 |
SOME PRINCIPAL AGENTS OF TROPICAL DEFORESTATION
Commercial Hardwood Logging
The role of commercial logging in the modification and
destruction of tropical forests varies strongly from region to
region. This regional variation is owing not only to the
relative ease of access into forested wilderness areas, but to
the biological nature of the forest itself. A key factor is the
number of tree species harvested, which hinges partly on the
tree species composition of the forest, and their lumber
characteristics.
|
|
Direct impacts of logging probably
hinge largely on
the degree of canopy opening.
|
In southeast Asia, often many species are harvested,
sometimes resulting in tremendous opening of the canopy. In
South America, many tree species are also currently harvested,
often leading to serious canopy opening. In the Selva Maya
region, most harvesting is typically restricted to mahogany (Swietenia
macrophylla), and to a lesser extent Cedro (Cedrela
odorata). Hence canopy opening is often minimal.
Logging in Southeast Asia
In terms of direct impacts, logging in the southeast Asian
tropics tends to be at the opposite end of the scale from
mahogany logging. Many trees in southeast Asia belong to a
single plant family, the Dipterocarpaceae, and many of these
have wood characteristics valued by the lumber trade. As a
result, logging operations in this part of the world tend to
remove more species, more individual trees per unit area, and
hence create much greater canopy opening in comparison with a
typical mahogany logging operation. While careful felling,
harvest limitations, and other practices can help reduce
impacts, in many cases a southeast Asian forest after logging is
a fundamentally different habitat than was the pristine forest
prior to logging.
Logging in Amazonia
While some logging in South America is of mainly or strictly
mahogany, in many cases logging is directed toward a larger
number of species, and canopy opening and structural damage tend
to increase in parallel.
In the eastern Amazon Basin of Brazil, up until the
mid-1900s, only six tree species comprised most of the wood
harvested. Since then, this number has blossomed, and some 140
species are now commercially harvested (Uhl and Vieira
1989). In
one study of a typical logging operation here, while fewer than
2% of the trees in the sampling area were harvested, 26% were
killed or damaged. Sixteen percent of the stand's total basal
area was removed, and an additional 28% of the basal area was
destroyed or damaged. While unlogged forest in this area had a
canopy cover of about 80%, this was reduced by nearly half, to
43% in the logged area (Uhl and Vieira 1989). About 30 tree
species were harvested, though three species made up more than
half the harvest.
|
|
This mahogany tree is a
prime candidate for felling.
|
Mahogany Logging
In the Selva Maya most logging has been of a single species,
mahogany. Because mahogany trees are often few and far between,
their removal often causes minimal canopy opening. However, in
the process of creating a bulldozer road to each such tree, and
creating larger feeder roads and scattered clearings that serve
as log loading yards, the total canopy opening is far greater
than that resulting from the felling of the target trees. In
addition, damage to remaining trees can be severe, due both to
mechanical injury by heavy machinery and breakage resulting from
heavy vines tying the felled trees to surrounding trees.
Despite these several contributors to canopy opening, in many
cases when one walks through a forest a few years after such
logging, it is difficult to distinguish logged from virgin
forest, except by the reduced number of large mahoganies,
occasional stumps, and the presence of old logging roads.
In one mahogany logging operation in Belize, only one tree
was harvested per two hectares of forest. For the forest as a
whole, canopy cover was decreased by only 2%, and 4.8% of the
adult trees and 1.9% of the saplings were damaged. Logging
directly affected 12.9% of the forest area (Whitman et al.
1998).
|
|
Site where a mahogany
tree was recently felled.
|
In another study of mahogany logging, in Bolivia, densities
of commercially valuable trees was even lower--0.12 trees per
hectare, or one tree per 8.3 hectares--and 4.4% of the forest
area was damaged (Gullison and Hardner
1993).
Mahogany logging is sometimes more intensive than suggested
above. In our own study at Tikal, logging removed 3.6 trees per
hectare and affected 12% of the forest (Schulze and
Whitacre, unpubl. data). Still, such single-species mahogany logging
provides an example of the lower intensities of direct impact
found among tropical hardwood logging.
Direct Biological Impacts of Logging
Research on the direct effects of logging on tropical forest
biota has been limited, and this is an area of much ongoing
research activity. Still, some comments are possible.
The magnitude of direct effects on the forest biota is
related in part to the severity of canopy opening resulting from
logging. As a working hypothesis one may envision that, the
greater the canopy opening, the greater the modification of the
forest's microclimate, of the future trajectory of the forest
vegetation, and of living conditions for animals. As noted
above, the magnitude of canopy opening in turn hinges not only
on the number of trees harvested per unit area, but on the
design and construction of the road network and log-loading
yards used in logging.
The few studies to date suggest that the direct effects of
selective mahogany logging on forest biota are minimal. While a
difference in the bird community can at least sometimes be
detected between logged and unlogged sites, studies to date have
found such differences to be subtle (Whitman et al. 1998,
Schulze and Whitacre, unpubl. data).
|
|
Often creation of a logging road is but the first step
in a
process that leads eventually to an expanse
of cattle pastures
and abandoned fields supporting
young second-growth as well as
degraded
areas no longer used for farming or cattle.
|
Indirect Effects
The indirect or secondary effects of mahogany logging are
often far more serious than the direct effects. The main
secondary effect of logging is the access provided by logging
roads into hitherto inaccessible wilderness areas. Such access
often leads to uncontrolled immigration of peasant farmers, and
to attendant high rates of deforestation. While in theory
immigration over such roads should be preventible, in many cases
the long arm of the law scarcely reaches into these remote
frontier regions, and little government regulation is achieved.
