Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 471

This topic advocates management of weeds by manipulation of cropping
systems, and it is therefore worthwhile asking to what extent weeds vary
genetically in response to cropping system characteristics. Unfortunately, few
relevant data exist. Naylor & Jana (1976) compared seed dormancy of Avena
fatua in fields with different cropping histories.One site had been cropped in a
rotation with summer fallow in one year out of three. A second site had only
experienced occasional summer fallows, none of them recent. Naylor & Jana
(1976) established that percentage dormancy was a heritable characteristic in
these populations, and that dormancy was much more frequent in seeds of
plants from the first site, probably because of strong selection against plants
that emerged during the summer fallow years. In a follow-up experiment,
artificial populations established from mixtures of dormant and non-
dormant lines were subjected to either continuous cropping or a rotation with
summer fallow in alternate years (Jana & Thai, 1987). After seven years (three
summer fallows) the continuously cropped fields still had an equitable
balance of dormant and non-dormant types whereas most individuals in
fields with summer fallow were dormant. Similarly, Wilkes (1977) observed a
population of weedy Zea mexicana with strong seed dormancy in a location
where maize was consistently rotated with pasture. Elsewhere in its range,
seeds of Z.mexicana apparently do not persist beyond the next growing season.
Several studies have compared weed with nonweed races, or compared
weed populations subjected to different disturbance regimes.These give some
idea of the type and magnitude of adaptive response that might be expected
from changes in cropping systems or weed management practices. For
example, Hodgson (1970) compared responses of 10 populations of Cirsium
arvense to frequent cultivation with goosefoot shovels. After seven cultiva-
tions,some ecotypes were nearly eliminated,but one retained 32% of its origi-
nal shoot density.
Theaker & Briggs (1993) analyzed several life history and morphological
characteristics of Senecio vulgaris in a common garden experiment to test the
hypothesis that frequent intensive weeding in botanical gardens selects for
precocious development. As hypothesized, plants from botanical gardens
flowered and set fruit earlier than plants from field edges or seminatural hab-
itats (Table 10.6a). Also, plants from the botanical gardens were shorter and
senesced earlier than plants from other habitats (Table 10.6a), indicating a
trade-off between early fruiting and stature. Exposure to frequent intensive
weeding had similar effects on plant size and rate of development in
Arabidopsis thaliana (Jones, 1971) and Stellaria media (Sobey, 1987; Briggs,
Hodkinson & Block, 1991).
Theaker & Briggs (1993) also found that a field population of Senecio vulgaris