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