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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.
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Direct impacts of logging probably
hinge largely on
the degree of canopy opening.
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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.
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This mahogany tree is a
prime candidate for felling.
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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).
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Site where a mahogany
tree was recently felled.
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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).
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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.
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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.
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Mahogany experts examine a sapling at a
community
forestry concession in Quintana Roo, Mexico.
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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
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Synergisms between selective
logging, forest
fragmentation, and escaped fire present an
alarming vision of the future.
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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.
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