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

C.II.3.1.2 Dynamics of weed community composition
After 2.5 years of contrasted crop management, the species composition of the emerged weed
communities differed most strongly between all annual crop and all perennial crop treatments,
as shown by the pairwise multivariate comparisons using the MRPP permutation technique
(Table 9). The magnitude of the differences, indicated by the MRPP-A statistic (‘within group
agreement’), was strongest and always significant for all pairwise comparisons between
annual crops and perennial cocksfoot crops (A-values varied between 0.48**** and 0.51****,
see the first nine lines in Table 9). Pairwise differences between annual crops and perennial
alfalfa crops were also highly significant and their magnitudes came at the second place (A
varied between 0.36**** and 0.45****) followed by the differences between cocksfoot and
alfalfa crops (A = 0.28**** - 0.41****, Table 9).
The differences between annual and perennial crops increased with time (see first 18 lines in
Table 9).
In contrast, differences between spring and autumn sown perennial crops were very
strong during the first year (A = 0.27**** - 0.44****) and decreased with time. At the end of
the experiment, differences between spring and autumn sown crops were lower than all other
contrasts mentioned above, and only significant for alfalfa crops (A = 0.23****) but not for
cocksfoot (A=0.05 ns). For alfalfa crops, weed communities differed significantly between the
treatments with high and low cutting frequency at nearly all observation dates since these
contrasted cutting treatments were set up in 2007. However, the amplitude of these differences
were mostly weaker than the other differences mentioned above (A = 0.10** - 0.27****). In
contrast, cutting frequency had no significant impact on species composition for cocksfoot
crops at most observation dates (see Table 9 for details).
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