Agriculture Reference
In-Depth Information
2005, Brazil has an estimated 3.2 M ha of eucalyptus plantations (Neto 2005),
the exotic species most abundant in that country's plantation program
(McNabb 2005).
Exotic trees dominate the country's tree planting program for the following rea-
sons: (1) wide adaptability and tolerance to stress; (2) fast growth and high yield;
(3) available researches and technologies and; (4) availability of abundant and
superior germplasm.
15.2.1
Wide Adaptability and Tolerance to Stress
An obvious advantage recognized by most foresters and tree farmers planting
exotic trees is their versatility especially to unfavorable conditions which are char-
acteristic of many target areas for restoration. The adaptability of exotics to
degraded sites (e.g. acidic, low soil fertility, fire-prone areas) and their ability to
colonize even marginal grasslands is an attractive feature of these trees that makes
them widely planted. As an example, the exotic legumes (e.g.
Acacias,
Paraserianthes
) are nitrogen-fixing trees that permit optimum growth and develop-
ment even in nitrogen-depleted soils. Additionally, the wide adaptability of exotic
trees enables them to grow in a new environment free from the usual pests and dis-
eases common in its natural habitat (Pryor 1978; Evans 1992) making them almost
pest and disease-free at least for the first rotation. Gmelina and
A. auriculiformis
are fire resistant and will coppice after a fire. Likewise, their fast growth enables
them to compete even with the obnoxious
Imperata cylindrica
(ERDB 1998).
15.2.2
Fast Growth and High Yield
Concomitant with the ability to survive and grow under a wide range of environ-
mental conditions, exotics exhibit exceptionally fast growth and high wood yield.
These characteristics make them very attractive for smallholder tree farmers desir-
ing quick income and immediate returns to their investments. Some estimates
revealed that the yield of exotics trees ranges from a low of 5 m
3
ha
−1
year
−1
in poor
sites to as much as 40 m
3
ha
−1
year
−1
in good sites (ERDB 1998; Table 15.1).
However, most of these species exhibit impressive growth yields averaging from 30
to 35 m
3
ha
−−1
year
−1
. This is almost similar to the average growth performance of
Eucalyptus
species in Brazil which is 20 to 40 m
3
ha
−1
year
−1
. Additionally, a phe-
nomenal growth of 75 m
3
ha
−1
year
−1
was accordingly observed (Kageyama 1980
cited by McNabb 2005). In comparison, many indigenous trees are notoriously
slow growing making them less attractive to many smallholder tree farmers. In the
case of dipterocarps, an important ITS in the Philippines, growth rates vary from
different diameter classes ranging from a low 0.44 cm year
−1
(10 cm dbh class) and
peaking at 0.86 cm year
−1
(70 cm dbh class) (Weidelt and Banaag 1982). In
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