Recommendations for depletion modelling of granivorous birds
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Table 4.2. Density-dependent weed seed production in crops and winter stubbles, for species considered in the modelling exercise
4.3.2 Seed bank mortality and germination Table 4.3 summarises the estimates of seed mortality and germination from seedbank studies. Fig. 4.2 shows how these parameters translate into rates of seedbank decline. As shown in Fig. 4.2a, species vary considerably in the rate at which seedbank numbers decline through time. Species such as Chenopodium album and Fallopia convolvulus have extremely long lived seedbanks. The likely persistence of populations, however, depend not only on the rates of decay of seedbanks shown in Fig. 4.2a, but also depend on the input of seed into the soil. Fig. 4.2b shows the rate of decline in seedbank density following an initial seeding from a single individual. It is clear in Fig. 4.2b that whereas the density of some species may decline to less than one seed in under 20 years, declines in species such as Chenopodium album take many years longer to reach such a level. F  ig. 4.2 The long-term dynamics of seedbanks of arable weed species valuable as food for birds. (a) The poportion of seeds remaining following an initial sowing. (b) The total number of weeds remaining following the seeding of a single plant.
Table 4.3 Estimates of parameters determining seed dynamics of annual weeds. The key parameters are the rate of germination of seeds (g), the rate of seed mortality (m), which have been estimated separately from uncultivated and cultivated soil. Also shown is the proportion of seed lost to germination and mortality over 6 years in cultivated soil. These figures summarise information collated from the following sources: Barralis et al. (1988); Fround-Williams et al. (1983, 1984); Lawson & Wright (1993); Lonchamp et al. (1988); Roberts (1959, 1964, 1968, 1980, 1986); Roberts & Dawkins (1967); Roberts & Feast (1970, 1972); Roberts & Neilson (1980); Wilson & Lawson (1992).
Fig. 4.3 compares the rate of decline in arable weed seed densities in the soil (Robinson & Sutherland 2001), with the rates of seedbank decline in Fig. 4.2. It is clear in this figure that observed rates of seed density decline are many orders of magnitude lower than rates of seedbank depletion. This indicates very clearly that the observed rate of decline in weed seed numbers is the consequence of reductions in weed seed production resulting from increases in the efficiency of control. The relatively shallow decline in mean soil seed densities, compared with the very rapid decline in the densities of seeds already in the soil seedbank, indicates that considerable seed production must occur in order to account for this difference. By comparing these rates of decline, we estimate that at least 25% of the seedbank must (this figure is generated using the rate of decline for Chenopodium album), on average, be renewed to account for this difference. This implies that average seed production per square metre in arable crops has declined from appoximately 1000 m-2 to 50 m-2 over the course of less than a century. Table 4.3 also presents estimates of seed mortality and emergence in uncultivated soils. Both rates of emergence and mortality are considerably lower in uncultivated soils. Clearly arable fields are generally cultivated, hence these estimates are not of general relevance. However, emergence from uncultivated soils does occur in the autumn and winter when stubbles occur. Hence in the modelling below we used the rate of emergence of weeds in uncultivated soils to estimate the rate of germination of seeds in winter stubbles. F   ig. 4.3 Observed rate of decline of seeds in arable soils (Robinson & Sutherland 2000), compared with the rates predicted by the fitted models (Fig. 4.2). The large difference between these two rates implies that considerable seed production occurs, despite increases in control. 4.2.4 Baseline densities and management effects on winter seed production Table 4.5 summarises the information on the efficacy of control supplied by ADAS for the species used in the model. These data summarise the ease with which different weed species are managed in different crops, and hence indicate how management affects the density of plants producing seed at the beginning of winter. Crops differ in the species which are generally easiest to control, and similarly the ease with which individual weed species are controlled depends on the crop within which they are growing. Most species, apart from Alopecurus myosuroides, are readily controlled in winter wheat. Alopecurus myosuroides is also difficult to control in spring barley. Most species appear to be readily controlled in winter rape, as are most species other than Polygonum spp. readily controlled in peas. The other two crops, sugar beet and spring barley, both of which are spring sown, appear to present greater difficulty for weed control, with herbicidal control being only moderate for many species. In general in these two crops, it would be expect that the control of grass weeds would be easier in the borad-leaved crop (sugar beet) and conversely that the control of broad-leaved weeds would be easier in the cereal (spring barley). Table 4.5 Summary of information on the relative ease with which weeds are controlled in different crops, together with typical proportions of weeds killed by herbicide application. Information supplied by ADAS, based on experience with herbicide trials.
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