively, of pastures relative to total land in landscape sectors of 1 km around each farm). Therefore, the larger number of grass species may result from farming practices that introduce fodders in their rotational schemes and/or from the spillover of seeds from adjacent farms, but not from the abandonment of herbicides and mineral fertilizers.
Relative importance of β-diversity
The comparison of diversity components at both farm and regional scales (for all weeds and for the different functional groups) showed that βfarm/region contributed most to overall species richness. This finding indicated that substantial differences in weed species composition occurred among farms. Apparently, the local weed plant assemblies and weed species composition change markedly with local site characteristics, such as differences in soil properties and type, microclimate and/or water regime. The effects of environmental heterogeneity were higher at large spatial scales, i.e. higher between than within localities. In addition, the use of farm machinery may have served to homogenize weed diversity among fields within farms. Organic farming exhibited the greatest contribution to overall observed weed species richness in both regions, despite substantial dissimilarities in community composition across regions. The largest contribution of this farming system may be related to the less intensive agricultural practices and higher heterogeneity in management among farms compared with conventional farming (Clough et al. 2007; Armengot et al. 2011).
Implications for farmland management and arable weed species diversity
Heterogeneity in community composition among organic farms (β-diversity) accounts for the majority of arable weed species diversity in agricultural landscapes, despite substantial differences in community composition across regions. Therefore, future European-wide agri-environmental schemes should recognize the dissimilarity of local communities and focus on the conversion to organic farming at landscape scale to optimize limited budgets and thereby maximize arable weed species diversity across regions.