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

Although the reason for differences in predation rates among species is still unknown, such a
preference order indicates that weed seed predation may contribute to changes in the weed
community composition (White et al., 2007). Moreover, weed seed predation would have
lower impacts on perennial species and species that may reproduce vegetatively. Even if direct
evidence is difficult to obtain, seed predation is probably an important factor underlying the
impacts on weed population dynamics and weed community changes in and after PFCs.
However, it is not yet clear which proportion of seeds is directly eaten by the predators and
how many seeds are only removed and dispersed.
D.II.6 Overview of the underlying mechanisms
The results suggest that the impacts of PFCs on weeds are probably produced by several
factors affecting different phases of the weed life cycle including seed germination and
emergence, plant survival and vegetative growth, seed production and seed survival. The
absence of soil tillage probably favoured perennial over annual weeds (at least for the
broadleaved species), the temporarily extended competition probably reduced vegetative weed
growth and seed production, mowings suppressed in particular the upright weeds while
favouring several grasses and broadleaved species with rosettes and seed predation probably
affected annuals more than perennial species. The importance of the different mechanisms
probably varies with weed life stage: The impacts of cuttings increases with plant age (Article
4), the impact of competition is probably highest for young weed plants (Magda et al., 2006),
and soil tillage has both high impacts on seed germination but also on all other plant stages.
PFCs thus impose diverse selection pressures to wild plants. As the concept of IWM itself, the
weed regulating function of PFCs is probably based on several mechanisms. Therefore, the
risk of selecting ‘resistant’ weed biotypes among the suppressed arable weed species would be
lower than for ‘single-mechanism’ weed control techniques. It is more likely that the imposed
selection pressures modify the weed community assembly as observed in Articles 1-3 on the
general impacts and Articles 4-8 on specific mechanisms.
Some of the mechanisms causing the impacts of PFCs on weeds could be clearly demonstrated
in this project. However, the relative importance of them is difficult to determine and may
strongly vary between local situations. Moreover, there may be other mechanisms and
interactions that could not be investigated:
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