Agriculture Reference
In-Depth Information
of the cropped land on the Canadian Prairies is seeded with minimum-tillage
and 31% with zero-tillage practices [11].
Despite the successful adoption of zero tillage on the Canadian Prairies,
concerns have been expressed regarding potential shifts in weed species or
increased weed densities with adoption of reduced tillage practices [2]. An
increase in annual grass and perennial weed species with reduced tillage has
been documented in several studies around the world [12-14], and this has led
to the overall prediction that these species will become the predominant weeds
in reduced tillage cropping systems. However, Pollard et al. [15] reported an
increased density of four annual species, a decreased density of six annual spe-
cies, and no change in density of seven other annual species with zero tillage.
Additionally, reduced tillage has resulted in an increase in volunteer crops in
some studies [16] but not others [17]. Indeed, several studies found that gen-
eralizations regarding reduced tillage effects on weed community dynamics
were far from robust [12, 18]. Thus, further efforts aimed at gaining a better
understanding of weed community responses to changing tillage practices are
warranted.
Invasive weeds and weed communities
Plant invasion refers to the geographical expansion of a species into an area not
previously occupied by that species [19]. Successful invasion requires that a
species arrive, establish, spread and integrate with other members of a com-
munity. Plant invasions occur at many scales from introduction of a non-native
plant into a continent to the establishment of a new weed in an agricultural
field. Three habitat characteristics that are generally thought to encourage
invasions are disturbance (tillage), low species richness, and resource avail-
ability. Agricultural ecosystems possess all of these characteristics.
The main interest of farmers, and many weed scientists, is how best to con-
trol a newly invasive weed species on agricultural lands. However, farmers
should also be concerned with stopping the invasion process as early as possi-
ble (i.e., preventing weed establishment). Most research has concentrated on
the control of individual weed species once they become established in crop-
ping systems. A greater understanding is needed as to why weeds occur where
they do and how they respond to the external stimuli around them.
The study of weed communities, rather than just problem weed species,
may have merit in understanding and managing weeds in agroecosystems [20,
21]. Individuals do not act independently; they are embedded in community
fabric. A community is most commonly defined as an assemblage of species
that occur in the same space and time [22]. Communities are dynamic; con-
tinually changing in response to external and internal forces. As a community
develops, it follows a trajectory through time. A trajectory is a series of com-
munity states. As a community progresses along its trajectory, a series of
changing constraints and processes control its pathway. Belyea and Lancaster
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