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

2007 and 2008, while big numbers of weed plants of various species survived in the OSFs
resulting in higher weed plant densities (Fig. 13) and biomasses (Fig. 18) in this treatment.
However, all perennial crop treatments, which were not tilled during the whole experiment,
had considerably lower final weed densities than all annual crop treatments. Other processes
with compensating effects on weed population dynamics must thus have taken place.
Empirical evidence for the lack of stimulation for weed germination will be discussed in the
following. Other compensating effects may be linked to the fact that more weed seeds stayed
on the untilled soil surface of PFCs. This might result in reduced seed dormancy and increased
numbers of (fatal) germinations (Benvenuti et al., 2001), increased seed decay by microorganisms
(Wagner and Mitschunas, 2008) or increased seed predation rates (which will be
analyzed in Chapter C.IV).
While the destruction of established weed plants was obvious for several soil tillage events,
there are also strong indications that tillage favoured the germination and establishment of
new weed plants. First, strong weed emergences were observed after most soil tillage periods
except in winter (Fig. 13). As expected, the species composition that emerged after the tillage
events varied strongly according to the tillage period. The soil tillage in April 2007 (applied
for seed bed preparation of the spring-sown perennial crops) had stimulating effects on several
spring and summer emerging weed species including C. bursa-pastoris, Chenopodium album,
Amaranthus retroflexus, Echinochloa crus-galli, Polygonum lapathifolium, and P. persicaria.
In contrast, these species were absent form all other treatments that were not tilled in this
season.
After the first month of the experiment, the perennial crops often showed much lower weed
emergence rates compared to the annual crops. This is another strong indicator for the
importance of soil tillage for weed recruitment. In this way, soil tillage in February 2008 and
2009 (seedbed preparation for the annual crops) stimulated the emergence of weed species
that are known to emerge preferentially in winter and early spring (G. aparine, A.
myosuroides, Polygonum sp., S. arvensis, and V. persica), while the soil tillages after cereal
harvest in August 2007 and August 2009 stimulated the emergence of species including A.
retroflexus, L. multiflorum, A. myosuroides, G. aparine, S. arvensis (Fig. 13) that are typical
for summer or autumn sown crops.
The last indication for a strong impact of the lack of soil disturbance in perennial crops is the
reduced weed emergence compared to the regularly tilled annual crops. Among the perennial
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