Читать книгу Ecology - Michael Begon - Страница 122

The general tendency for final yield to saturate – not continue to increase in line with initial density – is something that underlies many activities throughout agriculture, horticulture and even animal husbandry. Generally, we wish to avoid investing unnecessarily in high stocking rates that will not translate ultimately into higher yields. A more unusual example of its application, though, is provided by a study seeking to conserve three species of plant facing extinction in large parts of Europe that may also sometimes be weeds, reducing the productivity of crops with which they may coexist (Lang et al., 2016). The species concerned were European Venus’ looking glass, Legousia speculum‐veneris, forking larkspur, Consolida regalis, and field gromwell, Lithospermum arvense, which can all act as weeds in fields of winter rye, Secale cereale. How might we combine the conservation of these species with avoidance of crop loss? It is clear in Figure 5.9a that as the sowing densities of the plants was increased, significant levelling off was apparent at sowing densities above a few hundreds per square metre (though a strictly ‘constant’ yield was not reached). We can also see that below these kinds of levels, the yield of the rye was not affected (Figure 5.9b), whereas with further increases in weed seed density, crop yield showed a significant drop (though at densities above around 1000 weed seeds per square metre it again levelled off). We will not discuss interspecific competition, between the weeds and the crop, in detail until Chapter 8. Nonetheless, we can understand that very low sowing rates of the three plants would be ineffective in establishing viable populations of these threatened species. However, a levelling off of the yield of new seeds was well underway before sowing densities were reached that affected crop yield. Lang and her co‐workers were therefore able to recommend ‘optimal’ sowing rates of 100 seeds per m² for C. regalis and L. arvense and 50 seeds per m² for L. speculum‐veneris, combining successful establishment, the avoidance of wasted seeds, and negligible crop loss.

Figure 5.9 Optimal sowing rates can conserve rare weedy plants without threatening crop yields. (a) In experimental plots near Munich, Germany, seeds of three plant species, Legousia speculum‐veneris (top), Consolida regalis (middle) and Lithospermum arvense (bottom), were sown at a range of densities amongst a growing crop plant, rye, Secale cereale, and the yield of seeds of the three plants monitored at the end of the growing season. (b) Results for the same plots in terms of the biomass yield of the rye (dry tons per hectare).

Source: After Lang et al. (2016).