Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 82

contrast,Kelley & Bruns (1975) found no increase in seeds for water flowing in
a canal whose banks were kept free of weeds by grazing, tillage, and burning.
Thus, weed control on canal banks can reduce the dispersal of weeds into
fields.
Weed dispersal as a management issue
The many studies cited in the preceding sections largely agree that
the density of weed propagules dispersing into an area is usually insufficient
to create substantial competitive pressure against crops. Instead, weed popu-
lations appear to reach competitively effective densities primarily through
local population growth. The central problem that human-facilitated weed
dispersal poses for management is therefore the prevention of new infesta-
tions,including both the arrival of new species onto farms and the multiplica-
tion of foci for local population growth within fields. Consequently, from the
farmer's perspective, movement of weeds that are already widespread on the
farm can largely be ignored, and efforts instead concentrated on preventing
the spread of particular weeds that are both competitive and currently absent
from all or much of the farm. From the weed scientist's perspective, the key
issue with weed dispersal is prevention of the spread of economically damag-
ing species through the landscape. Surprisingly little research directly
addresses this problem. Effectively preventing the spread of weeds probably
requires region-wide co-ordination of education and containment efforts
analogous to infectious disease control activities of public health agencies.
This is discussed further in Chapter 10.
Dispersal and spatial pattern
Weed populations usually have noticeably clumped spatial patterns.
The most common pattern is for the frequency of occurrence of individuals to
follow a negative binomial distribution (Zanin, Berti & Zuin, 1989; Wiles et
al .,1992; Mortensen,Johnson & Young,1993; Mulugeta & Stoltenberg,1997).
For wandering perennial species, much of the clumped pattern is the result of
vegetative growth. Variation in soil fertility, tilth, drainage, and the density
and vigor of the crop causes variation in plant size of both perennial and
annual weeds.In addition,variation in the size,burial depth,and genetic con-
stitution among seeds, and in the time of emergence of seedlings, leads to
great variation in the size of annual plants. The resulting clumped distribu-
tion of weed biomass in any given year leads to a clumped distribution of
density the following year, since reproductive output is correlated with plant
size (e.g.,Mohler & Callaway,1995),and most weed seeds only disperse a short
distance. Even if a weed is initially distributed uniformly across a field, as