Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 56

Figure 2.3 Decline in density ofviable seeds through time in a field annually tilled
and planted with winter wheat or winter oilseed rape.(After Wilson & Lawson,
1992.)
Sources of seed mortality
Factors affecting the rate of seed mortality in the soil include (i) the
action of seed predators, including vertebrates, invertebrates, fungi, and bac-
teria, (ii) physiological aging and exhaustion of reserves through respiration,
and (iii) germination at depths in the soil or times of year that are unsuitable
for emergence. Strictly speaking, the latter involves the death of seedlings
rather than seeds, but it is customarily treated as a source of seed mortality.
Several studies have partitioned the sources of weed seed mortality, and of the
three factors listed above,inappropriate germination often causes the greatest
reduction in seed density (Roberts, 1972). For example, Schafer & Chilcote
(1970) found that after burial for 60 days at 10 cm depth,11% to 13% of Lolium
multiflorum seeds were nonviable whereas 40% to 64% had died after germina-
tion. Zorner, Zimdahl & Schweizer (1984 a , 1984 b ) and Gleichsner & Appleby
(1989) found that in situ germination was the largest source of mortality for
deeply buried Avena fatua , Kochia scoparia , and Bromus rigidus ( B. diandrus ) seeds,
but that loss of viability increased as a cause of mortality with shallower place-
ment. Wilson (1972) noted (i) that A. fatua lost dormancy more quickly at the
soil surface and (ii) that the seeds on the soil surface rapidly lost weight