Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 42

2.2, below). Small seed size allows for production of many seeds by mature
individuals. This facilitates colonization of new sites. Moreover, a high repro-
ductive 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.
The life history of weeds
Weeds progress through a series of stages in the life cycle: germina-
tion, establishment, growth, reproduction, dispersal, and dormancy.
Management tactics generally apply to a particular stage. Moreover, differ-
ences in the behavior of species in each stage lead to differences in susceptibil-
ity to control by a particular approach. Thus, life history is an organizing
principle for the integration of weed management tactics.
Although agricultural weeds commonly share many ecological attributes,
they are by no means a homogeneous group of species. In particular, four
broad categories of life history can be distinguished (Table 2.2).Annual weeds
grow from germination to reproduction within a single growing season.With
few exceptions, their seeds persist in the soil for at least a few years and in
many cases for decades.
Stationary perennials live from two to several growing seasons (biennials
are included in this group).Because they generally do not rejuvenate via vege-
tative reproduction, they eventually die. As with the annuals, their seeds
usually persist in the soil for at least a few years, and often much longer.
Under favorable conditions some of these species may set seed the year of