Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 41

Table 2.1. Ecological characteristics of agricultural weeds and crops

Character Weed Crop

Maximum relative growth rate (g g 1 d 1 ) Very high High

Early growth rate (g d 1 ) Low High

Shade tolerance Low Low

Tolerance ofnutrient stress Low Low

Nutrient uptake rate Very high High

Seed size Mostly small Mostly large

Size at establishment Mostly small Mostly large

Reproductive rate High Varies with crop

Seasonal innate seed dormancy Frequent Very rare

Germination in response to tillage related cues a Common Rare

Seed longevity in soil Often long Usually short

Dispersal Mostly by humans By humans

Notes:
a Light,fluctuating temperature,nitrate.

ments for plants: to get high productivity from crops, the grower reduces
stress through seedbed preparation, fertilization, irrigation, and artificial
drainage. Moreover, in annual cropping systems, resources greatly exceed the
needs of both crop and weeds for several weeks after the crop is planted, and
during this period competition has a negligible effect on seedling establish-
ment.The species that do well in these conditions,namely agricultural weeds,
prosper because they have very high maximum relative growth rates (see
section “Vegetative growth and crop-weed competition” below). This allows
them to grow large rapidly and occupy space before resources are monopo-
lized by crops and any ruderal species that happen to be present.The very high
relative growth rates of agricultural weeds are coincident with inefficient
resource use. Weeds are more susceptible to the negative effects of shade than
are species commonly found in less disturbed conditions (Fenner, 1978).
Weeds typically accumulate higher concentrations of mineral nutrients than
crop species when nutrients are plentiful, but often suffer greater relative
declines in growth than crops when nutrients are in short supply (Vengris,
Colby & Drake, 1955; Alkämper, 1976). Inherent physiological trade-offs
appear to prevent plants from fully adapting to both high and low light levels
(Givnish, 1988), or to both high and low nutrient availability (Schläpfer &
Ryser, 1996). Agricultural weeds are at one extreme of these adaptive
continua.
Because agricultural weeds establish primarily in conditions of low compe-
tition, only minimal provisioning of offspring by the mother plant is
required.Hence,weed seeds usually weigh only a few milligrams or less (Table