Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 44

establishment, but often they require longer to mature. Most stationary per-
ennials 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 frag-
mentation, 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 ecologi-
cal rather than physiological trade-offs. For example, propagation by rhi-
zomes 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 of weed 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 distur-
bance, 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 germi-
nated 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,