Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 71

similarly low or even lower survival rates for perennial weeds in perennial
vegetation (Forcella & Wood, 1986; McEvoy et al ., 1993; Qi, Upadhyaya &
Turkington, 1996). In contrast with annual weeds, perennial weeds in peren-
nial vegetation like pastures and hay meadows remain in the same location for
an extended period. This makes them easier for herbivores to find and allows
the build-up of enemies to effective levels. Even more importantly, the dense,
diverse crop provides shelter for the herbivores that attack young weeds, and
the shady,moist conditions within the sward facilitate attack by pathogens.In
addition,the weed suffers competitive pressure from the crop,and in pastures
suffers from grazing and trampling by livestock.These factors can be manipu-
lated to decrease weed survival in perennial systems (Chapters 8 and 9).
Life span and seed production
The potential postgermination life span of weeds in agricultural
systems varies from a few months to decades.In most arable cropping systems,
actual life span is rarely more than a few years due to periodic tillage. Some
annuals are truly monocarpic: resources in vegetative tissues are remobilized
to fill seeds and the plant senesces after seed set (e.g., Chenopodium album ,
Setaria faberi ).However, many annuals shed seeds continuously through much
of the growing season and for a substantial proportion of the weed's life span
(e.g., Galinsoga ciliata , Digitaria sanguinalis ).
Continuously fruiting annuals tend to dominate the weed flora of fall-
sown cereals (Figure 2.5), perhaps because most sprawling species are contin-
uously fruiting and a sprawling habit is well adapted for surviving winter
conditions. The early seed production of continuously fruiting annuals like
Portulaca oleracea and Stellaria media adapts them well to cropping systems in
which disturbance occurs throughout the growing season; consequently, they
are common weeds in gardens and vegetable farms. In contrast, the true
monocarpic annuals are more sensitive to frequent weeding or cultivation,
but because they do not expend resources on reproduction early in life, they
are better able to grow tall and compete with large-statured crops.
Consequently, monocarpic annuals tend to dominate the weed flora of tall
crops such as maize, and crops such as oat that are rarely cultivated (Figure
2.5).
The seed production capacity of weeds varies greatly both within and
between species. In most populations, a few individuals produce many seeds,
whereas most individuals produce far fewer (Figure 2.6) (Salisbury, 1942;
Mack & Pyke, 1983). This variation in seed production is largely the result of