Geography Reference
In-Depth Information
government actions also strongly influenced
deforestation. In this case, the problem was neither
colonisation schemes nor economic subsidies for
cattle ranching but the monopolistic practices of
the Dictator Rafael Trujillo and the chaos
following his assassination. Brothers concludes that
the national government bears a major
responsibility for deforestation and contradicts that
government's suggestion that the problems are
caused by farmers (
cf
. Kustudia 1998). The
continued opposition between the government
and the rural population also militates against any
remedy to the problem.
as the strength of the rock, tree roots or soil, are in
balance with those that encourage its failure, such
as gravity, depth of soil, weight of trees, etc. Where
rock and debris slopes lie on the very threshold of
failure, they exhibit behaviour associated with
'self-organised criticality' (Bak 1997; Noever
1993; Haigh
et al
. 1988). Vulnerable to very small
disturbances, these slopes fail unpredictably in
individual cases, but predictably in statistical terms.
Their vulnerability may be predicted as a
characteristic log linear association between
landslide volume and landslide frequency (Haigh
1988).
Trees are increasingly employed by
bioengineers for the stabilisation of steep slopes
(Barker 1984). The reason is that tree roots have
a tensile strength that can be equivalent to, or
greater than, steel. In forest environments, tree
roots provide a significant component in the
capacity of a hillside to resist failure. Tropical
forest data are not to hand, but research in
Alaska's declining yellow cedar forests suggests
that tree roots can be as important a control on
landsliding as pore water pressure on shallow soils
(Johnson 1993). Here, the application of simple
map overlay techniques demonstrated that steep
slopes suffering cedar decline have three times as
many landslides as steep slopes in adjacent forests
(Johnson and Wilcock 1998). These authors
investigated the possibility that increased
landslide activity was due to increased soil
saturation following the reduced transpiration of
the dying cedars but, ultimately, rejected this
possibility for the more conventional explanation
of loss of root strength. Where trees had been
dead for 14-51 years, 70-90 per cent of roots
with diameters from 1-30 mm had decayed. This
reduced soil cohesion by up to 80 per cent (4.6
to 0.9 kPa). The effect of root decay on the factor
of safety of the slope was as great as that of
changes in pore water pressure, and greater on
slopes with shallow soils.
The bulk of the landslide activity that follows
deforestation is the generation of shallow slumps
of the deep soils and debris covers that forests
preserve on even quite steep slopes. The process is
affected by slope angle. Studies at Taranaki, New
ENVIRONMENTAL IMPACTS OF
DEFORESTATION
Japan's 'Basic Plan for Forest Resources, 1996'
ascribes five main functions to forests:
conservation of water resources, environmental
conservation, timber production, health/cultural
activities and disaster prevention (Price 1998).
Trees play a major role in mitigating extreme
events: floods, avalanches and landslides
(Charoenphong 1991; Duffy and Ursic 1991).
When the trees are gone, negative environmental
changes may follow. However, the message offered
by the section that follows is that not all of the
environmental impacts of deforestation need be
disastrous and that many can be mitigated by
careful environmental management, by good land
and water husbandry (Shaxson 1995; Shaxson
et al
.
1989; Pereira 1989).
Landslides
Deforestation has its greatest geomorphological
impact in mountain regions. Price (1998) argues
that the best way of minimising risks from natural
hazards in mountains is to ensure a stable forest
cover. In steep lands, where there is rapid uplift
and/or stream channel incision, hill slopes evolve
to a condition that lies close to their margins of
stability. In geotechnical terms, these slopes have a
'factor of safety' close to unity. This means that the
forces that preserve the slope and resist failure, such
