Environmental Engineering Reference
In-Depth Information
pollination web has successfully reassembled during the restoration process. In
other words, restitution of the facilitation role played by pollinators posed no
problem for the meadow restoration timetable.
8.2.6
Invoking
enemy-interaction
theory
Herbivores can be a potent infl uence on succession if they preferentially consume
plants from a particular successional stage. They can be expected to speed up suc-
cession if they fi nd pioneer species most tasty unless, of course, these are plants
that normally facilitate later species. On the other hand, succession may be slowed
if herbivores have their greatest negative infl uence on later species. Thus, removal
of insect predators of seeds during an old-fi eld succession led to dominance of the
meadow goldenrod (
Solidago altissima
) after only 3 years, when this species nor-
mally appears about 5 years into the timetable (Carson & Root, 1999). This hap-
pened because release from seed predation allowed the goldenrods to outcompete
earlier colonists more quickly.
Vertebrate herbivores can be just as infl uential as insects. Thus, Lai and Wong
(2005) improved the growth and survival of seedlings of thick-leaved oak (
Cyclo -
balanopsis edithiae
), a late-successional species of the original primary forest of Hong
Kong, by preventing access to browsing mammals (Figure 8.10). In their study
the total cost per surviving seedling was reduced from US$6.76 in the control
(access to browsers) to US$4.05, despite the cost of providing the tree guard (and
weed mat).
Within the patchy mosaic of forest in the Missouri region of the USA, 'Ozark
glades' are rocky outcrops where pH, nutrient concentrations and soil moisture are
low, limiting plant productivity and producing characteristic prairie-like patches in
the forest. Historically, these glades were maintained by fi re cycles that prevented
tree encroachment. But decades of fi re suppression have resulted in the intrusion of
fi re-intolerant species such as Eastern red cedar (
Juniperus virginiana
). Cur rent res-
toration practices, which include cutting down the cedars and burning the waste
in 'brush' piles, seem sensible enough, but the brush piles represent habitat for
(a)
(b)
100
120
Control
Weed mat
Tree guard and weed mat
100
80
80
60
60
40
40
20
20
0
0
0
10
20
30
40
0
10
20
30
40
Time (months)
Time (months)
Fig. 8.10
Protection from browsing mammals by a plastic sleeve ('tree guard') increased both (a)
seedling growth and (b) survivorship of thick-leaved oak in restored Hong Kong forest. The
provision of a hessian 'weed mat', to reduce competition with pioneer plants, improved
survivorship but to a lesser extent than when combined with a tree guard. (After Lai & Wong,
2005.)
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