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