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Notes:

a Seeds were mixed with the top 15 cm of soil and either left undisturbed or mixed four times per year. b No seeds viable after five years. Source: From Roberts & Feast (1972).

Notes:

a Seeds were mixed with the top 15 cm of soil and either left undisturbed or mixed four times per year. b No seeds viable after five years. Source: From Roberts & Feast (1972).

seed survival (Villiers, 1973), whereas seed longevity is much less in agricultural fields (Table 2.5). Often the logarithm of seed density plotted against time follows a straight line for weed seed banks (Roberts & Dawkins, 1967; Roberts & Feast, 1973), although in some cases the mortality rate is somewhat higher or lower during the first year (Figure 2.3). Thus, seed survival times are often better characterized by the half-life of the population rather than the maximum age achieved by the most persistent individual. Studies on weed seed survival lead to two general conclusions. First, a substantial portion of the seeds of even relatively persistent species disappears from the soil each year, and second, soil disturbance increases the rate of disappearance (Table 2.5) (Roberts & Feast, 1972; Lueschen & Andersen, 1980; Froud-Williams, Chancellor & Drennan, 1984; Warnes & Andersen, 1984; Barralis, Chadoeuf & Lonchamp, 1988).

Figure 2.3 Decline in density of viable seeds through time in a field annually tilled and planted with winter wheat or winter oilseed rape. (After Wilson & Lawson, 1992.)
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