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

irradiation,…), and (ii) biotic interactions such as parasitism (diseases), symbioses and
competition: a plant surviving a cutting treatment in ‘optimal conditions’ may show a reduced
regrowth rate or even die in suboptimal conditions. In the present study, we focus on
competition, which is of particular importance in temporary grasslands, pastures or field
margin strips, which may all contain highly competitive perennial species (Schoofs and Entz,
2000; Dear et al., 2006).
When acting separately, competition and physical disturbances will likely both have
negative effects on plant fitness. When acting together, both processes may interact. These
interactions may be considered from two directions, each with several mechanisms that may
act simultaneously:
1) Physical disturbances may change the actual community composition and thus the
outcome of competition (Weigelt and Jolliffe, 2003) through three mechanisms:
a) creating “open” habitats by destroying disproportionate parts of the species actually
dominating the community [generally the competitive, K-selected species (Pianka,
1970)], thus giving a “chance” to other species,
b) favouring species that are not or are less affected by the specific type of disturbance
due to special morphological or phenological traits (resistant species), or
c) favouring species that can grow quickly after the disturbance [resilient or
opportunistic, r-selected species (Pianka, 1970)].
2) Competition may change the impact of the physical disturbance
a) by modifying the plant’s morphology (height, specific leaf area,…) and phenology
(stage) before the disturbance event: Mowing at a given height removes more
biomass and buds from tall plants that were etiolated owing to competition compared
to short plants grown in less competitive environments (Ballare and Casal, 2000).
b) by modifying the performance of the damaged plants after the disturbance event. The
likelihood of plant survival and the regrowth speed will thus probably decrease with
increasing competition after the disturbance. The level of competition afterwards
depends on the survival and regrowth of the neighbouring plants (forage crop). Due to
the asymmetric nature of competition for light (Weiner and Damgaard, 2006), small
differences in the regrowth speed between different species may have a big influence
on the outcome of competition.
The possible effect (or sign) of such interactions on plant fitness may lie on a gradient
between a positive interaction (disproportionate enhancement of the effects) and a strict
negative interaction (compensation of negative effects) defined in Fig. 1.
In the cases A, B, and C of Fig. 1, the negative effect of one treatment is (more or
less) enhanced by the addition of the other treatment. This may arise when competition
before and/or after the cutting treatment increases the impact of cutting (mechanisms 2a and
2b). In contrast, the mechanisms 1a, 1b, and 1c might lead to all four cases defined in Fig. 1:
If the strong repressive effect of the dominant species is broken by the cutting treatment,
other species (weeds) may have a chance to establish, which would lead to a strict negative
interaction (case D of Fig. 1). This corresponds to the ‘classical view’ founded by Grime
(1974) that opposes disturbances and competition (‘competitive’ vs. ‘disturbed habitats’ or
‘competitive’ vs. ‘ruderal species’. But if the actually dominant species (forage crop) has also
the best regrowth ability (resilience) after cutting, its competitive advantage might also be
increased by the disturbance, which would lead to cases A, B or C of Fig. 1.
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