Biology Reference
In-Depth Information
Fig. 8.2 Inter-row soil surface environment in a juvenile barley crop direct-seeded into canola
stubble (Lacombe, AB). Weed seedling germination and survival is often more successful in low
surface-residue environments vs. the high-surface residue conditions depicted here
level after 4 years (O'Donovan et al. 2001). Overall, farmers in western Canada
often report lower weed densities after 5-10 years of no-till crop production.
8.5
Weed Vulnerabilities to Resource Availability
Fortunately, both native and alien weeds have exploitable vulnerabilities. Weeds
may suffer greater relative declines in growth than crops when nutrients are lim-
ited because they are adapted to exploit high resource levels (Mohler 2001). Their
generally small seed size is often associated with higher relative growth rates and
greater biomass partitioning to thin leaves (Mohler 2001). However, successful
weed phenotypes are selected by almost infinite, random environments that they
encounter in agroecosystems. As a consequence of this selection, the relatively
greater genetic diversity of weeds compared with most crop species can make
them more responsive or plastic to varied resource availability (Dekker 1997).
Nevertheless, whether weed or crop, some genotypes are simply more plastic than
others and the capacity for phenotypic plasticity is genetically controlled (Via
et al. 1995).
In resource-limited or resource-abundant environments where artificial resources
such as fertilizers are applied, weeds may have the advantage when crop managers
are not careful to somehow sequester the applied resources for the crops. Accordingly,
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