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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(Fig. 1A). The or<strong>de</strong>r of the four groups of fields (a–d)<br />
corresponds to the or<strong>de</strong>r of the phases during the <strong>crop</strong><br />
rotation and thus to the hypothetical temporal trajectory<br />
of weed communities. The cyclic trajectory of weed<br />
communities was also confirmed by following the<br />
individual fields of young and old lucerne and wheat<br />
following lucerne in subsequent years (Fig. 1A). Pairwise<br />
comparisons showed that weed species composition<br />
differed significantly between all treatments (Table 2).<br />
Differences were greatest between wheat following<br />
annual <strong>crop</strong>s (group a) vs. 2–6 year old lucerne (c)<br />
(Table 2). In CDA, these two groups were mainly<br />
separated on the first axis (Fig. 1A). The second highest<br />
difference appeared between first-year lucerne (b) and<br />
wheat following lucerne (d), which was mainly separated<br />
on the second CDA axis. Differences were thus stronger<br />
between groups lying on opposite si<strong>de</strong>s of the circle and<br />
lower, but still significant, between adjacent groups,<br />
which would follow each other in the rotation (Table 2,<br />
Fig. 1A).<br />
Indicator Species Analysis (ISA) showed that nine<br />
weed species were significantly associated <strong>with</strong> wheat<br />
following annual <strong>crop</strong>s (a) (see Table 3). First-year<br />
lucerne (b) was associated <strong>with</strong> 16 other species, while<br />
most of the 9 species typical for wheat after annual <strong>crop</strong>s<br />
had reduced Indicator Values. As a result, species<br />
richness and Shannon diversity indices at the field scale<br />
(a-diversity) were increased and young lucerne had also<br />
a higher dissimilarity between the fields (b-diversity)<br />
than wheat following annual <strong>crop</strong>s (Fig. 2). Ol<strong>de</strong>r<br />
lucerne (c) was significantly associated <strong>with</strong> 10 species<br />
and four additional species had similarly high IV in<br />
young and old lucerne (Table 3). In contrast, most weed<br />
species that were typical of annual <strong>crop</strong>s had further<br />
reduced frequencies or disappeared. As a result, mean<br />
a-diversity dropped down again to the level of annual<br />
<strong>crop</strong>s (about 20 species per field), while mean b-diversity<br />
remained high (Fig. 2). Only four species showed highest<br />
IV in wheat following perennial lucerne (d), but IV of<br />
some species were nearly the same as in group c,<br />
<strong>de</strong>monstrating the similarity between old lucerne and the<br />
Table 2 Pairwise comparisons of weed communities in four <strong>crop</strong><br />
treatments (Table 1). ANOSIM-R varies between 1 (groups do not<br />
share any species) and 0 (no differences between groups) (Clarke,<br />
1993)<br />
Contrast ANOSIM-R<br />
(c) Lucerne 2–6 years vs. (a) wheat after annuals 0.326****<br />
(b) Lucerne 1 year vs. (d) wheat after lucerne 0.171****<br />
(c) Lucerne 2–6 years vs. (d) wheat after lucerne 0.109****<br />
(b) Lucerne 1 year vs. (a) wheat after annuals 0.107****<br />
(b) Lucerne 1 year vs. (c) lucerne 2–6 years 0.062****<br />
(a) Wheat after annuals vs. (d) wheat after lucerne 0.045****<br />
****P < 0.0001 (Bonferroni-corrected).<br />
following annual <strong>crop</strong>s. On the contrary, wheat following<br />
lucerne also contained several species typical for<br />
annual <strong>crop</strong>s, but always <strong>with</strong> lower IV (see Table 3).<br />
Interestingly, none of the three diversity measures<br />
differed between old lucerne and the following wheat;<br />
b-diversity was thus significantly higher than in wheat<br />
following annual <strong>crop</strong>s (Fig. 2). The weed species which<br />
were significant in ISA (P < 0.05 and IVmax ‡ 15) are<br />
represented on the CDA plot illustrating the trajectories<br />
of individual weed species during the <strong>crop</strong> rotation<br />
(Fig. 1B).<br />
Functional groups<br />
Relative frequencies of the a-priori <strong>de</strong>fined weed FG<br />
varied between the four treatments. Differences were<br />
significant for six out of the eight FG (Fig. 3). Relative<br />
frequencies of both upright and climbing annual broadleaved<br />
species (FG 1 and 2) were highest in wheat after<br />
annual <strong>crop</strong>s (a), reduced in young lucerne (b), more<br />
strongly reduced in old lucerne (c), and increased again<br />
in wheat after annual <strong>crop</strong>s (d), though they did not<br />
reach the level of (a). The opposite pattern was observed<br />
<strong>with</strong> four other FG: annuals <strong>with</strong> rosettes, broad-leaved<br />
species <strong>with</strong> ÔintermediateÕ life cycles, perennial broadleaved<br />
species and perennial grasses (FG 4, 5, 6 and 8),<br />
which were two to three times more frequent in old<br />
lucerne (c) than in wheat after annual <strong>crop</strong>s (a). Two FG<br />
did not show any significant difference: annual broadleaved<br />
species <strong>with</strong> ÔotherÕ morphologies (FG 3) and<br />
annual grasses (FG 7) (see Fig. 3).<br />
Discussion<br />
Ó 2010 INRA<br />
Journal Compilation Ó 2010 European Weed Research Society Weed Research 50, 331–340<br />
Arable weeds and perennial lucerne 335<br />
This study indicates that weed community dynamics are<br />
strongly affected by perennial lucerne. In accordance<br />
<strong>with</strong> Ominski et al. (1999), wheat fields following<br />
pluriannual lucerne (d) vs. several years of annual <strong>crop</strong>s<br />
(a) showed strong differences in weed species composition.<br />
These differences were consistent <strong>with</strong>, and may<br />
thus be explained by, the weed community trajectories<br />
during the perennial <strong>crop</strong>s (b and c), which, to our<br />
knowledge, have not been shown to date. Below we<br />
compare the observed community shifts to findings<br />
in previous studies and discuss possible un<strong>de</strong>rlying<br />
mechanisms.<br />
Lucerne had strong negative impacts on broad-leaved<br />
weeds <strong>with</strong> an upright morphology (FG 1, e.g. Mercurialis<br />
annua L., Chenopodium album L., Solanum nigrum<br />
L.) and on species climbing on neighbouring plants (FG<br />
2, e.g. Galium aparine L., Fallopia convolvulus (L.) A ´ .<br />
Lo¨ve). In previous studies, species <strong>with</strong> similar morphologies,<br />
including Abutilon theophrasti Medik., Amaranthus<br />
sp., C. album, and G. aparine, reacted in a similar way