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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and higher production costs or eventually reduced <strong>crop</strong> yields may be compensated by higher<br />
marked prizes of the labelled products.<br />
A.III.3 Combining high weed diversity <strong>with</strong> low weed abundance?<br />
One strategy for alleviating the tra<strong>de</strong>-off between the harms and functions of weeds might be<br />
the increase of weed species diversity <strong>with</strong>out increasing the total weed abundance, thus<br />
increasing the evenness of the plant community. Higher plant diversity may increase the<br />
diversity of other trophic levels (animals and micro-organisms) as <strong>de</strong>tailed in section A.II.3<br />
above. However, weed abundance and weed diversity are generally positively related as both<br />
<strong>de</strong>pend on the intensity of weed control. Hyvonen et al. (2003) observed always positive<br />
linear relationships when plotting the weed species richness against the number of weed<br />
individuals (both on logarithmic scales). However, the intercepts of these regressions were<br />
higher for organic <strong>crop</strong>ping systems compared to conventional systems indicating that the<br />
abundance-diversity relationship may differ between <strong>crop</strong>ping systems. At equal abundances,<br />
organic systems had about 2 more species compared to conventional systems. Such an<br />
improvement might be caused by the replacement of one dominating weed control technique<br />
that is very efficient on most weed species (e.g. herbici<strong>de</strong>s <strong>with</strong> broad ranges) by several less<br />
efficient techniques and principles (IWM, see section A.III.2 above) or due to the rotation of<br />
dissimilar <strong>crop</strong>s (see section A.III.8 below).<br />
A.III.4 ‘Good’ vs. ‘bad’ weeds?<br />
Another approach goes beyond the former approach in consi<strong>de</strong>ring differences between the<br />
weed species according to (i) their potential harm for <strong>crop</strong> production, (ii) their difficulty to be<br />
controlled and (iii) their role for biodiversity (habitat and trophic resource) and ecosystem<br />
functioning. Fig. 4 shows that weed species may consi<strong>de</strong>rably differ in ‘harmfulness’<br />
(competitive ability) and ‘biodiversity value’ (number of associated insect species). But when<br />
combining these two criteria, many weed species are neither ‘very good’ nor ‘very bad’ but<br />
somehow intermediate.<br />
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