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

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