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

Table 12: Factors differing between five crop treatment comparisons (as in).
Factor
Crop type Species
96
Treatment comparison
Sowing
season
Cutting
frequency
Tillage in
intercrop
Soil disturbance (superficial tillage, sowing)* An >> P
D >~ M
(roots?)
Season = T+ > T-
Aboveground vegetation disturbance
(hay cuttings /crop harvesting)*
P > An = A >~ Sp C+ >> C- T+ >~ T-
Fertilisation (N-fertilizer vs. N-fixation) ≠ ≠ = = =
Vegetation cover (competition for light)*
≠ ≠ growth
dynamics
≠ growth
dynamics
≠ ≠ crop
establishm.
C- > C+ =
Allelopathy ?? ?? = = =
* Impacts of these factors on the weed life cycle are detailed in Table 13.
C.II.4.1.1
Plant densities
Emerged plant densities and biomasses of many common annual weed species showed stable
or decreasing tendencies in all perennial crop treatments. Such weed population dynamics
may be surprising given the complete absence of soil tillage and specific weed control
techniques in the PFCs. (Possible mechanisms of these declines will be discussed below).
Interestingly, reductions in weed abundances in PFCs were much more pronounced than
reductions in species richness leading to improved diversity/density-ratios ( Fig. 12) that may
be useful for reducing the ‘weeds trade-off’ (see § A.III.3 in the general introduction).
In contrast, some weed species showed strongly increasing population sizes in the succession
of annual crops, as indicated by the increasing field emergence densities (Table 10, Fig. 12,
Fig. 13) and weed biomasses (Fig. 17) during the 2.5 years experiment compared to the
perennial crops. These strong weed increases in the annual crops are probably due to events of
weed seed multiplication during the experimental period. It can not be explained by dense
background seed densities or the initial weed seed addition, which would be visible in all
treatments of the randomized block experiment, which was not the case for any of the 16 sown
species. In both spring-sown perennial crops, considerable weed seed production occurred
probably during the first month after crop sowing due to the slow initial crop establishment
and the late first cutting (see discussion on the mechanism below).
C.II.4.1.2
Species composition
Due to these heterogeneous weed population dynamics, species composition varied most
strongly between annual and perennial crops at the end of the experiment, ( Table 9). After the