Environmental Engineering Reference
In-Depth Information
suitable for the project, then it will be necessary to take
actions to actively increase colonization. In this section,
I discuss considerations for choosing methods to intro-
duce and manage these species.
ments of many tropical tree species (e.g. timing of
fruiting, germination requirements and light and soil
requirements), and to screen which of these species
establish and grow rapidly in plantations (e.g. Rod-
rigues
et al
. 2009; Kettle 2010). Some studies have
shown that planting fast-growing
exotic species
is a
cost-effective approach to accelerating recovery, as
these species may facilitate establishment of native
species in the understorey (Ashton
et al
. 1998 ; Janzen
2002). These exotic trees can be logged after roughly a
decade, allowing the native species to grow while also
providing a source of income to landowners if there
are nearby markets. Concerns, however, have been
raised about potential long-term negative effects of
these species on soil chemistry (Boley
et al
. 2009 ; Wei
et al
. 2009 ).
A few different native species-planting approaches
are used (Lamb
et al
. 2005). Most commonly, a small
subset of rapidly growing species are planted in hopes
that they will create a closed canopy within 1-3 years
and thereby facilitate forest recovery by attracting dis-
persers and creating microhabitat conditions favoura-
ble for forest seedling establishment (Lamb
et al
. 2005 ).
The specifi c species selected, particularly when only a
few are used, can strongly infl uence the composition
of species that recruit in the understorey (Cusack &
9.4.1
Tree planting
The most commonly used strategy is to
reintroduce
woody species, as seedlings, cuttings or seeds, which
can help to overcome several of the obstacles to recov-
ery. Specifi cally, trees provide the canopy architecture
to encourage seed dispersal by birds, shade out light-
demanding pasture grasses, ameliorate stressful micro-
climate conditions, and often improve soil structure
and soil nutrient availability (Holl 2002; Cusack &
Montagnini 2004; Figure 9.4); these effects in turn
serve to enhance seedling establishment of forest
species. There are several choices to be made when
designing a planting strategy, including species selec-
tion, planting density, propagation method and seed-
ling maintenance protocol.
Native species
should be used preferentially for
tropical forest restoration. Over the past two decades,
there has been considerable research to increase
knowledge of the natural history and nursery require-
Figure 9.4
Former pasture lands in Costa Rica 3 years after abandonment. The two sites looked identical at the start of the
study. Control treatment (left) and plantation of four tree species to shade out grasses and facilitate seedling establishment
(right). (Photographs by Karen D. Holl.)
