Environmental Engineering Reference
In-Depth Information
and thereby propagate the plant vegetatively. Above- and below-ground modules
are relatively autonomous such that herbivory or physical damage may harm the
plant but not kill it. For example, in most plants removal of vegetative branches will
lead to the production of new tissues rather than death. In addition, fragmenta-
tion of an individual into independent clones (ramets) may arise through physical
means, including many control techniques. Cloning is an important characteristic
leading to the persistence and dispersal of many perennial weeds (Silvertown and
Charlesworth 2001; Radosevich
et al
. 2007).
Flowering plants reproduce sexually by means of seeds, which are usually dis-
persed physically away from the maternal plant and may remain dormant for very
long time periods in the soil. h e soil seed bank is a reservoir of seeds in which
both deposits and withdrawals are made (Fig. 9.2) (Harper 1977; Allessio Leck
et al
. 1989). Deposits occur by seed rain from seed production and dispersal both
on-site and from off -site locations, whereas withdrawals occur by germination,
predation, senescence, and death. Storage of seeds results from the vertical distri-
bution of seeds through the soil profi le, although most weed seeds occur at shallow
depths (Allessio Leck
et al
. 1989; Baskin and Baskin 1998; Benech Arnold
et al
.
2000). As noted above, the soil may also contain a reservoir of vegetative buds
that can revegetate a site following weed removal. h e dynamics of seed and bud
banks should be considered when developing a plant management programme
(Altieri and Liebman 1988; Baskin and Baskin 1998; Swanton and Booth 2004)
(Fig. 9.3).
The objectives of invasive plant management in wildland ecosystems are similar
to those of weed management in agroecosystems, although the specifi c con-
siderations in evaluating whether to control invasive plants differ from those
pertaining to agricultural weed control (Holt 2004). Economic cost:benefi t
analysis is more diffi cult in ecosystems where no harvestable commodity is pro-
duced and the value of the land or services it provides are intangible (Pimentel
et al
. 1997, 2000). The principles for setting priorities for management of inva-
sive plants are based on the risk of invasion and the value of the land (Hobbs
and Humphries 1995; Hiebert 1997). Assuming some form of management is
indicated, many of the same approaches and tools for weed control in agroeco-
systems can be used in wildlands, as described below (Holt 2004; Smith
et al
.
2006).
containment, and control
Management of weeds and invasive plants is a general strategy that encom-
passes the approaches of prevention, eradication, and control (Table 9.1) (Ross
