Diversifying crop rotations with temporary grasslands - Université de ...

crops (Clay and Aguilar, 1998; Cardina et al., 2002a; Teasdale et al., 2004) and mown field
margin strips (De Cauwer et al., 2005). However, other previous studies reported reduced
abundances of (annual) grass species including Apera spica-venti (L.)P.Beauv., Avena fatua L.
and Setaria sp. (Norris and Ayres, 1991; Gill and Holmes, 1997; Schoofs and Entz, 2000;
Cardina et al., 2002a; Albrecht, 2005) while the perennial grasses Elymus repens (L.) Gould
and Poa sp. sometimes profited from perennial crops (Andersson and Milberg, 1996; Clay and
Aguilar, 1998; Teasdale et al., 2004; Albrecht, 2005). Heterogeneous reactions of different
grass species might be due to two opposed effects: most grasses probably have a good
vegetative regrowth capacity (see chapter C.III), but hay cuttings may considerably reduce
seed production of grassy weeds, especially all grass species that have tall and upright
reproductive spires, which was also observed by Dalbies-Dulout and Dore (2001) for A.
myosuroides in mown set-aside fields. Therefore, grasses may sometimes have high biomasses
in perennial forage crops but this does not necessarily lead to high seed production and
increasing populations (see chapter C.II). The success of seed production thus depends on the
species’ phenology and the exact cutting dates.
D.I.6 Weed abundance and diversity
While the community composition varied strongly between annual and perennial crops, and
during the crop rotation of the space-for-time substitution design, variations in species
diversity were smaller. At the field scale, higher species diversities were observed only for
young perennial crops (Fig. 2 of Article 2), when species typical for annual and perennial
crops co-existed temporally. This corresponds to the field experiments, where plant diversities
were highest during the first month of the perennial crops (Fig. 12). Afterwards, weed
diversities decreased at the field scale with the age of the perennial crops in both the large and
the small-scale studies (which was mainly due to the reduction of typical arable weeds).
Interestingly, these decreases were less strong than the decrease in weed abundances,
improving the richness/abundance ratios compared to annual crops (Fig. 12). Moreover, the βdiversity
(dissimilarity between the fields in the large-scale surveys) remained high (Fig. 2 of
Article 2).
Sjursen (2001) observed a similar decrease in weed species numbers (and weed abundance) in
the established vegetation during 3-year forage crops. In this study, Sjursen (2001) observed
reduced weed abundances in the soil seed bank, but species diversities remained unchanged in
the soil. In long-term experimental studies conducted by Sosnoskie et al. (2006), seed bank
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