Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 45

wandering perennial agricultural weeds like Elytrigia repens and Cirsium arvense
are commonly found in well-vegetated, abandoned fields, roadsides, and even
disturbed forest.Tospread under highly competitive conditions in closed veg-
etation, they were selected to substantially provision vegetative propagules.
Seeds probably served primarily for the risky enterprise of long-distance dis-
persal.
The advent of tillage greatly changed conditions of life for wandering per-
ennials. 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 com-
petitive pressure. Consequently, many of the world's worst weeds are wander-
ing 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 con-
sequently, synchronizing establishment with an annual crop is rarely pos-
sible. 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 pre-
vents 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 topic 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., germi-
nation in the presence of host roots, source of nutrition), their study is a spe-
cialized field beyond the scope of this topic. Several recent topics treat the