Weeds from an ecological perspective

Weeds share certain ecological characteristics that distinguish them from other plants. Specifically, weeds are plants that are especially successful at colonizing disturbed, but potentially productive, sites and at maintaining their abundance under conditions of repeated disturbance. That is, weeds are the plants that thrive where soil and climate are favorable to plant growth, but disturbance frequently reduces competition among plants to low levels. Unlike previous conceptions of weediness (Baker, 1965; Harlan & de Wet, 1965; Buchholtz et al., 1967), this ecologically based definition lacks reference to humans and human disturbance. The species people refer to as weeds mostly existed prior to human disturbance, and the repertoire of behaviors that makes them invasive and persistent in human-dominated habitats largely evolved independently of human society. Nevertheless, as discussed in Chapter 10, human activities selectively modify weed characteristics such that weeds are becoming better adapted to human disturbance regimes.

The subcategory of weeds dealt with in this book consists of the weeds of agriculture - specifically, the plants that colonize and increase in the disturbances created by farming. These are sometimes termed agrestal weeds, as distinguished from the ruderal weeds of roadsides, waste piles, and other non-agricultural disturbances (Baker, 1965). Agricultural weeds share certain life-history characteristics that adapt them for life on farms (Table 2.1). The thesis of this chapter is that understanding life-history characteristics provides insights into how weed management practices work and how they can be improved. In particular, differences between weeds and crops in germination characteristics, seed size, growth rate, and susceptibility of different life stages to stress provide weed management options.

Relative to most ecosystems, agricultural fields are not stressful environ-

Table 2.1. Ecological characteristics of agricultural weeds and crops

Character

Weed

Crop

Maximum relative growth rate (g g_1d_1)

Very high

High

Early growth rate (g d-1)

Low

High

Shade tolerance

Low

Low

Tolerance of nutrient 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 cues2

Common

Rare

Seed longevity in soil

Often long

Usually short

Dispersal

Mostly by humans

a Light, fluctuating temperature, nitrate.

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 establishment. 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 monopolized 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; Alkamper, 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 (Schlapfer & Ryser, 1996). Agricultural weeds are at one extreme of these adaptive continua.

Because agricultural weeds establish primarily in conditions of low competition, only minimal provisioning of offspring by the mother plant is required. Hence, weed seeds usually weigh only a few milligrams or less (Table

2.2, below). Small seed size allows for production of many seeds by mature individuals. This facilitates colonization of new sites. Moreover, a high reproductive rate is necessary to compensate for high mortality caused by (i) repeated disturbance during the growing season, and (ii) the environmental unpredictability created by crop rotation and variation in weather. Weeds avoid some unpredictability via dormancy mechanisms and germination cues that allow synchronization of establishment with favorable conditions. They also spread risks across years with different environmental conditions by means of perennation and seed banks. Although all these characteristics allow agricultural weeds to prosper in farm fields, they also provide opportunities for weed management.

Each of the properties of agricultural weeds mentioned above is discussed further in the following sections, with a focus on how the nature of weeds indicates their vulnerability to control. The following discussion focuses on broad patterns and generalities regarding various sorts of weeds. Naturally, exceptions exist for each of these generalizations. To avoid undue digression, however, these exceptions are usually not discussed explicitly. Hopefully, understanding of the usual properties shared by many weed species will also clarify the functional significance of the exceptional properties of unusual species.

Growing Soilless

Growing Soilless

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