Often creation of a logging road is but the first step in a
process that leads eventually to an expanse of cattle pastures
and abandoned fields supporting young second-growth as well as
degraded areas no longer used for farming or cattle.
Logging roads also provide access for hunters, and
unregulated hunting can have serious effects on fauna even
within areas but lightly touched by logging.
In summary, the intensity of direct forest modification by
logging varies strongly with region, and in some cases the
secondary impacts of uncontrolled immigration, as well as
hunting, are far worse than the immediate effects of logging.
Moreover, there is a question of silvicultural and economic
sustainability of logging, and conservationists are divided on
whether extractive forest reserves have a part to play as an
aspect of conservation policy in the globe's tropical forests.
Challenges to Sustainability of Tropical Hardwood Logging
A basic problem is that tropical hardwoods grow, and hence
increase in dollar value, at a rate that is slow compared to
returns on capital that are available through any number of
alternative investments. Left completely to market forces,
loggers can often maximize their profits by heavily logging an
area once, without regard to sustainability, then investing that
money in the stock market or elsewhere. Hence government
regulation is often critical to any hopes of attaining
sustainable forestry practices.
At least two solutions to this dilemma may be seen. First, if
an area, once logged, is placed into a completely protected
status for the long term, this might be acceptable to
conservationists. This is true because, as noted above, the
detrimental direct effects of logging to biodiversity values of
the forest are often but slight.
Second, loggers might simply be restrained by law to a
certain level of impact, and forced to accept harvest schedules
and intensities that will be sustainable in terms of
silviculture (tree growth) and ecologically acceptable in terms
of overall impact on the forest. If logging companies find these
limitations unattractive, they would not be forced to accept
them, and could forego logging opportunities. However, if
logging is not then a viable means of extracting some economic
gain from the forest, this may increase the probability that the
forest will be converted wholesale to some other activity such
as farming or ranching.
|
|
Mahogany experts examine a sapling at a
community
forestry concession in Quintana Roo, Mexico.
|
In the case of mahogany, loggers often effectively mine it
from the forest, harvesting it at a rate exceeding the species'
natural rate of reproduction and growth. This scenario is not
sustainable, and will ultimately lead to the collapse of
mahogany production. A further complication is that mahogany
generally requires fairly large clearings or tree-fall gaps
within which to reproduce. Some foresters believe that, in order
to make mahogany logging silviculturally sustainable, we will
need to intentionally create large disturbances within the
forest as regeneration sites, and/or increase regeneration by
other artificial means. Other forest ecologists feel that such a
level of induced disturbance would not be ecologically
acceptable as a route toward sustained logging.
In summary, issues surrounding the silvicultural
sustainability, economics, and ecological desirability of
mahogany logging are quite complicated, and tropical foresters
and conservationists are divided in their stance toward this
industry. While it is questionable whether an outright boycott
of tropical hardwoods is a helpful reaction on the part of
consumers, there is no question that ecologically concerned
consumers would do well to support efforts toward "green
certification," which recognizes forestry operations in
which sustainable forest management practices are promoted.
Synergistic Effects
|
|
Synergisms between selective
logging, forest
fragmentation, and escaped fire present an
alarming vision of the future.
|
Synergistic effects among logging, fragmentation, and fire
can also be serious.
A recent discovery of synergistic interactions in the Amazon
Basin provides a chilling warning. Selective logging makes
Amazonian forests more fire-prone, and wildfires often result
from escape of agricultural fires. Trees of tropical moist
forests are not fire-adapted, and many trees die as a result of
burning. Once burned, these forests become yet more fire-prone,
and so on in a vicious positive-feedback loop. After three or
more fires, these forest stands often are no longer forest, but
more closely resemble brushy pasture (Cochrane et al. 1999,
Nepstad et al. 1999). Clearly, once such a forest has gone
through such a change, a large fraction of the species that once
made it their home must have disappeared from the forest.
Industrial-scale logging is now accelerating in Amazonia and
other large remaining forests. Hence this synergy between
logging and wildfire could result in catastrophic loss of
remaining forests (Cochrane and Schulze 1998,
1999).
Another frightening prospect is the following. Patterns of
deforestation in the Brazilian Amazon have already fragmented
the forest in some areas to such an extent that very little of
the forest is more than a few kilometers from the forest edge.
Fragmentation is known to cause increasing dryness within the
forest, due to penetration of wind along edges. Such drying is
widely expected to increase the incidence of wildfires in
Amazonia.
Finally, a recent study attempted to predict the future of
Brazil's Amazon forests, given projected patterns of development
(new roads, etc.) in combination with observed rates of
deforestation after similar developments in the recent past. The
optimistic scenario predicted that 42% of the Brazilian Amazon
forest may be destroyed or heavily degraded by the year 2020,
whereas the non-optimistic scenario predicted that as little as
5% of the Brazilian Amazon may remain as pristine forest by the
year 2020 (Laurance et al.
2001).
 Literature Cited, Tropical Logging and the Logging/Fire
Synergy and Sources on Tropical Deforestation and the Impending
Extinction Crisis.
NEXT
14
|
|
