It is becoming clear that rainforest ecosystems may collapse when a certain “critical mass” of forest has been cleared or damaged. That is, ecosystems cannot maintain themselves when too much forest has been removed, due to the loss of interrelationships among species. When a number of environmental factors act together (synergism), they may cause the demise of the forest ecosystem. For instance, in Borneo, where much of the forest has been cut, burned, or leased for logging, a number of factors – drought, deforestation, fragmentation – have combined synergistically to endanger the remaining rainforests. The past 20 years have been years of severe and prolonged drought and uncontrolled logging. Unusually dry conditions have led to innumerable forest fires, further damaging the forests.
1) Southeast Asian Dipterocarps: As mentioned above, dipterocarp species (the large canopy trees) of Southeast Asia fruit sporadically and synchronously, and only during El Niño years. Within six weeks of the onset of El Niño, the dipterocarps flower, fruit and disperse seeds, a bounty for fruit-feeding animals. Some of the seeds will sprout into seedlings, the next potential generation of dipterocarps. Synchronous reproduction requires large areas of intact forest; as this area is reduced, fewer viable seeds are produced. According to recent data, on the island of Borneo this intricate reproductive system is in trouble. Many species of dipterocarp in protected forests surrounded by logged areas have failed to produce viable seedlings over the last decade. This is in contrast with 1991, when the same area produced 155,000 seedlings per hectare after a mast fruiting. The loss of reproductive ability may be due to the synergistic impacts of El Niño droughts, logging activities, and the establishment of plantations on former forest land. Areas of intact forest surrounded by degraded ones can provide food and shelter for animals fleeing damaged areas, but they consume all of the fallen seeds, leaving none on the ground to sprout. Because there is not enough food for the additional wildlife, populations of seed- and fruit-eating species will crash (Curran, et al., 1999; Hartshorn, 1999). As an example, orangutan populations have declined by 50% in the past 10 years (Wuethrich, 2000).
2) Fires: Fires destroy seeds (as well as killing animals and trees), and smoke from fires kills seedlings, is unhealthy for animals, and can inhibit rainfall. Additionally, trees which are killed in fires provide fuel for future, more devastating fires. During the El Niño droughts of 1997-1998, 10 million acres of forest in Central and South America were consumed by fire; another 11.5 million acres burned in Southeast Asia. Some of this land had been logged; some was primary forest. Many of these fires were set deliberately to clear pasture or cropland, and then burned out of control because of the dry conditions. Forty per cent of the El Ocote Biosphere Reserve in Chiapas, southern Mexico, was destroyed in 1998 by fires set by hunters to drive prey and by farmers and ranchers in the vicinity. These fires burned for almost four months, as did fires in Chimalapas Biological Reserve, in Oaxaca state (Cochrane, 2001). More than two million acres of rainforest burned in southern Mexico in that year (Stolzenburg, 2001). Unsustainable logging in East Kalimantan (Indonesian Borneo) has exacerbated the risk of spontaneous fire, already severe due to dessication from El Niño and the small fires normally set by people for forest conversion, hunting, and animal collection, as well as disputes over land ownership. In 1997-1998, approximately 5.2 million hectares (52,000 km2) were affected by fires, 42% with “severe damage” and 34% with “total damage.” Pulpwood plantations and recently-logged areas suffered the most, pristine forests the least, as the latter are naturally resistant to fire (Siegert, 2001).
The fragmentation of forests encourages fires; most fires begin near or at forest edges. Trees near the edges also suffer more from drought and many more of them die during dry periods than trees in the forest interior. These synergistic effects are being seen in many tropical areas, such as the Amazon, which is becoming increasingly drier. Many parts of the Amazon are now barely wet enough to support tropical rainforests and so are hard-hit by drought and are vulnerable to fire. Drought, even more than logging, may eventually doom many Amazonian tropical forests through changes in the hydrological environment.
According to Cochrane, et al., (1999), almost 50% of the remaining Amazon rainforest has been disturbed by accidental fires. Fires reduce the understory vegetation, increase temperatures and decrease humidity under the canopy, all of which conditions lead to greater risk of additional future fires. As Cochrane (2001) points out, the temperature in an intact forest usually remains below 28oC, but where the canopy has been reduced by fire, temperatures may reach 38oC, leaving the forest ripe for more fires. Similarly, reduction of canopy cover by 40%, such as occurs after a fire, reduces the humidity greatly, providing ideal conditions for more fires. Recurrent fires are much more destructive than are initial fires, because their intensity tends to be greater, and even large trees will be consumed. Thus, a destructive cycle begins.