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