Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 58

insects,and fungi (Wilson & Cussans,1972; Grant,1983; Brust & House,1988;
Fellows & Roeth, 1992). Seed predation, and the manipulation of agricultural
systems to increase predation on weed seeds,are discussed further in Chapters
5 and 8.

What types of species survive better in soil?
Species vary greatly in their ability to survive in the soil seed bank,
and certain broad patterns exist that predict which species survive well in the
soil.Understanding trends in seed survival across species provides a means for
targeting ecological management strategies at particular classes of weeds. It
also provides a basis for guessing the likely seed persistence of unstudied
species.
A first broad pattern across species is that many broadleaf weeds are able to
survive in the seed bank for several decades (Chancellor, 1986), whereas seeds
of only a few grass species survive in substantial numbers for more than 5 to
10 years (Dawson & Bruns, 1975; Froud-Williams, 1987, Baskin & Baskin,
1998 b ). Moreover, the seeds of some grass weeds do not persist longer than a
single year (e.g., Bromus diandrus - Harradine, 1986; Gleichsner & Appleby,
1989; Bromus sterilis - Roberts, 1986), whereas few, if any, broadleaf agricultu-
ral weeds appear to have seed survival times as short as that. Both of the
annual grasses tested by Conn & Deck (1995) ( Avena fatua and Hordeum jubatum )
were reduced to 1% viability by 3.7 years of burial and were completely gone

after 9.7 years. In contrast, mean survival of the 13 annual broadleaf species
they tested was 32% after 3.7 years,and all but one of the broadleaf species had
at least a few surviving seeds after 9.7 years. Burnside et al . (1996) counted
seedlings emerging from seed samples that had been left in the soil for 1 to 17
years. After 12 years of burial, the 14 annual broadleaf species averaged 10%
germination,whereas germination of the 11 annual grasses averaged only 2%.
Nevertheless, some economically important weedy annual grasses, notably
Echinochloa crus-galli , Bromus secalinus , and several species of Setaria , retained
moderate viability into their second decade of burial.
A second broad pattern among species is that annual and stationary peren-
nial weeds tend to form persistent seed banks, whereas wandering perennials
and woody weeds usually do not. For example, of the 22 terrestrial grass
species tested by Roberts (1986), the weedy, short-lived Avena fatua , A. sterilis
ssp. ludoviciana ,and Poa annua had the greatest emergence 2 years after sowing,
whereas many of the rhizomatous species were completely gone by that time.
In the study of Burnside et al . (1996), seven stationary perennial broadleaf
species (including biennials) averaged 37% germination after 8 years of burial
and 25 annual broadleaf species averaged 15%, whereas nine wandering