Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 480

pests. Use of cropping system diversity to limit weed adaptation will likely
work best if farmers and crop advisors learn to recognize incipient adaptive
responses in weed populations and know how to respond appropriately.
Although diversification of cropping systems should in general slow the
adaptive response of weeds to specific crops and control tactics, weeds can
potentially cope with complex rotations and polycultures by several means.
First, long gaps between years with a particular class of crop should select for
species and genotypes that have great seed longevity. A testable corollary is
that annual species whose seed store well in the soil should make up a greater
proportion of the seed bank in fields that have been regularly rotated into sod
crops. Moreover, populations of particular species from such fields should
have greater seed longevity than populations from fields that have been in
continuous monoculture of an annual crop. Second, weeds may evolve addi-
tional mechanisms for cueing germination to particular cropping conditions,
and conversely, mechanisms for remaining dormant through unfavorable
periods.Third,complex cropping systems may select for greater plasticity and
polymorphism. Biotypes that can survive at low stature and abundance
through several unfavorable years but produce abundant seed in the occa-
sional good year should be favored. Similarly, a diverse cropping system may
select for increased polymorphism, especially in germination characteristics.
Is a trade-off between selection for general adaptations by diverse cropping
systems and selection for specific adaptations by continuous monoculture
inevitable? In principle, if multiple tactics that attack the weed population at
several life stages keep population densities perennially low,then evolution of
both specific and general adaptations should be slow due to the limited
number of genotypes available for selection and the chance loss of adapted
types when they do occur. Additional strategies for limiting evolution of
general adaptations within the context of diversified cropping systems need
to be identified.
Seed banks and the management of weed adaptation
Presence of a seed bank tends to prevent genetic drift by increasing
effective population size during years in which the above-ground population
is small (Epling, Lewis & Ball, 1960). More importantly, a seed bank can
prevent loss of genetic variability due to transient selection in a fluctuating
environment by shielding a subset of individuals from the selection pressure
(Templeton & Levin, 1979). A seed bank thereby slows adaptation to
management.
Due to more rapid turnover of seed banks in reduced tillage systems rela-
tive to conventional tillage, reduced tillage, and especially no-till, probably