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

C.II.4.3 Underlying mechanisms
Decreasing weed population dynamics in PFCs may be quite astonishing, given the complete
absence of herbicides and soil tillage and the sowing of 17 weed species at the beginning of
the experiment. Other mechanisms must thus be responsible for these reductions.
Annual and perennial crops differ in several aspects that may affect the growth of weeds (cf.
Table 4 in the general introduction). Thanks to the rather complex study design comparing
nine crop treatments and the high temporal resolution of weed density and biomass
observations, this experimental study allows to learn more about several of the mechanisms
potentially underlying the impacts of PFCs on weeds. This has, to our knowledge, rarely been
investigated previously. In the following three paragraphs (C.II.4.3.1-3), I will discuss three
main factors governing the impacts of PFCs on arable weeds: the absence of soil tillage (A),
the strong and temporally extended competition (B), and the frequent hay cuttings (C). Table
12 gives an overview how these factors varied with the experimental treatments. Our results
suggest that these three factors affected several important parts of the weed life cycle (Fig.
20). These impacts are also summarized in Table 13.
†
(3)
vegetative
growth
Seedlings
Emergence
(2)
†
(1)
Initial
growth
Germination
Seed aging, decay
Post-dispersal seed predation
†
Survival of
perennials
(7)
†
Vegetative
plants
(7)
(4)
Survival in
the seed bank
Seeds
Fig. 20: Life cycle of annual and perennial weeds.
Grey boxes, four stages; arrows, transitions between stages; †, mortality. The thickness of the arrows corresponds
to approximate densities of individuals (varies between species and populations). The survival of established
plants (4) will be more important for perennial species, seed rain and the survival of seeds in the seed bank will
be more important for annual species. The numbers (1-6) corresponds to the weed life stages in Table 13 showing
the possible impacts of PFCs.
99
(4)
Flowering
Immigration
Emigration
(5)
Pollen
Reproducing
plants
Seed
rain
(6)
†
†
Pre-dispersal
seed predation
(4)