Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 83

might happen, for example, if it were introduced in contaminated seed, the
high variance in reproductive output among individual plants (Figure 2.6)
guarantees that the species will quickly become clumped.However,the initial
distribution of a species within the field is usually far from uniform.Often the
weed expands from an initial point of establishment, either as an expanding
front, or with new inoculation points appearing elsewhere in the field by
intermediate-distance dispersal from the original site (Cousens & Mortimer,
1995,pp.217-42).Some weed populations may be entirely maintained by dis-
persal from an adjacent habitat that is more suitable for the plant's reproduc-
tion. In such cases, the species is likely to be more common along the edge of
the field (Marshall, 1989; Wilson & Aebischer, 1995). However, even weeds
that are well adapted to farm fields are often more abundant along field edges
due to soil compaction, lower crop competition, and inefficient application of
herbicides and cultivation (Wilson & Aebischer, 1995).
The persistent storage organs of perennial weeds and the persistent seed
bank of many annual weeds insures that weed patches tend to remain in the
same locations in successive seasons (Dieleman & Mortensen, 1999).
Moreover, all the factors that disperse seeds within a field leave most seeds
within a few meters (or less) of the parent plant.Consequently,once a clumped
distribution of weeds is formed, it tends to persist. For example, Wilson &
Brain (1991) found that Alopecurus myosuroides tended to occur in the same loca-
tions within a farm year after year. Patches tended to persist even through
several years of sod.This is reasonable for a species with a persistent seed bank
since dispersal forces are particularly weak after the seeds are in the soil, espe-
cially if the ground is not tilled. Thus, although variation in the soil condi-
tions within a field doubtless contributes to the maintenance of some weed
patches, the dynamics of reproduction seem sufficient to explain many of the
clumped patterns observed.
Dispersal also creates patchiness at larger scales. For example, McCanny,
Baugh & Cavers (1988) repeatedly surveyed wild Panicum miliaceum popula-
tions in two Ontario townships, and found it present in 10% to 20% of the
tilled fields. They found that the number of infested fields did not change
much, but that the species had a probability of local extinction of 17% to 48%
depending on the year. As some populations went extinct, new ones formed
by colonization of previously uninfested fields. Perennial weeds and species
with persistent seed banks probably behave similarly, but on longer time
scales.