Anak Krakatau (son of Krakatoa) eruption in 1992. (Photo by N. Butler)
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NATURAL FORCES
Throughout their existence, tropical rainforests have been affected by natural forces like fire, drought, and storms. These events occur on a random basis and can damage large stretches of rainforest. However, the damage caused by these natural occurrences is generally different from that caused by human activities; namely in that the forest loss is not complete and parts of the ecosystem continue to function. From the surviving remnants of the ecosystem, the forest can usually rapidly regenerate. Within a few years, the forest diversity can return to or exceed the diversity that existed before the disturbance. Some studies have suggested that these periodic occurrences are an important ingredient to a forest's diversity. Without these events, scientists believe, some forests cannot reach their fully dynamic state. Researches have found that forest turnover rates may be as short as 65-135 years.
FIRE [news]
Natural forest fires occur in rainforests, despite their humid nature. Generally, when forest burns in smaller fires, ground vegetation, shrubbery, saplings, and smaller trees are eradicated, while the larger canopy species are spared. The fire has the effect of clearing the forest floor and weaker trees, thus allowing new growth.
A forest, stressed by drought or some other factor, is most susceptible to fire, usually sparked by lightening or small human fires that escape neighboring agricultural zones and burn out of control. For example, the great Bornean fires of 1982-83, destroyed more than 9 million acres (3.6 million hectares) of forest (2.81 million ha of dipterocarp forest) in Kalimantan (Indonesia). These fires, like the wildfires of 1997-98, was ignited by land-clearers setting fires in and around forest heavily degraded by logging under el Niņo-induced drought conditions. Research has recently found that over half the Amazon rainforest is at risk of burning during extreme droughts, like the one that struck from July—November in 2005.
VOLCANIC ACTIVITY
Volcanic eruptions and subsequent lava flows sometimes burn large tracts of forest, while the gases released from the activity can kill wildlife. An interesting phenomenon, documented by a National Geographic expedition in Zaire's (now the Democratic Republic of Congo) Virunga National Park, and undoubtedly in other areas with volcanic activity, involves carbon dioxide, a gas that is poisonous in high concentrations. Since carbon dioxide is heavier than air and tends to hug the ground, it forms deadly sink areas when released via volcanic vents. In these areas, green grass grows vigorously due to the abundance of C02 and the lack of herbivores, which are attracted to the untouched grass, but are quickly asphyxiated by the toxic gas. Other animals come, including predators attracted to the dead animals, but they, too, perish. The sink becomes a death trap. However, these carbon dioxide sinks are rare, so they do not pose a major threat to most animals. In 1999, lava flows in Cameroon destroyed sections of virgin rainforest, while there is evidence of large-scale lava damage in the rainforests of Costa Rica.
DROUGHT [news]
Drought can affect forests by weakening the system to the point where it is more susceptible to other events like fire and disease. Drought is often partially induced by human activities like deforestation, but it is also a natural occurrence, like El Niņo, which periodically causes drought in Asian and American tropical rainforests. El Niņo is usually named as one of the influences behind the great Bornean fires of 1982-1983 and 1997-1998.
Drought initially weakens canopy plants by reduction of local humidity and rainfall. If the drought is long enough, the leaf litter dries out, killing off decomposers and reducing the effectiveness of nutrient recycling within the system as a whole.
Drought can bring benefits to the forest. Studies have shown that El Niņo may enhance fruit production in forests affected by seasonal differences in rainfall. El Niņo events produce dry sunny years in Central America, northern South America, the Amazon, Malesia (Indonesia and Malaysia), New Guinea, parts of equatorial west Africa, and India. The reduced cloudiness is beneficial to fruit production in that higher levels of photosynthetically active radiation (PAR) reach plants. El Niņo and high levels of PAR have been associated with mast fruiting.
However it is a different case in the years following El Niņo events since dry, sunny El Niņo events tend to alternate with wet, cloudy years in the tropics. In addition, high fruit production consumes stored reserves for plants, limiting the next reproductive event.
Some scientists are concerned that climate change could worsen droughts in the Amazon. In 2005, the accumulation of warm waters in the tropical Atlantic helped fuel a record hurricane season while reducing the availability of moisture to the Amazon basin. These conditions could worsen as global temperatures increase. Research published in December 2005 suggests that warmer sea temperatures in the tropical Atlantic are likely to become increasingly prevalent as currents normally carrying warm water from equatorial regions towards the Arctic weaken. One simulation by researchers at the University of Illinois, Urbana-Champaign, found that 45 percent chance that existing water currents in the North Atlantic Ocean could shut down by the end of the century if nothing is done to slow greenhouse gas emissions. Such a collapse of the so-called "Atlantic heat conveyor belt" could plunge parts of Europe into significantly colder winters while exacerbating drought conditions in equatorial Africa. Back in the Amazon, the usually moist rainforest could become drier and more susceptible to forest fires.
