Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 59

broadleaf perennials had an average germination of only 5%. In a study of the
flora of northwestern Europe that emphasized arable weeds and grassland
species, Thompson et al . (1998) showed greater persistence of monocarpic
(single fruiting - in this context, annual and biennial) species relative to poly-
carpic (multiple fruiting) species.
A third pattern across species is that those with weak dormancy mecha-
nisms often have shorter longevity than species with well-developed dor-
mancy mechanisms (Bostock, 1978; Roberts, 1986). The greater persistence of
species with dormancy is due to reduction of inappropriate germination
rather than to greater retention of viability (Roberts, 1972). Experimental
support for the importance of innate dormancy for seed survival comes from
studies showing that seed lots of several weed species that were dormant
when buried retained viability in the soil better than initially nondormant
seed lots (Taylorson, 1970; Naylor, 1983; Zorner, Zimdahl & Schweizer,
1984 b ).
A fourth general pattern is that small, round-seeded species tend to persist
in the seed bank longer than species with large or elongate seeds. Thompson,
Band & Hodgson (1993) surveyed 97 species and found that few species with
seeds larger than 2 mg or with variance in diaspore relative dimension greater
than 0.2 persisted longer than 5 years in the soil. In the non-agricultural con-
ditions in which seed persistence evolved, incorporation into the soil was
probably more difficult for large or elongate seeds than for small, round ones.
The latter can wash into cracks,or be ingested by earthworms more easily than
the former. Since seed survival is lower on the soil surface than deep in the
profile, species with a low probability of incorporation into the seed bank
probably experienced little selection favoring mechanisms that allow long
persistence. Large seeds are also more likely to be eaten by small mammals
(Hulme, 1994), and possibly selection rarely favors mechanisms that allow
long residency in the soil of large-seeded species for this reason. The excep-
tions to the general pattern occur primarily in the Leguminosae,
Convolvulaceae, and Malvaceae, which include many species with large, hard
seeds (Taylorson, 1987). Seeds of most of these species have toxins that may
help defend them against mammalian seed predators (e.g., Senna obtusifolia ,
Ipomoea purpurea ) (Kingsbury, 1964, p.314; Friedman & Henika, 1991).
Seed persistence and weed management
Persistence of seeds in the soil has consequences for many aspects of
weed management. By allowing a given generation of seeds to test the suit-
ability of several growing seasons,a seed bank buffers annual species against a
year in which little reproduction is possible (Cohen, 1966). This protects the