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Setariafaberi

Poa annuab

Convolvulus arvensis

Toxicodendron radicans

Notes:

* Seed size ranges and medians from an analysis of 39 annual, 18 stationary perennial, and 16 wandering perennial British weed species of arable land and well-drained grassland reported in Salisbury (1961) were 0.02-35 (1.1), 0.2-3 (1.2) and 0.13-14 (0.7) mg, respectively, indicating no difference in seed size between the three categories.

6 Because Poa annua sets seed during its first season of life, it can behave as an annual in annual cropping systems. However, if left undisturbed it usually lives at least two seasons, and often sets more seed the second year (Law, Bradshaw & Putwain, 1977).

establishment, but often they require longer to mature. Most stationary perennials are broadleaf species. Perennial bunchgrasses are intermediate between stationary and wandering perennials in that most can rejuvenate indefinitely, but have limited capacity for spreading vegetatively.

Because wandering perennials reproduce by vegetative spread and fragmentation, the life span of a genetic individual is indefinite and potentially very long. Although most of these species produce seeds, most reproduction is by vegetative propagation. Seeds of many wandering perennials persist for more than one year, but relatively few have great seed longevity; consequently, wandering perennials are rarely well represented in the seed bank.

Woody weeds are perennials that develop persistent shoot structures. Although some species spread clonally, most reproduce primarily by seed. Their seeds are often relatively large, short-lived, and dispersed by wind or birds. As explained later, they are problems in long-lived crops like orchards and permanent pastures, and are increasingly problematic in no-till planted annual crops (J. Cardina, personal communication). Woody weeds with a vining growth habit (lianas) are often the most difficult to control because they can sprawl laterally and can rapidly reach an orchard canopy by using the crop trees for support.

The contrasting life-history characters of the four groups express ecological rather than physiological trade-offs. For example, propagation by rhizomes is probably not physiologically related to lack of persistence in the seed bank. Basically, adaptation to different stages of ecological succession has grouped characteristics into suites, thereby forming four ecologically distinct types ofweed species.

Following a severe disturbance like tillage, annuals predominate because they can survive the disturbance event in a physiologically dormant state as seeds. The stationary perennials are similarly tied to establishment shortly after tillage. However, because they persist in a vegetative state for a longer period, their allocation of resources to roots is greater, and consequently, their seedling growth rate tends to be lower. Thus, in the first year after disturbance, annuals often predominate even if stationary perennials are abundant in the seed bank. However, because stationary perennials start growth with greater reserves during the second season of life compared with newly germinated annuals they are better able to compete with established perennial crops after the first year. Consequently, they are particularly common in hay fields.

In pre-agricultural landscapes, wandering perennials were probably found primarily after the first year of regrowth in fertile, disturbed locations like areas where animals congregated or flood deposited soil along streams. Today, wandering perennial agricultural weeds like Elytrigia repens and Cirsium arvense are commonly found in well-vegetated, abandoned fields, roadsides, and even disturbed forest. To spread under highly competitive conditions in closed vegetation, they were selected to substantially provision vegetative propagules. Seeds probably served primarily for the risky enterprise of long-distance dispersal.

The advent of tillage greatly changed conditions of life for wandering perennials. Tillage separates daughter plants from the parent and spreads them within and between fields. Simultaneously, tillage removes the competing vegetation. This puts spreading perennials in the advantageous position of having well-provisioned propagules establishing with relatively little competitive pressure. Consequently, many of the world's worst weeds are wandering perennials (Holm et al., 1977). Essentially, these species have characteristics that evolved in response to conditions quite different from present-day agriculture, but they were fortuitously pre-adapted to thrive under moderate tillage. However, deep and frequently repeated tillage is often detrimental to these species (Chapter 4).

Woody perennial weeds are primarily problems in orchards and pastures. Although they help restore soil and eliminate pests and disease during the regenerative phase of shifting agricultural systems (swidden), they can also reduce crop productivity during the cropping phase (Staver, 1991). Woody perennials are poorly adapted to cropping systems with annual tillage for two reasons. First, only a few of these species form persistent seed banks, and consequently, synchronizing establishment with an annual crop is rarely possible. Second, wood is an energetically expensive way to hold up a plant relative to fiber and turgor pressure. Consequently, woody plants grow more slowly than herbs when young (Grime & Hunt, 1975), and as young plants they are rarely competitive with herbaceous annual crops. Because tillage prevents woody weeds from surviving more than one year, they are poorly adapted to most annual cropping systems. Nevertheless, their long life span, tall stature, and vigorous resprouting after cutting and browsing make them serious weeds in tree crops and pastures, and a significant nuisance in no-till planted annual crops.

Parasitic weeds constitute an additional life-history category not included in Table 2.2. To the extent that they behave like other vascular plants (e.g., seed production, seed dispersal), the principles discussed in this book apply to them as well as to weeds that affect the crop primarily through competition for resources. To the extent that they behave more like pathogens (e.g., germination in the presence of host roots, source of nutrition), their study is a specialized field beyond the scope of this book. Several recent books treat the

Figure 2.1 Dormancy/germination states of weed seeds. (Redrawn from Egley (1995) based on the concepts of Baskin & Baskin (1985,1998«).)

ecology and management of parasitic weeds (Musselman, 1987; Pieterse, Verkleij & ter Borg, 1994; Hosmani, 1995).

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