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

Such diversified crop rotations could therefore be considered a valuable component of
Integrated Weed Management.
D.II UNDERLYING MECHANISMS
The results suggested that the impacts of PFCs on arable weeds are governed by several
mechanisms that affect different phases of the weed life cycle. Evidence comes from i) the
analyses of weed species functional groups favoured and suppressed in and after PFCs in the
large-scale weed surveys (chapter C.I), ii) the differences between weed community and
population dynamics between annual and perennial crops and between the different crop
managements of the field experiment (chapter C.II), and finally from the separate analyses of
two potential mechanisms, namely iii) the post-cutting plant regrowth studied in the
greenhouse experiments (chapter C.III) and iv) the weed seed predation tested in different
field experiments (chapter C.IV).
The absence of soil tillage (A), the strong and temporally extended competition (B), and the
frequent hay cuttings (C) were probably the most important factors governing the impacts of
PFCs on arable weeds. In the following four paragraphs, I will briefly discuss how these
factors (and interactions between them) probably affect the different phases of the life cycles
of different weed species and species functional groups (see also the overview in Table 13 and
the illustration in Fig. 20).
D.II.1 Absence of soil tillage (A)
The large-scale surveys and the small-scale field experiment both suggested that the absence
of soil tillage in PFCs have two opposing impacts on weeds, to which different weed species
may react differently. The absence of soil disturbance may i) reduce the germination and
establishment of several typical annual weed species (through several mechanisms, Huarte and
Arnold, 2003) while it may ii) increase the survival of established plants, which may be
particularly favourable to longer living biennial and perennial species. These two mechanisms
are probably a main cause of the reduced frequencies and abundances of typical annual weed
species observed in the large-scale surveys and the field experiment and the increased
frequencies of perennial (broadleaved) species in the large-scale surveys. Annual weed species
(therophytes, Raunkiær, 1905) may survive the regular soil tillage typical for annual crops as
seeds germinate preferentially after soil disturbance, while perennial species
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