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

A
Richness S
B
Shannon H’
C
Distances to centroids
50
40
30
20
10
0
3
2
1
0
0.8
0.7
0.6
0.5
0.4
0.3
B
B
B
a)
Wheat
after
annuals
P = 0.0041
P = 0.0033
P < 0.0001
b)
Lucerne
1 year
c)
Lucerne
2–6
years
Groups of fields
d)
Wheat
after
lucerne
Fig. 2 Weed species diversity measures for four groups of fields
(see Table 1). (A) species richness (B) Shannon diversity index,
(C) Bray–Curtis distance to group centroids (beta-diversity). Broad
lines are medians, black dots are means, boxes show the interquartile
range and whiskers extend to the last data point within
1.5 times the inter-quartile range, open circles are further outliers.
P-values of ANOVA (A, B) and permutation tests (C) are given.
Groups not sharing the same letter are significantly different
at a = 0.05.
perennials Taraxacum spp. and Rumex spp. have
already been reported (Ominski et al., 1999; Albrecht,
2005; Hiltbrunner et al., 2008; Ulber et al., 2009). Our
surveys suggest that other perennial species ⁄ genera react
A
A
A
Ó 2010 INRA
Journal Compilation Ó 2010 European Weed Research Society Weed Research 50, 331–340
B
B
A
B
B
A
Arable weeds and perennial lucerne 337
similarly (Reseda spp., Malva spp., Picris spp., Crepis
spp., Silene latifolia Poir., Falcaria vulgaris Bernh.,
Table 3), leading to a strong response of the whole
functional group (Fig. 3). Perennial species likely profited
from the absence of soil tillage. In this way, weed
growth conditions in pluriannual lucerne may be similar
to no-till systems (e.g. Zanin et al., 1997). Some perennials
might also be more tolerant to prolonged competition.
Conversely, most annual weeds are probably best
adapted to annual crops and survive the yearly soil
tillage as seeds in the soil. Soil tillage may even promote
recruitment of annual weeds, while it may be inhibited
under dense canopies of perennial crops (Huarte &
Arnold, 2003). Annual crops would therefore correspond
to very early successional stages favouring
r-selected species with shorter life cycles and generative
reproduction (therophytes), whereas established lucerne
would correspond to later successional stages favouring
slower growing and longer living biennial and perennial
(K-selected) species (hemicryptophytes and geophytes).
Cirsium arvense (L.) Scop. was the most important
exception (Table 3), despite its perennial life cycle.
Negative impacts of forage crops on C. arvense have
repeatedly been observed (e.g., Ominski et al., 1999) and
might be linked to the repeated mowings exhausting the
carbohydrate reserves needed for regrowth (Graglia
et al., 2006).
While the a priori division of broad-leaved species
into six functional groups according to life cycle and
morphology has proven to be quite successful in our
study, we have no explanation for the heterogeneous
reactions of the different grass species. Grasses may
show a better regrowth after cutting than broad-leaved
species, but cutting may strongly reduce seed production,
especially of tall grasses. In previous studies, some
grass species including Apera spica-venti (L.) P.Beauv.,
Avena fatua L. and Setaria faberi F.Herm. were
suppressed by forage crops (Schoofs & Entz, 2000;
Albrecht, 2005), while others including Elymus repens
(L.) Gould and Poa sp. increased (Andersson &
Milberg, 1996; Clay & Aguilar, 1998; Teasdale et al.,
2004; Albrecht, 2005). In our study, the FG of perennial
grasses was slightly increased in lucerne, which may be
compared with the positive reaction of perennial broadleaved
species (see above). In contrast, the FG of annual
grasses showed no significant differences. Looking at
each individual species might be more informative.
Species favoured by lucerne included both perennial and
annual grasses (Poa trivialis L. and Bromus spp.) and
some species that are sometimes included in the sown
mixtures (Lolium spp. and Festuca spp., Andersson &
Milberg, 1996), while other annual and perennial grasses
showed no differences or were suppressed (Alopecurus
myosuroides Hudson., A. fatua, P. annua and E. repens).