Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 67

limiting resource to the lowest level (Tilman, 1982). Thus, in unfertilized
grasslands, the superior competitor is usually the species that reduces the
nutrient element in shortest supply (often nitrogen) to the lowest concentra-
tion when the several species are grown in monoculture (Tilman & Wedin,
1991 a ,1991 b ).However,the ability to compete on low-nutrient soils is usually
associated with a low ability to adjust growth rate in response to resource
availability (Lambers & Poorter, 1992). Consequently, when grassland is ferti-
lized, large, fast-growing species tend to overtop and shade out species
adapted to low-nutrient conditions (Smith, Elston & Bunting, 1971). Since
fast-growing species grown under favorable conditions tend to have higher
concentrations of nitrogen (protein), minerals, and water, and lower concen-
trations of secondary compounds than slow-growing species (Lambers &
Poorter, 1992), their palatability is high. That is, the potentially fast-growing
species tend to be desirable forage species (i.e., the “crop”). Thus, fertilizing a
weedy grassland tends to have the opposite effect on the competitive balance
between crops and weeds of fertilizing a weedy annual crop. However,
legumes complicate this picture somewhat. In low-nitrogen soils, nitrogen-
fixing legumes tend to have an advantage relative to other species (Donald,
1961), despite relatively slow growth (Grime & Hunt, 1975). Where legumes
have been abundant for a while, however, nitrogen accumulates, and taller-
growing grasses tend to displace them (Turkington & Harper, 1979). Dung
deposition also favors tall-growing grasses that displace legumes and contrib-
utes to a shifting mosaic of species in pasture communities (Lieth, 1960).
Weed management in pastures is discussed further in Chapter 9.
Survival after emergence
Rates of natural mortality due to disease, herbivory, and drought are
usually low for established weeds in annual crops. In the absence of post-
emergence weed control, survival rates for annual weeds from the cotyledon
stage to maturity usually lie between 25% and 75% (Table 2.8) (Chancellor &
Peters, 1972; Naylor, 1972; Sagar & Mortimer, 1976; Weiss, 1981; Mack &
Pyke, 1983; Lapointe et al ., 1984; Mohler & Callaway, 1992). Sometimes,
however, rates of survival to maturity exceed 90% (Young, 1986) or approach
0% (Lindquist et al ., 1995) (Table 2.8). Given the high reproductive rates of
annual weeds (see the following section), their reported survival rates seem
surprisingly high, and probably indicate that most weed mortality occurs
prior to emergence or is due to post-emergence control measures. Mortality
rate usually decreases with increasing plant size or age (Weiss,1981; Mohler &
Callaway, 1992; Buhler & Owen, 1997).