TROPICAL STORMS
Storms can cause extensive damage in the rainforest through tree falls. When a canopy or emergent tree falls, dozens of other neighboring trees, attached by lianas, are brought down with it. Surveying the forest following a storm can reveal numerous tree falls, light gaps, and fallen matter including epiphytes and branches. However, a healthy forest can recover from moderate storm damage in a matter of months or years. The "light gaps" are quickly colonized and soon filled by canopy trees, while the fallen matter is decomposed and reabsorbed back in to the system.
Larger tropical storms, such as hurricanes (cyclones or typhoons), can cause substantial damage to rainforest and recovery may take decade to centuries. For example, in 1880 unusually strong winds damaged large areas of Kelantan forest in the northwestern part of peninsular Malaysia. A study in the 1950s found the most common tree species were still characteristic of secondary forest and dipterocarps—typical of southeast Asian rainforest—remained sparse. Similarly, recovery from Hurricane Hugo (1989) for the Luquillo montane forest in Puerto Rico is expected to take at least 250 years.
Review questions:
- What are some natural causes of rainforest destruction?
- How could climate change threaten rainforests?
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Continued: Deforestation Threats from Humankind
This article was written by Rhett A. Butler [bibliographic citation for this page] and was last updated on the most recent date listed in the column on the right side.
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WEEKLY NEWSLETTER
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Recent news
Some toilet paper production destroys Indonesian rainforests, endangering tigers and elephants
(02/09/2012) American consumers are unwittingly contributing to the destruction of endangered rainforests in Sumatra by purchasing certain brands of toilet paper, asserts a new report published by the environmental group WWF. The report, Don't Flush Tiger Forests: Toilet Paper, U.S. Supermarkets, and the Destruction of Indonesia's Last Tiger Habitats, takes aim at two tissue brands that source fiber from Asia Pulp & Paper (APP), a paper products giant long criticized by environmentalists and scientists for its forestry practices on the Indonesian island of Sumatra. The brands — Paseo and Livi — are among the fastest growing, in terms of sales, in the United States.
Tropical ecologist: Australia must follow U.S. and EU in banning illegally logged wood
(02/09/2012) Australia should join the widening effort to stamp out illegal logging, according to testimony given this week by tropical ecologist William Laurance with James Cook University. Presenting before the Australian Senate's rural affairs committee, Laurance argued that the massive environmental and economic costs of illegal logging worldwide should press Australia to tighten regulations against importing illegally logged timber at home.
Humans drove rainforest into savannah in ancient Africa
(02/09/2012) Three thousand years ago (around 1000 BCE) several large sections of the Congo rainforest in central Africa suddenly vanished and became savannah. Scientists have long believed the loss of the forest was due to changes in the climate, however a new study in Science implicates an additional culprit: humans. The study argues that a migration of farmers into the region led to rapid land-use changes from agriculture and iron smelting, eventually causing the collapse of rainforest in places and a rise of grasslands. The study has implications for today as scientists warn that the potent combination of deforestation and climate change could flip parts of the Amazon rainforest as well into savannah.
More big companies disclosing impacts on forests
(02/07/2012) More companies are reporting on the impact of their operations on global forests, finds a new report. Eighty-seven global corporations disclosed their "forest footprint" in 2011, according to the third Forest Footprint Disclosure (FFD), which asks companies to report on their impact on forests based on their use of five commodities: soy, palm oil, timber and pulp, cattle, and biofuels. This is a 11 percent rise from the companies that reported in 2010, including the first reports by companies such as the Walt Disney Company, Tesco UK, and Johnson & Johnson. However a number of so-called "green" companies continue to refuse to disclose, including Patagonia, Stonyfield Farms, and Whole Foods Markets Inc.
New meteorological theory argues that the world's forests are rainmakers
(02/01/2012) New, radical theories in science often take time to be accepted, especially those that directly challenge longstanding ideas, contemporary policy or cultural norms. The fact that the Earth revolves around the sun, and not vice-versa, took centuries to gain widespread scientific and public acceptance. While Darwin's theory of evolution was quickly grasped by biologists, portions of the public today, especially in places like the U.S., still disbelieve. Currently, the near total consensus by climatologists that human activities are warming the Earth continues to be challenged by outsiders. Whether or not the biotic pump theory will one day fall into this grouping remains to be seen. First published in 2007 by two Russian physicists, Victor Gorshkov and Anastassia Makarieva, the still little-known biotic pump theory postulates that forests are the driving force behind precipitation over land masses.
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