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

When both treatments were combined, weed biomass production was lowest and
plant mortality was highest compared to all other treatments (Fig. 2 and Fig. 3). In the global
ANOVA model pooling all weed species, the interaction between cutting and competition was
not significant but there was a slightly significant 3-way interaction (p=0.0476, Table II)
indicating that the species reacted differently. When analyzing each species separately, the
interaction term was nearly significant for V. persica (p=0.055) and G. aparine (p=0.062). For
these two species, the interaction tended to be positive as the combination of cutting and
competition produced plants that were rather smaller than expected supposing only additive
effects (Fig. 3). The interaction term was not significant for the 4 other species. For C. album,
the cutting treatment alone reduced biomass production very strongly so that the increased
competition could not reduce plant growth any further (Fig. 3).
DISCUSSION
As expected from the literature (Andreasen et al., 2002; Graglia et al., 2006; Mager et
al., 2006) and our previous experiments (Meiss et al., 2008), cumulated weed biomass
production was reduced by cutting. In Fig. 3, we compared the biomass of uncut plants with
the cumulated biomass production of cut plants (summing up the cut and the regrown dry
matter). When considering only the biomass regrown, the impact of cutting was even stronger
(Fig. 2).
Increased competition also had a negative effect on weed biomass production, but the
amplitude of this effect was rather small at the beginning of the experiment, probably caused
by the slow initial development the lucerne plants. Competition became more and more
important as lucerne regrowth increased with the consecutive cuttings.
The combination of both treatments resulted in the lowest weed biomasses. The nonsignificant
interaction terms and the graphical analysis (Fig. 3) suggest that the negative
effects of both treatments are mainly additive. These results are in accordance with Graglia et
al. (2006) who did not find any interaction between the negative effects of competition by
grasses or clovers and the number of mowings on C. arvense biomass production in the
following crop. For the two weed species where we observed a nearly significant interaction
term (V. persica and G. aparine), the interaction tended to be positive (Fig. 3). There was
thus no evidence that one treatment is counteracting or compensating the negative effect of
the other treatment at this stage, but rather a tendency towards a mutual amplification.
Even though each cutting definitely reduced competition by removing the biggest part
of the aboveground biomass, lucerne showed a good regrowth capacity rapidly restoring
strong levels of competition after the disturbance events. Nevertheless, lucerne growth was
also slightly affected by the additional early cutting treatment and produced slightly less
biomass than the lucerne trays without the early cut. This lower lucerne biomass was
probably the reason for slightly (but not significantly) higher weed biomass in the combined
treatment compared to competition alone (compare Fig. 2D & 2B). This may be called an
‘indirect impact’ of the early cutting treatment. When lucerne is cut too early or too often, its
regrowth ability may be reduced (Teixeira et al., 2007) which may lead to reduced
competition and thus a ‘strict negative interaction’ between cutting and competition.
Lucerne was already becoming the dominant species after the first cutting. It may thus
be considered a ‘key stone species’ of the experimental system. Before the first cutting, total
weed biomass was higher than lucerne biomass. The lucerne’s competitive advantage thus
appears only after the cutting treatment. Without this disturbance, the lucerne would probably
become the dominant species later. All 12 weed species showed less regrowth capacity than
lucerne (Fig. 2). This corresponds to our previous experiments on plants grown in individual
pots without competition (Meiss et al., 2008). The present results show that (low) competition
between different weed species and (higher) competition with additional lucerne plants will
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