2. ESTABLISHMENT AND REPRODUCTIVE SUCCESS OF BRASSICA NAPUS (L.) UNDER
2.4 D ISCUSSION
2.4.5 Conclusions
Overall, I showed that OSR is capable of reproducing even under stressful conditions such as low-quality sandy ruderal soils with competing vegetation. My results confirm the importance of vegetation cover for establishment but demonstrate that the immediate vicinity of the seeds is crucial: Sites with generally high vegetation cover may still facilitate establishment if gaps of 30*30 cm are present. Such small disturbances may well be caused by burrowing animals, e.g. moles. Some of these animals, e.g. rabbits, may however contribute to plant mortality through herbivory. I further found that high soil quality may have direct positive effects on establishment success, but that these can either be ineffectual if other factors become limiting or be overridden by indirect negative effects in the course of secondary succession, namely the simultaneous promotion of competing vegetation. These findings in combination with my observations for grassland soils indicate that an intermediate soil quality may be most suitable for OSR, as it is less easily outcompeted by other vegetation. Crawley et al. (1993) observed similarly that microsites for OSR establishment disappear more quickly with increasing soil fertility. Free microsites appear to be of greater importance than the amount of seeds dispersed.
Establishment success was mostly very low (treatment means between 0 and 1.6%), in spite of a substantial number of emerged seedlings, and cannot quite explain the great number of ruderal populations found in Bremen in a previous survey (Menzel 2006). On the one hand, my study likely underestimated success due to suboptimal conditions (see 2.4.1). On the other hand, I suspect that successful long-term establishment is limited to very specific conditions:
bare sites with intermediate soil quality. The absence of slugs and rabbits also appears to be vital. This may be the reason why OSR survived well on railway tracks with gravel beds in another study (Menzel 2006). My chosen sites only partly fulfilled the requirements for OSR establishment. Randomly spilled seeds seem to have limited chances of hitting an appropriate habitat. So where do all the feral OSR plants come from? All things considered, I think it most likely that a majority of OSR populations establishes from seeds imported with topsoil from former agricultural areas. This method of dispersal is known to generate OSR roadside populations (Crawley & Brown 1995). In this case, seeds are automatically brought to an
appropriate habitat: bare soil in larger areas which vegetation and herbivores usually do not immediately recolonise. Furthermore, OSR seeds can persist in the seed bank for up to 11 years (Lutman et al. 2003) and thus outlast periods with unsuitable conditions. My study further showed that establishment success of OSR ferals can be higher in the ruderal than in the agricultural environment, stressing the importance of including ruderal populations in GMP risk assessment.
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